Note: Descriptions are shown in the official language in which they were submitted.
s
CA 02315438 2000-07-27
LITTONP.003A PATENT
ACOUSTIC SENSING SYSTEM FOR DOWNHOLE SEISMIC APPLICATIONS
UTILIZING AN ARRAY OF FIBER OPTIC SENSORS
BackQround of the Invention
Field of the Invention
The present invention relates generally to acoustic sensing systems, and more
specifically relates to a system for sensing acoustic waves comprising an
acoustic sensor
array.
Description of the Related Art
Typically, to obtain oil, a well or hole is dug by drilling and removing earth
from the ground to form a shaft known as a "borehole," which extends to the
bottom of
the well. Generally, a large metal pipe or casing will be inserted into the
borehole.
Smaller pipes, known as production tubes, are inserted into the casing. These
production tubes allow access to the bottom of the well. For example, oil may
be drawn
from the well through the production tubing.
Ultimately, the well will appear to go dry. Despite the apparent lack of oil
within the well, vast supplies of oil are often trapped in pockets in the
earth nearby the
well. These pockets, however, are generally inaccessible to the drilled well.
To locate
such pockets, known in the art as "in-place" reserves, geologists conduct
surveys of
swaths of earth sunounding the wells. Geologists employ techniques like cross-
well
tomography in which acoustic waves are transmitted through a volume of earth
to
characterize properties, such as density, in that volume. Knowledge of the
density of
the earth helps determine the presence or absence of oil in the region of the
earth being
characterized.
To survey the transmission characteristics of a region of the earth, an
acoustic
wave source can be used to generate acoustic waves, i.e., sound, while an
array of
acoustic sensors detects these acoustic waves. Generally, each of the sensors
in the
array will be situated at a different location. The acoustic waves emitted
from the
acoustic source are thus sampled at a plurality of points which typically make
up a line.
-1-
CA 02315438 2000-07-27
By changing the location of the acoustic source, the location of the sensor
array, or both,
the transmission characteristics of a volume of earth may be measured. In this
manner,
a three-dimensional map of the density throughout a region of earth can be
produced.
Although some prior art techniques rely on acoustic sources and/or sensor
arrays
situated on the surface of the earth, placing the acoustic sources and sensor
arrays deep
within the earth is more effective for surveying lower regions of the earth.
To conduct
measurements deep within the earth, a probe can be lowered into the well.
However, the frailty of conventional prior art sensors prevents prior art
sensor
arrays from being employed deep within a well. Conventional sensor arrays
employ
piezoelectric transducers (or piezos) to convert vibrations originating from
the acoustic
waves into electronic signals. Since a piezoelectric transducer outputs only a
small
signal, an electronic preamplifier must be mounted near the piezo to prevent
noise from
overwhelming the small transducer signal. Electronics, however, are
incompatible with
the harsh environmental conditions, such as high temperature and pressure,
that prevail
deep within the earth. Even preamplifiers designed to survive high temperature
have a
short lifetime and may last, for example, only for one hour under harsh
conditions.
Thus, the requirement for an electronic preamplifier prevents piezoelectric
transducers
from being employed deep within a well.
Fiber optic sensors, on the other hand, are electrically passive devices. That
is,
they do not require electrical components or external electrical connections.
Thus they
are less susceptible to the harshness associated with high temperature, high
pressure
environments. Furthermore, fiber optic sensors avoid the environmental
problems
associated with electrical components, e.g., the electromagnetic interference
that arises
when electrical components are placed in the presence of transmission lines.
For these
reasons, fiber optic sensors are sometimes used in hydrophones operating under
harsh
environmental conditions.
Fiber optic hydrophones can generally be classified into two categories.
Hydrophones of the air backed mandrel design have a hollow, sealed cavity that
deforms in response to acoustic pressure, so that strain is transferred to the
fiber
wrapped around the mandrel. Other, less sensitive, fiber optic hydrophone
designs
record the effects of pressure directly on the fiber itself, e.g., the fiber
may be wrapped
-2-
CA 02315438 2000-07-27
around a solid body. Fiber optic hydrophones with high sensitivity (i.e., air
backed
mandrel hydrophones) are generally limited to operating pressures of less than
about
5000 pounds per square inch (psi) and temperatures of less than about 120 C.
Outside
this range, the materials used in the mandrels of air backed mandrel
hydrophones
deform excessively. For example, polycarbonate plastic deforms at these
temperatures,
whereas metals such as aluminum buckle inelastically when subjected to high
pressures.
On the other hand, fiber optic hydrophones utilizing solid bodies or fiber for
acoustic
transduction typically have much lower sensitivities.
In addition to operating limitations on pressure and temperature, current
fiber
optic hydrophones are generally bulky, and may have large cross sections that
do not
lend themselves to use in applications where compactness is essential, e.g.,
in
commercial petrochemical wells and boreholes. Thus, there is a need for a
fiber optic
hydrophone having a relatively small cross section and the ability to
withstand high
pressures and temperatures.
In addition to restrictions on the placement of the prior art acoustic arrays,
limitations exist on the number of sensors that may be employed in prior art
acoustic
arrays. With a larger number of sensors more information must be processed.
Limitations on the amount of information that can be processed within a
reasonable
amount of time restrict the number of sensors that can be used. Higher
resolution maps,
however, can be achieved with a larger number of sensors.
Thus, a need exists for a system for sensing acoustic waves that is rugged
enough to operate in the harsh downhole environment and accommodates a large
number of sensors. .
Systems accommodating a large number of sensors may benefit from the use of
multiplexing, in which multiple signals are communicated within a single line.
One
common approach, known as frequency division multiplexing (FDM), operates by
modulating a carrier wave at a number of different frequencies equal to the
number of
signals that are to be multiplexed. When FDM is applied to a system using
interferometric sensors, the multiplexed signal includes signal components not
just at
the modulation frequencies, but at all harmonic frequencies of the modulation
frequencies as well. For such a system, the multiplexed signal may be
demultiplexed
-3-
CA 02315438 2000-07-27
through detection of the signal components at the modulation and first
harmonic
frequencies, provided these components do not overlap (in frequency) one
another or
any components at the higher harmonics. Such overlap may be prevented by
selecting
modulation frequencies that are sufficiently large and separated that the
lowest second
order harmonic component exceeds the highest first harmonic component. This
leads to
large bands of unused frequency between DC and the highest frequency signal
component detected. However, to keep the signal processing electronics simple
it is
preferable to keep the maximum frequency detected as low as possible. Thus, a
need
exists for a method of selecting a set of FDM modulation frequencies having as
low a
maximum frequency as possible while maintaining fundamental and first harmonic
signal components that are not overlapped by other signal components.
Summary of the Invention
The present invention provides a frequency division multiplexed fiber optic
sensor
array system, and method of modulation frequency selection, having a reduced
system
1s bandwidth. The fiber optic sensor array system incorporates N light sources
modulated at
different frequencies. The N light sources connect up to N fiber optic
acoustic sensors via
N fiber distribution lines. The N fiber optic sensor outputs are coupled to a
single fiber
return line connected to a detector that senses the multiplexed signal at the
N modulation
frequencies and at the N first harmonic frequencies associated with the
modulation
frequencies. The modulation frequencies are selected so that none of the N
modulation
frequencies and N first harmonic frequencies is closer in frequency to any
other
modulation frequency or to any other harmonic frequency of any order than some
minimum separation frequency, M. The N modulation frequencies are selected so
that at
least one of the second harmonic frequencies associated with the modulation
frequencies
is interleaved within the first harmonic frequencies. This has the effect of
allowing for a
smaller system bandwidth than can be accomplished in the absence of
interleaving. In
further embodiments, the N modulation frequencies are selected so that at
least one of the
first harmonic frequencies is interleaved among the modulation frequencies. In
still
further embodiments, the N modulation frequencies are selected so that at
least one of the
second harmonic frequencies is interleaved among the modulation frequencies.
In still
-4-
CA 02315438 2008-09-10
further embodiments, the N modulation frequencies are selected so that at
least one of the
third harmonic frequencies is interleaved among the modulation frequencies.
In accordance with an aspect of the present invention, there is provided a
frequency
division multiplexed fiber optic acoustic sensor system for simultaneously
detecting
acoustical information at N spaced points, comprising: N light sources,
wherein each light
source is modulated at a different modulation frequency; N distribution fiber
lines, wherein
each distribution fiber line is connected to a different one of said N light
sources; a return
fiber line; a sensor group comprising N fiber optic acoustical sensors,
wherein each
acoustical sensor is connected between a different one of said N distribution
fiber lines
and said return fiber line; and a detector connected to said return fiber
line; wherein said N
modulation frequencies are selected such that: each modulation frequency has a
corresponding signal component that is not overlapped in frequency space by
any
other modulation frequency component or by any harmonic frequency components
of any order associated with any of said N modulation frequencies; a first
harmonic
frequency component associated with each modulation frequency is not
overlapped in frequency space by any modulation frequency component or by
any other harmonic frequency components of any order associated with any of
said N modulation frequencies; and at least one of said N second harmonic
frequencies is interleaved within said first harmonic frequencies.
In accordance with another aspect of the present invention, there is provided
in a
frequency division multiplexed fiber optic acoustic sensor system having N
light sources modulated at N different modulation frequencies, a method of
optimally selecting said N modulation frequencies, comprising: selecting N
modulation
frequencies, fl, fZ, ...fN, wherein: none of said N modulation frequencies and
their
corresponding N first harmonic frequencies is closer in frequency space to any
other modulation frequency or to any other harmonic frequency of any order
associated with any modulation frequency than some minimum frequency, Af, and
at
least one of said second harmonic frequencies is interleaved within said first
-5-
CA 02315438 2008-09-10
harmonic frequencies.
Brief Description of the Drawings
The present invention will be described in detail below in connection with the
attached drawings, in which:
FIGURE 1 illustrates a side elevational view of a downhole acoustic sensing
system that is the preferred embodiment of the present invention;
FIGURE 2 illustrates a perspective view of a cable comprising a downlead cable
and
a sensor array cable;
FIGURE 3A illustrates a schematic view of the first embodiment of the
acoustic sensing system of the present invention comprising six laser sources,
sixteen
optical detectors, and 96 acoustic sensors, wherein the sensors are contained
within a single
acoustic sensor array;
FIGURE 3B illustrates a schematic view of an embodiment of the acoustic
sensing
system of the present invention comprising six laser sources, 32 optical
detectors, and 192 acoustic sensors, wherein the sensors are contained within
two
separate acoustic sensor arrays;
FIGURE 4, comprising FIGURES 4A-4H, illustrates a schematic view of one
implementation of the distribution and return of the optical signal in the
first embodiment.
This implementation accommodates a 6 x 16 optical sensors array having sixteen
sensor
groups, wherein each sensor group has a dedicated return fiber line;
FIGURE 5 illustrates a schematic view of one preferred embodiment of the
acoustic sensor, a fiber sensor that is a Mach-Zehnder interferometer;
FIGURE 6 illustrates a block diagram of the detector/electronics assembly and
laser
drawer in the first embodiment of the acoustic sensing system having 96
sensors in the 6 x
16 sensor array of FIGURE 4;
FIGURE 7 illustrates a flow chart of the interaction of the acoustic source
and the
acoustic sensing system;
-5a-
CA 02315438 2000-07-27
FIGURE 8 illustrates a flow chart of the operation of the acoustic sensing
system, namely, the process by which acoustic waves are sensed and data is
output in
conventional industry standard seismic format; and
FIGURE 9, comprising FIGURES 9A-9B, illustrates a schematic view of the
detector/electronics assembly and laser drawer in the second embodiment of the
acoustic sensing system having 192 sensors in a 2 x (6 x 16) sensor array.
FIGURE 10, comprising FIGURES l0A and lOB, illustrates frequency
components for multiplexed signals in which the modulation frequencies have
been
selected so as to keep the fundamental, first harmonic, and second harmonic
sets from
overlapping. FIGURES l OA and l OB illustrate the components for systems with
five and
six modulation frequencies, respectively.
FIGURE 11, comprising FIGURES 11A and 11B, illustrates frequency
components for multiplexed signals in accordance with an embodiment of the
present
invention, wherein the modulation frequencies are selected to be equally
spaced, and
1s wherein the first harmonic and second harmonic sets overlap without
overlapping the
component signals within the two sets. FIGURES 11A and 11B illustrate the
components
for systems with five and six modulation frequencies, respectively.
FIGURE 12 illustrates frequency components for a multiplexed signal resulting
from five light sources in accordance with an embodiment of the present
invention,
wherein the modulation frequencies are evenly spaced beginning at 60f, except
for
skipping a modulation frequency at 90f.
FIGURE 13 illustrates frequency components for a multiplexed signal resulting
from six light sources in accordance with an embodiment of the present
invention,
wherein the modulation frequencies are evenly spaced beginning at 7Af, except
for
skipping a modulation frequency at 120f.
FIGURE 14, comprising FIGURES 14A and 14B, illustrates frequency
components for a multiplexed signal resulting from six light sources in
accordance with an
embodiment of the present invention, wherein the modulation frequency
components are
-6-
CA 02315438 2000-07-27
selected at Of multiples of 5z/3, 7, 8, 9, 10, and 12%Z.. For clarity, FIGURE
14A isolates
the fundamental frequency components.
FIGURE 15, comprising FIGURES 15A and 15B, illustrates frequency
components for a multiplexed signal resulting from six light sources in
accordance with an
embodiment of the present invention, wherein the modulation frequency
components are
selected at Af multiples of 3, 4, 5, 7, 11, and 13. For clarity, FIGURE 15A
isolates the
fundamental frequency components.
FIGURE 16 illustrates a cutaway view of a hydrophone embodiment that resides
within a cable.
FIGURE 17 illustrates a cross sectional view of the cable of FIGURE 16 at a
location away from the hydrophone.
FIGURE 18 illustrates mechanical support features used around the
hydrophone's sensor to protect it from breakage that might otherwise occur
during
bending of the cable.
FIGURE 19 illustrates an expanded view of the sensor showing a telemetry can,
a reference mandrel, and two sensing mandrels, as well as the optical fibers
that link
them.
FIGURE 20, comprising FIGURES 20A, 20B, and 20C, illustrates schematic
diagrams of the optical fiber routing within the sensor. In FIGURES 20A, 20B,
and
20C, the sensor functions as a Mach-Zehnder interferometer, a Michelson
interferometer, and a Fabry-Perot interferometer, respectively.
FIGURE 21 illustrates a perspective view of the reference mandrel including
its
hemispherical endcaps.
FIGURE 22 illustrates a cross sectional view of a hemispherical endcap.
FIGURE 23 illustrates a flexible interlink used to join two hemispherical
endcaps.
-7-
CA 02315438 2000-07-27
Detailed Description of the Preferred Embodiment
A system 100 for sensing acoustic waves 102 in accordance with a preferred
enlbodinient of the present invention is shown in FIGURE 1. The system 100
comprises
an acoustic array cable 104 attached to a downlead cable 106 which is held on
a first
spool 108 on a first truck 110. The downlead cable 106 passes from the first
spool 108
to a reel 112, also mounted on the first truck 110, and to a sheave 114
situated on a
surface 116 adjacent to a well 118. From the sheave 114, the downlead cable
106 runs
up to a pulley 120 fixed to a crane 122. The downlead cable 106 and the
acoustic array
cable 104 extend from this pulley 120 into the well 118. The well 118
comprises a first
borehole 124 formed in a layer of earth 126. A large metal pipe known as a
casing (not
shown) is inserted into the borehole 124. The downlead cable 106 on the spool
108 is
connected to a receiver processing electronics 128 housed in the first truck
110.
An acoustic source 130 is situated in a second borehole 132. This acoustic
source 130 is attached to an acoustic source cable 134, which is held on a
second spool
136 on a second truck 138. The acoustic source cable 134 passes from the
second spool
136 to a second reel 140, also mounted on the second truck 138, and to a
second sheave
142 situated on the surface 116 adjacent to the second borehole 132. From the
second
sheave 142, the acoustic source cable 134 runs up to a second pulley 144 fixed
to a
second crane 146. The acoustic source cable 134 extends from this pulley 144
into the
second borehole 132. Also housed in the second truck 138 is source electronics
148
associated with the acoustic source 130. The acoustic waves 102 emanate from
the
acoustic source 130 in the second borehole 132 and arrive at the acoustic
array cable
104 in the first borehole 124.
A perspective view of a cable 202 comprising the downlead cable 106 and the
acoustic array cable 104 is shown in FIGURE 2. An interface 204 connects the
downlead cable 106 to the acoustic array cable 104. The acoustic array cable
104 is
terminated by a gamma detector 206, which operates in a conventional manner to
produce an electrical signal responsive to the passage of the gamma detector
206
through each section of pipe forming the casing within the borehole 124. The
gamma
detector 206 provides a signal that is a processed to determine the depth to
the
termination of the acoustic array cable 104.
-8-
CA 02315438 2000-07-27
As shown in FIGURE 3A, a plurality of laser sources LS1, LS2, LS3, LS4, LS5,
LS6 are positioned to supply optical feed lines F1-F6, which are joined at an
optical
terminator 302. The optical terminator 302 connects to the downlead cable 106,
which
is connected to the acoustic array cable 104. The acoustic array cable 104
houses a
plurality of sensors, which in this exemplary embodiment total 96 and are
designated
Sl-S96. The optical terminator 302 also provides a link between the downlead
cable
106 and a plurality (e.g., 16) of return fibers R1-R16, which are coupled to
optical
detectors D 1-D 16. The outputs of the optical detectors D 1-D 16 are
electrically
connected to processing electronics 304.
Each laser source LSI, LS2, LS3, LS4, LS5, LS6 comprises a respective laser
L1, L2, L3, L4, L5, L6 and a modulator M1, M2, M3, M4, M5, M6. Each of the
lasers
L1-L6 generates an optical beam having a different optical wavelength. The six
optical
beams produced by these lasers L1-L6 are directed to respective modulators M1-
M6.
Preferably, these modulators M1-M6 comprise phase modulators, each
characterized by
a different modulation frequency. Accordingly, the laser sources LS 1, LS2,
LS3, LS4,
LS5, LS6 output six optical signals each having different optical wavelengths
and each
modulated at a separate modulation frequency.
FIGURE 3B shows an embodiment comprising 192 sensors S 1-S 192 contained
within two separate acoustic array cables 104a, 104b appended to two separate
downlead cables 106a, 106b. The two separate acoustic array cables 104a, 104b
and
downlead cables 106a, 106b could be inserted in two separate boreholes 124.
This
embodiment having 192 sensors will be discussed more fully below.
The plurality of feed lines F1-F6 are connected to a plurality of distribution
fiber
lines DFl-DF6 (shown in FIGURE 4A-4H) at the optical terminator 302 to
transfer the
optical signals outputted by the laser sources LS1-LS6 to the distribution
fiber lines.
These distribution feed lines DFl-DF6 run through the downlead cable 106 and
into the
acoustic array cable 104 as well.
FIGURE 4, which comprises FIGURES 4A-4H, shows the 96 sensors S 1-S96 in
a single acoustic array cable 104 similar to that shown in FIGURE 3A. These 96
sensors S 1-S96 are divided into eight sensor groups of twelve sensors each. A
first
sensor group, group 401, is shown in FIGURE 4A. The optical path from the
first
-9-
_.
CA 02315438 2000-07-27
sensor group 401 to the laser sources LS1, LS2, LS3, LS4, LS5, LS6 and to the
processing electronics 304 is shorter than for any of the other sensor groups
402-408.
Seven additional sensor groups 402-408 are shown in FIGURES 4A-4H. Each sensor
group 401-408 has at least one sensor coupled to each of the six distribution
fiber lines
s DF1-DF6. For example, in the first sensor group 401, the distribution fiber
lines DF1-
DF6 are connected to respective standard 1 x 2 input couplers 420, which are
in turn
connected to respective sensors S 1-S 12. Similarly, in the second sensor
group 402, the
distribution fiber lines DF 1-DF6 are connected to respective sensors S 13-S24
via
additional standard I x 2 input couplers 420.
All the sensors S 1-S 12 in the group 401 are coupled to two return fiber
lines
RF1, RF2. Similarly, each of the sensor groups 402-408 has two of the return
fiber
lines RF2-RF16 dedicated solely to its use. For example, sensors S7-S24 are
all
coupled to two of the return fiber lines RF1-RF16, namely, the third and
fourth fiber
lines RF3, RF4. As a further example, the sensors S85-S96 are coupled to the
last two
fiber lines RF15, RF16. In this embodiment, no adjacent sensors S 1-S96 share
a
common return fiber line RF 1-RF 16.
The return fiber lines RF 1-RF 16 are connected to return fibers R 1-R16. The
return fiber lines RF1-RF16 and the return fibers R1-R16 direct the optical
outputs of
the acoustic sensors S 1-S96 to the optical detectors D 1-D 16.
In FIGURE 5, the acoustic sensors S1-S96 comprise an interferometer 502 that
is sensitive to acoustic pressure, pressure changes, or pressure waves. The
interferometer 502 depicted in FIGURE 5 is a Mach-Zehnder interferometer. This
interferometer 502 includes a sensor input line 504, which is connected to a
first coupler
506. A reference ann 508 and a test or sensing arm 510 are attached to this
first coupler
506. The reference arm 508 and the test arm 510, are optical fibers. The
optical fibers
508, 510 are connected to a second coupler 512 that is connected to a sensor
output line
514. The input coupler 420 and output coupler 430 are connected to the sensor
input
line 504 and sensor output line 514, respectively.
The optical signal that emanates from the laser sources LSI-LS6 is coupled
into
the sensor input line 504 of the interferometer 502 via the input coupler 420.
This
signal is split by the first coupler 506 into two beams. A reference beam
travels through
-10-
CA 02315438 2000-07-27
the reference arm 508, and a test beam travels through the test arm 510. The
two beams
are coupled into a single fiber 514, the sensor output line, at the second
coupler 512 of
the interferometer 504. The reference beam and the test beam interfere in the
second
coupler 512 to produce an output signal that is detected at one of the optical
detectors
D1-D16.
Acoustic vibrations that impinge on one of the acoustic sensors Sl-S96 cause
the optical fiber comprising the respective test arm 510 to be deformed, e.g.,
to be
stretched or contracted, which in turn changes the optical path length of the
test arm
510. In contrast, the reference arm 508 is shielded from the acoustic
vibration. Thus,
the optical path length of the reference arm does not change. Since the
optical path
length of the test arm 510 changes while the optical path length of the
reference arm 508
does not change, the phase difference between the beams traveling in the test
and
reference arms changes in response to the acoustic vibrations. The changes in
relative
phase between the test and reference arms 510, 508 result in time-varying
interference at
the second coupler 512. The time-varying interference results in a time
varying light
intensity of the signal output from the second coupler 512. The time-varying
light
intensity is detected by one of the detectors (e.g., the first detector Dl).
FIGURE 6 depicts a detector/electronics assembly 601 for the first embodiment
of the acoustic sensing system 100, which has sixteen return fibers R1-R16
that are
coupled to the sixteen optical detectors D1-D16. The detector/electronics
assembly 601
includes the optical detectors D1-D16 and the processing electronics 304.
FIGURE 6 also schematically shows an optic sensor array 602 and illustrates
how the detector/electronics assembly 601 is connected to the optical sensor
array and
to the laser sources LS1-LS6. As defined herein, the optical sensor array 602
comprises
a plurality of optical sensors coupled together using optical fibers. The
optical sensor
array 602 shown in FIGURE 6 includes the designation 6 x 16 corresponding to
the six
distribution fiber lines DF 1-DF6 and 16 return fiber lines RF 1-RF6 shown in
FIGURES
4A-4H.
Each of the optical detectors Dl-D16 is included as part of the four 24-
channel
digital receivers/demodulators 604. The optical detectors are separated into
four groups,
-11-
CA 02315438 2000-07-27
D1-D4, D5-D8, D9-D12, and D13-D16, wherein each group is situated in one of
the
four 24-channel digital receiver/demodulators 604.
As shown in FIGURE 6, the four 24-channel digital receiver/demodulators 604
are electrically connected to four 24-channel digital signal processors (DSPs)
606. Each
of the 24-channel DSPs 606 comprises twelve digital signal processing chips.
Accordingly, the term "12-DSP processing element" 606 may be used
interchangeably
with 24-channel digital signal processors.
Each of the 24-channel digital receiver/demodulators 604 is paired with one of
the 12-DSP processing elements 606. The four 12-DSP processing elements are
coupled to a PCI bus 608 (or other suitable bus), which is coupled to a
central
processing unit (CPU) 610, such as, for example, an Intel Pentium II or
Pentium III
processor.
The CPU 610 is coupled to a hard drive 612 via a SCSI bus 614. The central
processing unit 610 is also connected to an operator console 616 and a
recording and
processing system 618 via two Ethernet lines 620, 622.
Each of the 24-channel digital receiver/demodulators 604 acconunodates 24
signals because each of the four detectors D l-D 16 within one of the digital
receiver/demodulators receives six signals from a group of six sensors. The
six signals
that arrive at each of the optical detectors D 1-D 16 originate from the six
laser sources
LS 1-LS6 and have a different optical wavelength and have different modulation
frequency. Upon being irradiated by the six signals, each of the optical
detectors Dl-
D 16 outputs an electrical signal having components proportional to the
intensity of the
optical light incident thereon at each of the modulation frequencies and at
hannonics of
the modulation frequencies. The electrical signal from one of the optical
detectors, e.g.,
the first detector Dl, is separated into the six signals produced by the six
acoustic
sensors, e.g., the first six odd sensors S1, S3, S5, S7, S9, S11, whose
outputs are
channeled to the optical detector. The six signals are distinguished by
separating the
components according to the modulation frequencies. Although the light
incident on
the detector D1 comprises six different optical wavelengths, it is not
necessary to
separate the signals optically. The difference in optical wavelengths is used
to keep the
six signals from optically interfering with each other.
-12-
CA 02315438 2000-07-27
The total number of acoustic sensor signals processed by the
detector/electronics
assembly 601 employed in the embodiment depicted in FIGURE 6 is 96. Each of
the
24-channel digital receiver/demodulators 604 receives four optical signals
from four of
the return fibers RI-R16. The 24-channel digital receiver/demodulator 604
converts
each of the four optical beams into six separate electrical channels,
resulting in 24
electrical channels. Since the detector/electronics assembly 601 for the
embodiment
shown in FIGURE 6 has four 24-channel digital receiver/demodulators 604, a
total of
96 (4 x 24) electrical channels are utilized. Each of the 96 electrical
channels contain
information relating to the acoustic vibrations at a respective one of the 96
acoustic
sensors S 1-S 96.
As noted above, each of the acoustic sensors S 1-S96 comprises an
interferometer 502 that splits the coherent light source into two waves
following
separate paths that eventually converge. Upon convergence, the two waves
interfere
with each other such that the intensity I of the combination is given by I = A
+ B cos 0,
where A and B are constants and 0 is the phase difference between the two
waves upon
convergence.
In order to multiplex the six sensor signals associated with the six lasers L1-
L6
that are transmitted via each return fiber (e.g., RF1), the interferometer
phase angle of
each of the six sensors is modulated at a different frequency, con. The
interferometer
phase angle modulation may be represented as 0(t) = C. cosUJnt, where n = 1,
..., 6, and
C. is the amplitude of the phase modulation in radians. The phase angle in the
interferometer is modulated by sinusoidally varying the phase of each laser Ll-
L6. This
is accomplished by the modulator Ml-M6 by sinusoidally varying the voltage
across a
lithium niobate segment (not shown) of the optical path. A laser source phase
modulation, (D =(Docos((ot), where (Do is the phase amplitude in radians,
results in a laser
frequency modulation f = f, + Af sin(cot), where f, is the optical carrier
frequency and Af
=(Dow/2n. This frequency modulation, in turn, results in a modulation of the
interferometer phase angle, cp = 27rALOf / c sin((ot), where AL is the path
length offset
between the two interferometer paths and c is the speed of light in the fiber.
This modulation results in a time varying intensity for the output of the nth
interferometer given by: Iõ(t) = Aõ + Bõ cos [ Cõ cos((0nt)+cpõ(t) ], where
cpõ(t) is the time
-13-
CA 02315438 2000-07-27
varying phase created by the acoustical signal in the nth optical sensor (and
signal
noise). This equation may be expanded in terms of Bessel functions to give:
In (t) = A + Bn lj Jo(Cn) + 2E (-1)k J2k(Cn) cos(2k(Ont) D cos(cpn(t))
k=1,m
~ 2E (-1)k J2k+1(Cn) cos((2k+l )c)nt) D sln(Tn(t)) 1.
k=0,~
As noted earlier, the N lasers Ll-L6 are chosen to have sufficiently different
optical carrier frequencies to avoid optical interference. Thus, the total
intensity on the
detector, Itoõ connected to this particular return fiber (e.g., RF1) is then
given by I,Jt) =
In(t). The light intensities detected by each of the 16 detectors D 1-D 16 is
n=1,6
described by an analogous equation.
The above equations demonstrate that the interferometer intensity output
contains signal not only at the six modulation frequencies con, but also at
2con, 3ciln, etc.
The multiplexed intensity signal received by a given detector D 1-D 16 may be
fully
demultiplexed through detection of the signal components at con and 2coo using
the
following approach.
The total output signal, I,nl, may be mixed with a signal at con and a signal
at 2w,,,
and the results of the mixing may be low pass filtered to remove the signal at
all
harmonics above the first harmonic. This results in "direct" (I) and
"quadrature" (Q)
components, such that: In = BnGJI(Co) sincpo(t) and Q. = BnHJ2(Cn) coscpn(t),
where G
and H are the amplitudes of the mixing signals correspo.nding to the wo and
2con
components of the signal, respectively. The properties of Bessel functions are
such that
J,(x) and J2(x) are equal when the parameter x=2.6. See, e.g., Handbook of
Mathematical Functions, 1974, edited by M. Abramowitz and I. Stegun. Then, by
choosing G = H and Cn = 2.6 radians, the phase angle is given by: cpn(t) =
arctan(In/Q.
Thus, to demodulate, the 24-channel digital receiver/demodulators 604 mix the
electrical signals output by the optical detectors D 1-D 16 with sinusoidal
waveforms at
the six frequencies at which the output of the six lasers L1-L6 are modulated.
The 24-
channel digital receiver/demodulators 604 also mix the electrical signals
output by the
optical detectors D 1-D 16 with sinusoidal wavefonms having twice these six
frequencies.
Accordingly, the 24-channel digital receiver/demodulators 604 will mix the
electrical
-14-
CA 02315438 2000-07-27
signals output by the optical detectors D1-D16 with sinusoidal carriers at
frequencies of
(J)11 (02, (03, (J)41 (05, w61 and 2c)õ 2(1)2, 2cL)3, 2w4, 2w5, and 2(J)6.
As noted above, the demodulated signals produced as a result of this mixing
result in direct (I) and quadrature (Q) components. These components are
provided for
each channel as inputs to a circuit (not shown) that outputs the arctangent of
the two
components. In this manner, polar phase is obtained from the demodulated
signals.
This polar phase corresponds to the phase difference between the optical beams
in the
test and reference arms 510, 508. The time derivative of the polar phase is
generated
from digital circuitry (not shown) that is designed to implement
differentiation. The
derivative of the phase is proportional to the magnitude of the acoustic
vibrations sensed
at the sensors SI-S96.
The derivative of the phase produced by two channels of each 24-channel
digital
receiver/demodulator 604 is sent to one element of the corresponding 12-DSP
elements
606. The 12-DSP elements 606 filter and decimate the demodulated signals down
to
standard sample rates required by conventional seismic data recorders. These
12-DSP
elements 606 are coupled to the PCI bus 608 and use the PCI bus to communicate
with
the CPU 610. Accordingly, the filtered and decimated derivative of the phase
are fed
into the CPU 610. Note that each of the 12-DSP elements 606 processes the
phase
information from two acoustic channels, each of which is performed separately.
The CPU 610 formats the data corresponding to the acoustic vibrations such
that
it is compatible with industry standards (e.g., the SEG-D fonmat). For
example, the
CPU 610 stamps the acoustic data output with the time of system events such as
the
start of sensing. The CPU also adds any necessary information to identify the
data in
accordance with the industry standard format.
The CPU also handles interfaces with conventional seismic data recording
equipment. The CPU 610 sends the reformatted acoustic data to seismic data
recording
equipment at industry standard data rates. More specifically, the processed
and
formatted signals generated from the acoustic sensors S 1-S96 and optical
detectors D 1-
D 16 are transmitted over the PCI bus 608 to the CPU 610 and are outputted to
customer
supplied seismic processing equipment via the Ethernet line 622.
-15-
CA 02315438 2000-07-27
The host CPU 610 additionally provides system control and sequencing for the
operation of the individual components in the acoustic sensing system 100 .
The CPU also handles interfaces with an operator console 616. The operator
console 616 allows manual system intervention and is also used to display
system
status.
The detector/electronics assembly 601 additionally includes an auxiliary
inputloutput subsystem 624 that interfaces with the central processing unit
610 via the
PCI bus 608. This auxiliary input/output subsystem 624 interface with customer
supplied equipment (CSE) 626 to provide up to sixteen acoustic or non-acoustic
sensor
inputs for time marking or event triggering.
The detector/electronics assembly 601 additionally includes a global position
sensing (GPS) electronics card 628 that is electronically connected to an
antenna 630.
The GPS electronics card 628 interfaces with the CPU 610 via the PCI bus 608.
The
GPS electronics card 628 provides accurate time for the host CPU 610 to
facilitate time
stamping of system events.
In the embodiment shown in FIGURE 6, a frequency synthesizer card 632 is
included with the detector/electronics assembly 601. The frequency synthesizer
card
632 accepts a sync pulse from additional customer supplied equipment (CSE)
634.
Preferably, the frequency synthesizer card 632 accepts a sync pulse from the
source
electronics 148 associated with the acoustic source 130 in FIGURE 1. As shown
in
FIGURE 1, the electronics 148 associated with the acoustic source 130 is
located in the
second truck 138 adjacent the second borehole 132.
The frequency synthesizer card 632 is electrically connected to a laser module
controller/driver card 636, which is connected to the laseasources LS1-LS6,
both of
which are preferably located in a laser drawer 638. Additionally, the
frequency
synthesizer card 630 is electrically connected to an ISA bus 640 that is also
coupled to
the central processing unit 610.
As described above, the laser sources LSI-LS6 include lasers L1-L6 and
modulators Ml-M6, which provide signals to the optical feed lines Fl-F6 that
are
coupled to the acoustic sensors S1-S96. The frequency synthesizer card 632
provides
the modulators M l-M6 with periodic waveforms having the six modulation
frequencies
-16-
CA 02315438 2000-07-27
to modulate the outputs of the six lasers L 1-L6. The frequency synthesizer
card 632
also provides the 24-channel digital receiver/demodulators 604 with global
synchronization and timing signals to insure that the modulators Ml-M6 and
demodulator are phase locked. In particular, the frequency synthesizer card
632
provides a sync signal and a high speed clock signal to the 24-channel digital
receiver/demodulators 604. Using this sync signal and this clock signal, the
24-channel
digital receiver/demodulators 604 generate digital representations of
sinusoidal carriers
at the six modulation frequencies wõ c0z, w,, w4, w5, w6 and at twice the
modulation
frequencies 2w,, 2(L)2, 2w3, 2w4, 2ws, and 2w6. These digital carriers are
employed by
24-channel digital receiver/demodulators 604 for mixing and demodulation as
described
above.
The operation of the above-described acoustic sensing system 100 as presented
in FIGURES 1-6 is illustrated in FIGURE 7 in flowchart form. A first block 702
in a
source flow diagram represents the triggering event for the operation of the
acoustic
sensing system 100, wherein the acoustic source 130 transmits a sync pulse to
the
acoustic sensing system. (See FIGURE 1.) In an alternative preferred
embodiment, the
acoustic sensing system 100 can send a sync pulse to the acoustic source 130
to trigger
the source. This acoustic source 130 may comprise, e.g., a surface acoustic
source or an
underground acoustic source.
The acoustic sensing system 100 receives the sync pulse as indicated by a
first
block 704 in a series of blocks corresponding to the steps performed by the
acoustic
sensing system 100. In response to receiving the sync pulse, the acoustic
sensing
system 100 begins sensing. That is, the acoustic sensing system 100 begins
measuring
the level of acoustic vibration at the sensors S1-S96. The start of the
sensing is
represented by block 706 in FIGURE 7.
As shown in the source flow diagram, after a predetermined delay (block 708),
the acoustic source 130 starts producing acoustic waves 102 as indicated in a
block 710.
As represented by a block 712, the acoustic sensing system 100 continues
monitoring
the level of acoustic vibration at the sensors S 1-S96 and begins to sense the
acoustic
waves 102 emitted by the acoustic source 130 that reach the acoustic sensors.
A more
-17-
CA 02315438 2000-07-27
detailed discussion of the steps involved in sensing acoustic vibration are
presented in
FIGURE 8 in flow chart form, as discussed more fully below.
A block 714 represents the sensing system 100 sending the results of
measurements of the level of vibration at the acoustic sensors S1-S96 to
seismic
processing system as seismic data. At a block 716, the system 100 stops
sensing the
acoustic data. A determination as to when to stop sensing data is
advantageously based
upon the expiration of a predetermined time internal from the sync pulse.
The process for sensing acoustic data in the block 706 and the block 712 in
FIGURE 7 is depicted in more detail in FIGURE 8. As discussed above, the
sensing for
acoustic vibration at the acoustic sensors S 1-S96 starts immediately after
receiving the
sync pulse, although a delay exists between the time the sync pulse is
received and the
acoustic source 130 begins producing acoustic waves 102. This permits the
seismic
processing system to receive data indicative of the acoustic background noise
prior the
receipt of acoustic waves from the acoustic source.
In FIGURE 8, a first block 802 indicates that continuous wave light is emitted
from each of the laser sources LSl-LS6. The light from each source is
modulated, as
discussed above. In particular, the light from each of the laser sources LSl-
LS6 is
modulated at a different modulation frequency.
A block 804 represents the next step wherein the distribution fiber lines DF1-
DF6 propagate the light from the laser sources LS1-LS6 to the optical sensors
Sl-S96.
As discussed above, the light in the respective test arms 508 of the optical
sensors S1-
S96 is variably delayed when acoustic waves 102 strike the sensors. (See block
806).
The light in the reference arm 510 of each sensor S1-S96 is not variably
delayed. Each
of acoustic sensors Sl-S96 combines the light from the two arms 508, 510 in
the output
coupler 512.
A block 808 represents the return fiber lines RF1-RF16 carrying the light
outputted by the optical sensors Sl-S96 to the fiber receivers 604, i.e., the
24-channel
digital receivers/demodulators 604. The fiber receivers, which include the
optical
detectors D1-D16, convert the optical signals incident on the optical
detectors into
electrical signals as indicated in a block 810. As depicted by a block 812,
the
processing electronics 304 convert the electrical signal outputted by the
optical
-18-
CA 02315438 2000-07-27
detectors D1-D16 into SEG-D format, a standard format established by the
Society of
Exploration Geophysicists. The SEG-D format is conventional and is well known
in the
art.
The embodiment described above is particularly well suited for subterranean
geophysical surveys such as are employed in determining the presence of "in-
place" oil
reserves. The acoustic sensors S 1-S96 contained within the acoustic array
cable 104 are
capable of being lowered into the borehole of an oil well. The acoustic
sensors S1-S96
may also be employed for land seismic applications and in ocean bottom cables.
As used herein, the term borehole is defined as a shaft that extends to the
bottom
of a well 118 and a "well" is simply a hole dug by drilling and removing earth
from the
ground, often for the purpose of accessing oil or water.
Cable
The cable 202 shown in FIGURE 2 is designed to fit into a well 118 such as an
oil well. If the cable 202 is small enough, the cable can be inserted into the
production
tubing or in the gaps between the production tubing in the casing. However,
the cable
needs to be smaller than at least the inner diameter of the production tubing.
As described above, the term "casing" refers to a large metal pipe that is
typically inserted into the borehole. "Production tubes" are smaller pipes
inserted in the
casing that allow access to the bottom of the well 118.
The standard diameter for production tubing is two inches in the United States
and is 1.25 inches,in the North Sea. Consequently, to fit in the production
tubing or in
the gaps between the production tubing, the cable 202 needs to have a diameter
less than
two inches for use in the United States and less than 1.25 inches for use in
the North
Sea.
Conventional electronic acoustic sensor arrays range from 2.5 to 6 inches in
diameter requiring all the production tubing to be removed from the casing in
order to
insert a probe containing the array down into the well 118. After the probe is
removed,
the production tubing must be reinserted into the casing. The removal and
reinsertion
procedure is both costly, time-consuming, and inconvenient.
-19-
CA 02315438 2000-07-27
Accordingly, the cable 202, including the downlead cable 106, the interface
204,
and the acoustic array cable 104 have an outer diameter that is less than two
inches.
The diameter of the cable 202 is preferably than 1.25 inch. More preferably,
the
diameter of the cable 202 is less than 1.1 inches. Also, preferably the
diameter of the
acoustic array cable 104 does not vary more than 0.01 inch.
As shown above, the cable 202 includes a downlead cable 106 joined to an
acoustic array cable 104. The downlead cable 106 does not contain any sensors
S 1-S96.
Preferably, the downlead cable 106 has a length selected from the range
between 1,000
feet and 20,000 feet. In one particular embodiment, the downlead cable 106 is
approximately 10,000 feet long.
As described above, the acoustic array cable 104 contains the acoustic sensors
S 1-S96. Preferably, these acoustic sensors S 1-S96 are evenly spaced through
the
acoustic array cable 104. For example, in one particular embodiment each of
the
acoustic sensors S 1-S96 are advantageously spaced five feet apart within the
acoustic
array cable 104. The spacing, however, may vary 0.25 inches or by 0.5%
axially.
The spacing in the present invention, however, is not limited to spacings of
five
feet, rather, the spacing may be larger or smaller than five feet. For
example, in one
application, the acoustic sensors S 1-S96 may preferably be spaced 5 to 100
feet apart
within the acoustic array cable 104. Closer spacing provides better resolution
of the
acoustic signals. Greater spacing provides greater coverage of the acoustic
signals at
the expense of resolution. Although even spacing is preferable, the spacing
need not be
the same between each of the sensors S1-S96. The spacings described above
still apply
to the case where each of the sensors S1-S96 are not separated by the same
distance.
The length of the active portion of acoustic array cable 104 varies in
accordance
with the spacing between the acoustic array sensors S1-S96. The active portion
of the
array cable 104 is the aperture of the array. Preferably, the acoustic array
cable 104 has
a length selected from the range between 200 feet and 1000 feet. More
preferably, the
length of the acoustic array cable 104 is approximately 500 feet. By spacing
the sensors
farther apart, the aperture can be increased to as much as 10,000 feet.
Preferably, the cable 202 is durable enough to protect the distribution fiber
lines
DF 1-DF6, the return fiber lines RF 1-RF 16, and the acoustic sensors S 1-S96
against the
-20-
CA 02315438 2008-09-10
harsh downhole environment. As used herein, the term "downhole" is defined as
down
in the borehole. The downhole environment includes high temperature and high
pressure and may also include corrosive liquids commonly found in an oil well
environment.
In some cases, the cable 202 will be lowered into a pipe such as the
production
tubing or casing in the well where the pressure in a region of the pipe at the
top of the
well (i.e., at the surface 116) is higher than the ambient pressure at the top
of the well
(i.e., at the surface 116 but outside the well). The cable 202 may be lowered
through a
grease injection head capable of maintaining a pressure difference between the
ambient
pressure at the top of the well and the pressure within the region of the pipe
at the top of
the well. In the case where the cable 202 is lowered through a grease
injection head, a
cable 202 having a uniform diameter is required.
Distribution Fiber Lines
As shown in FIGURES 3 and 4A-4H, the distribution fiber lines DF1-DF6
couple the light from the laser sources LS1-LS6 into the optical sensors Sl-
S96 via the
input couplers 420. In each sensor group 401-408, a certain fraction of the
light from
the lasers sources LS 1-LS6 is coupled to one of the sensors S l-S96 in that
group. The
amount of light coupled into each sensor S 1-S96 is preferably chosen so as to
reduce
differences in the level of optical signal delivered to each sensor, and more
particularly,
to reduce the variations in the power level of the optical signals that are
delivered to the
different optical detectors D 1-D 16. A design for sensor arrays that enables
the signal
levels of the optical signals retumed from the sensor groups 401-408 to their
associated
detectors D1-D16 to be similar in magnitude is disclosed in the related
application of
entitled "Architecture for Large Optical Fiber Array Using Standard 1 x 2
Couplers",
U.S. Patent No. 6,249,622.
Although six distribution fiber lines DF 1-DF6 carry light beams emitted by
six
laser sources L1-L6 as shown in FIGURES 3 and 4A-4H, the number of
distribution
fiber lines that can be used is not restricted to six. Rather, the number of
distribution
fiber lines DF1-DF6 employed can range from two to twelve or more. Preferably,
-21-
CA 02315438 2000-07-27
however, the number of distribution fiber lines DF 1-DF6 will correspond with
the
number of laser sources LS1-LS6.
Similarly, in the embodiment shown in FIGURES 4A-4H, each of the
distribution fiber lines DF1-DF6 couples light into one of the sensor Sl-S96
in each of
the sensor groups 401-408. The present invention is not limited to this
arrangement.
Acoustic Sensors
The acoustic sensors Sl-S96 that are employed in the embodiment depicted in
FIGURES 1-5 are "optical" sensors and more particularly "all-optical" sensors.
As used herein the term "optical" means pertaining to or using light, which
corresponds to electromagnetic, radiation in the wavelength range extending
from the
vacuum ultraviolet at about 40 nanometers, through visible spectrum, to the
far infrared
at 1 millimeter in wavelength. More particularly, the optical sensors in the
present
invention operate in the range of visible or infrared wavelengths. Most
particularly, the
optical sensors operate in the infrared range at approximately 1319
nanometers.
As used herein the term "all-optical" means that the downhole portion of the
acoustic sensor array does not include any electronics. In particular, the
acoustic
sensors SI-S96 are electrically passive devices; they require no electrical
components or
electrical connections to the other components. Most notably, the acoustic
sensors S 1-
S96 do not rely on any semiconductor-based electronics, which are highly
sensitive to
temperature. Semiconductor-based electronics such as transistors are generally
not
compatible with the high temperatures that prevail in the downhole
environment, e.g.,
10,000 feet below the surface of the earth. For example, some preamplifiers
designed to
survive high temperatures have a short lifetime and may last only for one hour
under
harsh conditions. In contrast, the embodiment described above requires no pre-
amplifier in the borehole.
Each of the acoustic sensors S 1-S96 in the preferred embodiment comprises a
sensor that receives an optical beam as input and that outputs an optical
signal that
contains information corresponding to the level of acoustic vibration incident
on the
sensor. More preferably, the sensors Sl-S96 employed in the present invention
are
fiber-optic sensors wherein a beam of light is inputted into one end of a
fiber, the light
-22-
CA 02315438 2000-07-27
beam is altered in some manner while in the fiber, and this altered beam is
outputted at
another end of the fiber. As used herein, the term fiber-optic sensor is
defined as a
sensor for monitoring some physical property that comprises a length of
optical fiber
having light within it, wherein the fiber acts as a transducer that modifies
some attribute
of the light upon exposure to variation in the physical property being
measured.
Preferably, the acoustic sensors S1-S96 are optical interferometers. Most
preferably the sensors S1-S96 are Mach-Zehnder interferometers. While acoustic
sensors S 1-S96 as depicted in FIGURE 5 comprise Mach-Zehnder interferometers,
the
acoustic sensors of the present invention are not so limited but may comprise
other
interferometers as well as other types of optical sensors including sensors
other than
fiber-optic sensors. Other interferometers may include, for example, Michelson
interferometers, Fabry-Perot interferometers, and Sagnac interferometers.
In accordance with the present invention, the acoustic sensors S 1-S96 need to
be
capable of operating in a downhole. In particular, the sensors S 1-S96 need to
be able to
function and output a retrievable signal at a depth in the range of between
1,000 and
20,000 feet below the surface of the earth. More preferably, this depth is
approximately
10,000 feet.
In particular, the sensors S1-S96 must be capable of functioning within the
acoustic array cable 104 while the temperature surrounding the acoustic array
cable in
the range of between 100 C and 150 C.
Additionally, the sensors S1-S96 must be capable of functioning within the
acoustic array cable 104 while the pressure on the acoustic array cable is in
the range of
5,500 pounds per square inch (p.s.i.).
The acoustic sensors S1-S96 must be capable of functioning within the acoustic
array cable 104 when the acoustic array cable is immersed in water.
Accordingly, the
optical sensor S1-S96 may comprise a hydrophone. Alternatively, the optical
sensor
S1-S96 may comprise a geophone or a combination of a hydrophone and a
geophone,
e.g., one hydrophone and three geophones. A geophone is a vector sensor.
Consequently the preferred arrangement is to have three geophones employed
together,
possibly in combination with a hydrophone.
-23-
CA 02315438 2008-09-10
A hydrophone measures pressure, pressure changes, or both. A hydrophone
typically measures pressure or pressure changes in the audio or seismic range
corresponding to at least 1 Hz to 30 kHz. A geophone measures movement,
displacement, velocity, and/or acceleration. The geophone typically measures
movement, displacement, velocity, or acceleration in the audio or seismic
range
corresponding to at least 0.1 Hz to 10 kHz. One preferred hydrophone design is
disclosed below.
Although 96 acoustic sensors S 1-S96 are shown in FIGURES 3 and 4A-4H, the
number of sensors that can be used is not restricted to 96. As described
above, the
number of sensors can be doubled to 192. More generally, the number of
acoustic
sensors S 1-S96 can range from two to more than 200. If time division
multiplexing is
also employed, the number of acoustic sensors S 1-S96 can be increased 10 to
100 times.
Accordingly, the number of acoustic sensors S1-S96 can range from two to
20,000 or
more. Preferably, however, the number of acoustic sensors S 1-S96 corresponds
to the
product of the number of laser sources LS 1-LS6 and the number of optical
detectors
D 1-D 16 which also corresponds to the product of the number of distribution
fibers lines
DF 1-DF 16 and the number of return fiber lines RF 1-RF 16.
Return Fiber Lines
As shown in FIGURES 3 and 4A-4H, the return fiber lines RF 1-RF 16 couple
the light from the acoustic sensors Sl-S96 to the optical detectors D 1-D 16
via output
couplers 420. In each sensor group 401-408, a certain fraction of the light
from the
acoustic sensors S1-S96 is coupled to one of the optical detectors D1-D16. The
amount
of light coupled into each sensor Sl-S96 is preferably chosen so as to reduce
the
differences in the power level of the optical signals that are delivered to
the different
optical detectors D 1-D 16. In particular, the coupling ratios of the input
couplers 420
and the output couplers 430 are selected to reduce variations in the retuined
optical
signal levels at the detectors D 1-D 16. As discussed above, a design for
sensor arrays
that enables the signal levels of the optical signals returned from the sensor
groups 401-
408 to their associated detectors D1-D16 to be similar in magnitude is
disclosed in the
U.S. Patent No. 6,249,622, cited above.
-24-
CA 02315438 2008-09-10
The embodiment shown in FIGURES 3 and 4A-4H includes eight sensor groups
in which no two adjacent sensors have either a common distribution fiber line
or a
common return fiber line. The present invention is not limited to this
arrangement. For
example, sixteen sensor groups can be configured so that each sensor group has
one of
the return fibers R1-R16 dedicated to it as disclosed in U.S. Patent No.
6,249,622
cited above.
In accordance with the present invention, the return fiber lines RF 1-RF 16 as
well as the distribution fiber lines DF 1-DF6 need to be able to operate in a
downhole
and, therefore, need to be capable of functioning and outputting a retrievable
signal at a
depth in the range of between 5,000 and 20,000 feet below the earth's surface.
As
described above, the return fiber lines RF1-RF16 as well as the distribution
fiber lines
DF1-DF6 are contained within the cable 202. This cable 202 serves in part to
protect
the acoustic array from the harsh environment of the downhole. In particular,
the return
fiber lines as well as the distribution fiber lines must be capable of
functioning within
the cable while the temperature surrounding the cable in the range of between
100 C
and 150 C Additionally, the return fiber lines as well as the distribution
fiber lines must
be capable of functioning within the cable while the pressure on the cable is
as much as
5,500 pounds per square inch.
The return fiber lines RF 1-RF 16 as well as the distribution fiber lines DF 1-
DF6
must be capable of functioning within the cable when the cable is immersed in
water.
Although sixteen return fiber lines are shown in FIGURES 4A-4H, the number
of return fiber lines that can be used is not restricted to sixteen. For
example, the
number of return fiber lines can be doubled to 32, as described above. More
generally,
the number of return fiber lines employed can range from two to more than 32
Qptical Detectors
In the embodiment depicted in FIGURES 1-5, the optical detectors D 1-D 16
output an electrical signal whose magnitude is proportional to the intensity
of incident
light thereon. In particular, these optical detectors D 1-D 16 output a
voltage or a current
responsive to the intensity of incident light. In one embodiment, the optical
detectors
-25-
CA 02315438 2008-09-10
D 1-D 16 output a current responsive to the intensity of incident light, and a
transimpedance amplifier is employed to convert the current output into a
voltage.
As shown in FIGURES 3 and 4A-4H, each of the return fiber lines RF l-RF 16
directs light onto one of the optical detectors D1-D16. In one preferred
embodiment of
the present invention, each of the optical detectors D 1-D 16 comprises a
polarization
diversity receiver to guarantee the strongest optical interference signal is
taken and
processed. In this embodiment, each of the optical detectors D1-D16 includes
three
photodetectors, such as photodiodes, that sense a portion of light from the
beam incident
on the optical detector. In particular, the three photodetectors sense three
different
polarizations. The processing electronics 304 subsequently samples the signal
originating from each of the three photodetectors and selects the
photodetector that
yields the strongest signal for each acoustic channel. A polarization
diversity receiver
that employs three such photodiodes is described in U.S. Patent 5,852,507 to
Hall.
Although sixteen optical detectors D 1-D 16 are shown in FIGURE 3, the number
of optical detectors that can be used is not restricted to sixteen. For
example, the
number of optical detectors D1-Dl6 can be doubled to 32, as discussed above.
More
generally, the number of optical detectors D 1-D 16 employed can range from
two to
more than 32. Preferably, however, the number of optical detectors D 1-D 16
will
correspond with the number of return fiber lines.
24-Channel Digital Receiver/Demodulators (Fiber Receivers)
The 24-channel digital receiver/demodulators 604, alternatively referred to as
fiber receivers are displayed in FIGURE 6 described above, as well as in
FIGURES 9A-
9B.
FIGURES 9A-9B depict the detector/electronics assembly 601, laser drawer
638, and acoustic sensor array 602 for a second embodiment of the acoustic
sensing
system 100 of the present invention having 192 acoustic sensors S 1-S 192 (not
shown)
and six laser sources LS 1-LS6.
Such a system 100 having 192 acoustic sensors S 1-S 192 is shown in FIGURE
3B described above. The system 100 in FIGURE 3B comprises 192 sensors S I-S
192
-26-
CA 02315438 2000-07-27
contained within two separate acoustic array cables 104 appended to two'
separate
downlead cables 106.
The laser sources LSI, LS2, LS3, LS4, LS5, LS6 supply twelve optical feed
lines F1-F12, which are joined at optical couplers CI-C6. A first set of six
optical feed
s lines F 1-F6 extend from optical couplers C 1-C6 to a first terminator 306a
connected to a
first cable 202a. The first cable 202a comprises a first downlead cable 106a
and a first
acoustic array cable 104a. The first acoustic array cable 104a holds a first
set of 96
acoustic sensors S 1-S96. A second set of six optical feed lines F7-F12 extend
from
optical couplers CI-C6 to a second terminator 306b connected to a second cable
202b.
This second cable 202b comprises a second downlead cable 106b and a second
acoustic
array cable 104b. The second acoustic array cable 104b holds a second set of
96
acoustic sensors designated S97-S 192.
The first terminator 306a also provides a link between the first downlead
cable
106a and sixteen return fibers R1-R16, which are coupled to sixteen optical
detectors
D1-D16. The second terminator 306b also provides a link between the second
downlead cable 106b and sixteen additional return fibers designated R17-R32,
which
are coupled to sixteen additional optical detectors D17-D32. Such a system 100
has six
distribution fiber lines DF1-DF6 (not shown) and 32 return fiber lines RFI-
RF32 (not
shown) in each cable 202a, 202b. The outputs of the 32 optical detectors DI-
D32 are
electrically connected to processing electronics 304.
In an alternative embodiment comprising 192 acoustic sensors S 1-S 192, the
192
sensors S 1-S 192 may be contained in a single acoustic array cable 104
attached to a
downlead cable 106. Such a system 100 has six distribution fiber lines DFI-
DF6, 32
return fiber lines RF 1-RF32, and 32 optical detectors D 1-D32.
Either a system 100 comprising a single cable 202 or a system comprising two
cables 202a, 202b can be employed in conjunction with 192 sensors S I-S I 92
and the
detector/electronics assembly 601 depicted in FIGURES 9A-9B. As discussed
above,
the 192 sensors can be contained in the single cable 202 or a first set of
sensors S I -S96
can be contained within a first cable and a second set of sensors S97-S 192
can be
contained within second cable.
-27-
CA 02315438 2000-07-27
FIGURE 9B shows an optical sensor array 602 comprising fiber optic sensors.
This optical sensor array 602 is designated a 2 x (6 x 16) array because
various
configurations can be employed to accommodate 192 sensors S 1-S 192.
In FIGURE 9B, the 32 return fiber lines RF 1-RF32 are separated into eight
groups having four fibers each. Each group is connected to one of the 24-
channel
digital receiver/demodulators 604 via four of the return fibers R1-R32. The 24-
channel
digital receiver/demodulators 604 comprise circuitry formed on circuit boards,
and, are
hereinafter referred to as 24-channel digital receiver/demodulator cards or as
fiber
receiver cards. Each fiber receiver card 604 receives four of the return
fibers R1-R32
and, accordingly, contains four of the optical detectors D 1-D32 to sense the
light from
the four return fibers. Each of the return fibers R1-R32 contains the output
of six of the
acoustic sensors S 1-S 192. The six outputs are modulated at different
frequencies, as
described above.
The optical detectors D1-D32 within the fiber receiver cards 604 comprise
polarization diversity receivers as discussed above. Polarization diversity
receivers are
known in the art and one such polarization diversity receivers described in
U.S. Patent
5,852,507 to Hall was cited above. In this embodiment containing a
polarization
diversity receiver, each of the optical detectors Dl-D32 includes three
photodetectors,
such as photodiodes, that sense respective portion of light from the beam
incident on the
optical detector in accordance with the polarization of the light. The
processing
electronics 304 subsequently sample the signal originating from each of the
three
photodetectors and selects the photodetector output that yields the strongest
signal for
each acoustic channel. The output of this photodetector is then employed until
the
acoustic sensing system 100 is recalibrated.
The output of the photodetector is directed to a transimpedance amplifier and
converted from analog to digital via an analog-to-digital converter. This
output, now in
digital form, is mixed with a sinusoidal signal at the same modulation
frequency at
which the output of the six lasers Ll-L6 is modulated, c)õ w2, (03, (04, ws,
and w6,
resulting in six signals herein denoted 11, 12, 13, 14, 15, and 16. The
digitized output of
the photodetector is also mixed with a sinusoidal signal at twice the
modulation
a
frequency at which the output of the six lasers Ll-L6 is modulated, 2wõ 20)2,
2cwõ 2c04i
-28-
CA 02315438 2000-07-27
2co5i and 2Uo6, resulting in six signals herein denoted Q1, Q2, Q3, Q4, Q5,
and Q6.
These resultant signals individually pass through circuitry that performs
decimation and
through circuitry that provides gain.
For each of the optical detectors D 1-D32, twelve signals are generated. Six
signals are generated by mixing at the frequencies at which the six laser
sources LS 1-
LS6 are modulated, e.g., 11-16. Six signals are generated by mixing at twice
the
frequencies at which the six laser sources are modulated, e.g., Q1-Q6. Since
each fiber
receiver card 604 contains four of the optical detectors D1-D32 that each
receive light
from six laser sources LS1-LS6, then each fiber receiver card produces 48
resultant
signals. One set of 24, derived from demodulation at the frequencies wõ coZ,
w3, (04, (05,
c06 and are herein denoted I1-I24 and the other set of 24, derived from
demodulation at
the frequencies 2wõ 2(J)2, 2w3, 2w4, 2w5, and 2c06 are herein denoted, Q1-Q24.
The
eight fiber receiver cards 604 shown in the detector/electronics assembly 601
of
FIGURE 9A-9B produce a total of 384 such resultant signals, herein denoted I1-
I192
and Q 1-Q 192.
Preferably, the magnitudes of the signals resulting from mixing with
sinusoidal
signals having the modulation frequencies w,, w2, (03, (04, c05, and co6 are
equal to the
magnitudes of the corresponding signals resulting from mixing with sinusoidal
signals
having the frequencies 2caõ 2co2, 2w3, 2w4, 2w5, and 2w6i that is, preferably
IIi I=
I Ql I, 112 I= I Q2 (, 1131 = I Q3 (=== ( 1192 I= 11192 I. As described above,
the mixed
signals 11-1192, as well as QI-Q196, each individually pass through separate
circuitry
that can provide gain. In this manner the mixed signals can be set to have
equal
magnitude, i.e., I Il I can be set equal to I Q1 I, 112 I can be set equal to
I Q2 I,...and
11192 1 can be set equal to 11192 I.
Each fiber receiver card 604 contains two demultiplexers. One demultiplexer is
dedicated to selecting the signals resulting from mixing with a sinusoidal
signal at the
frequencies aOõ w2, w,, co4, c,u5, and w6, e.g. 11-124, the other
demultiplexer is dedicated
to selecting the signals resulting from=mixing with a sinusoidal signal at the
frequencies
2coõ 2(02, 20)3, 2w4, 2co5, and 2w6, e.g. Ql-Q24. The demultiplexers
sequentially read
the 24 resultant signals, e.g. 11-124 and Q1-Q24 and pairs the signals
together. In
sequence, each pair of resultant signals, i.e. I1 and Ql, 12 and Q2,...I24 and
Q24, are
-29-
CA 02315438 2000-07-27
then provided as inputs to circuitry that computes the arctangent of the ratio
of the two
inputted signals, e.g., tari-'[tj/Qj], tari'[12/Q2]...tari'[124/Q24]. This
circuitry outputs the
respective phase angles, ~ 1, ~2,...~24. Each phase angle, ~ 1-~24, etc.,
corresponds to
the output of one of the acoustic sensors S 1-S24, etc. These phase angles, ~
1, ~2.... ~24,
are then provided as input to circuitry that differentiates the phase angles
with respect to
time to produce d~l/dt, d~2/dt,...d~24/dt.
In the preferred embodiment, the arctangent circuitry outputs a 16-bit word
corresponding to phase. The circuitry that performs differentiation receives
the 16-bit
word and outputs a 32-bit word. This 32-bit word comprises two 16-bit words
io corresponding to the differentiated phase for two channels, e.g. dol/dt and
d~2/dt,
packed into one 32-bit word. Thus, in each of the 24-channel digital
receiver/demodulators 604, the results of two channels within the 24-channel
digital
receiver/demodulator are packed together into one word and the word is
outputted from
the receiver/demodulator 604.
With reference to FIGURE 9A and 9B, each 32-bit word outputted by one of the
eight 24-channel digital receiver/demodulators 604 is coupled to one of the
eight 12-
DSP elements 606 via the digital signal processor cluster local bus 902 and
accompanying link ports. This 32-bit word is unpacked into two 16-bit words in
the 12-
DSP elements 606. Since two of the channels are packed together, the output of
the 24-
channel digital receiver/demodulators 604 can serve as the input for the 12-
DSP
elements 606.
Although eight fiber receiver cards (i.e., 24-channel digital
receiver/demodulators) 604 are shown in FIGURE 9B, the number of fiber
receivers
that can be used is not restricted to eight. For example, the number of fiber
receiver
cards can be reduced to four. More generally, the number of fiber receivers
604
employed can range from one to more than eight. Preferably, however, the
number of
fiber receiver cards 604 corresponds to the number of return fiber lines RF 1-
RF32 and
the number of 12-DSP cards 606.
Additionally, although each fiber receiver 604 shown in FIGURE 9A contains
24 channels, each channel corresponding to the output of one of the acoustic
sensors S1-
S 192, the number of channels that can be used is not restricted to 24.
-30-
-__ _ -------.~.~.~.
CA 02315438 2000-07-27
12-DSP Cards
As discussed above, the eight 12-DSP elements 606 receive 32-bit words
outputted by the eight 24-channel digital receiver/demodulators 604. Each one
of the
12-DSP elements 606 is coupled to one of the eight 24-channel digital
receiver/demodulators 604 via the digital signal processor cluster local bus
902 and
accompanying link ports.
Each 32-bit word received by one of the 12-DSP elements 606 is unpacked into
the two component 16-bit words in the 12-DSP elements 606. Each 16-bit word
corresponds to the output of one of the acoustic sensors S 1-S 192.
The 12-DSP elements 606 decimate the incoming signal reducing the data flow
rate of the signals received by the 12-DSP elements to a rate more compatible
with the
sampling rate standard to conventional seismic recording equipment. The word
"decimate" is used herein in accordance with its conventional usage in the art
as
meaning to re-sample the signal at a lower rate to reduce the original
sampling rate for a
sequence to a lower rate. In particular, in the preferred embodiment, the 12-
DSP
elements 606 receive signals from the fiber receivers at a rate of 512,000
samples per
second and output a signal to the CPU 610 at a rate of 500, 1,000, 2,000, or
4,000
samples per second.
More specifically, the 12-DSP elements 606 convert the 16-bit words, which
were obtained from unpacking the two components of the 32-bit words, from 16-
bit
fixed point words to 32-bit floating point words. The these 32-bit words are
passed
through a multi-stage finite input response (FIR) filter, which serves as a
low pass filter.
This filter has a symmetric impulse response and introduces no phase
distortion or
introduces only linear phase distortion across the frequencies. The 32-bit
floating point
words are converted to 32-bit fixed point words and then passed to a RAM
(Random
Access Memory) buffer before being sent to the CPU 610. Each of these words
correspond to the output of one of the acoustic sensors S 1-S 192.
The 12-DSP elements 606 in the embodiment depicted in FIGURE 9A have
interfaces unique to the Analog Devices SHARC (Super Harvard Architecture)
2106x,
e.g., 21060, 21061, 21062, or 21065 DSP.
-31-
CA 02315438 2000-07-27
As described above, each of the 12-DSP elements 606 couples its respective
output signal to the CPU 610 via the PCI bus 608. The PCI bus 608 is a generic
bus
conventionally employed in personal computers. As such, a wide variety of
hardware is
readily available that interfaces with a PCI bus 608. Consequently, as
improvements
are made in hardware and electronics becomes faster, components in the
detector/electronics assembly 601 can be easily replaced with these faster PCI
compatible electronics.
Although eight 12-DSP cards 606 are shown in FIGURE 9A, the number of 12-
DSP cards that can be used is not restricted to eight. For example, the number
of 12-
DSP cards 606 can be reduced to four. More generally, the number of 12-DSP
cards
606 employed can range from one to more than sixteen. Preferably, however, the
number of 12-DSP cards 606 corresponds to the number of fiber receiver cards
604 and
return fiber lines RF 1-RF32.
Additionally, although each of the 12-DSP cards 606 shown in FIGURE 9A
contains 12 outputs, each output corresponding to the output of two of the
acoustic
sensors S1-S192, the number of outputs that can be used is not restricted to
12. The
number of outputs employed can range from two to more than 24. Preferably,
however,
the number of DSP outputs corresponds to one-half the number of
received/demodulator
channels.
CPU
The CPU 610 receives the 32-bit fixed point words corresponding to the output
of one of the acoustic sensors S i-S 192 from the RAM buffer in the 12-DSP
cards 606.
The CPU 610 truncates the 32-bit words down to 24 bits. The CPU 610 also
provides
any necessary scaling to comply with the SEG-D format.
Additionally, to comply with SEG-D format, the CPU 610 provides timing
information. In particular, the CPU 610 outputs the absolute measure of time
when the
processing electronics 304 received the sync signal from the acoustic source
130. This
absolute measure of time is acquired from the GPS electronics 628 at the time
the
processing electronics 304 received the sync signal. The GPS card can provide
1 part
per million (ppm) accuracy for time stamping events. The CPU 610 also includes
the
-32-
CA 02315438 2000-07-27
measure of time that lapsed between when the processing electronics 304
received the
sync signal and when the acoustic sensing system 100 began sampling, i.e.,
sensing for
acoustic vibration. The CPU 610 additionally provides the time separation
between the
samples.
FIGURES 6 and 9A show the CPU 610 outputting to the recording and
processing system 618 via the Ethernet bus 622. The signal output by the CPU
610
corresponds to the filtered differentiated phase and also includes the timing
information
described above. This output is compliant with conventional seismic data, and
more
specifically, with SEG-D format. Accordingly, the phase data, i.e., the rate
of change in
phase, output by the CPU 610 is readable by conventional seismic data
recording and
processing equipment, which e.g., can use the phase and timing information to
determine the amplitudes of the acoustic waves 102 at the sensors S 1-S 192.
The processing electronics 304 shown in FIGURES 6, 9A, and 9B can output
data at a sample rate of 500 hertz (Hz), 1 kilohertz (kHz), 2 kHz, and 4 kHz
upon the
user's selection. The output data resolution is 24 bits. Conventional systems
do not
provide the ability to select sample rates of, for example, 2 and 4 kHz.
Although, the processing electronics 304 shown in FIGURES 6, 9A and 9B
provides output in SEG-D format, the invention is not so limited. Other data
formats
can be employed, for example, SEG-Y or single precision (32-bit) ASCII.
Preferably,
such data formats are in conformity with conventional formats.
The CPU card 610 shown in FIGURE 9A is electrically connected to a mouse
904, a keyboard 906, an SVGA card 908 for display, and to a hard drive 612.
The CPU
card 610 also has Com 1 910 and Com 2 912 ports. As described above, the CPU
card
610 couples to an operator console 616 via Ethernet 620.
In the embodiment shown in FIGURE 9A, the CPU couples to the 12-DSP cards
606, the 16-channel A/D Auxiliary Input/Output Card 624 (denoted in FIGURE 6
as the
Auxiliary 1/0), and the GPS card 628 via the PCI bus 608. The CPU card 610
couples
to the frequency synthesizer card 632 through the ISA bus 640. The CPU 610
manages
the operation and interaction of these cards.
The PCI bus 608 as well as the ISA bus 640 are generic buses conventionally
employed in personal computers. As such, a wide variety of hardware is readily
-33-
CA 02315438 2000-07-27
available that interfaces with these buses 608, 640, and in particular with
the PCI bus.
Consequently, as improvements are made in hardware and electronics becomes
faster,
components in the processing electronics 304 can be easily replaced with these
faster
PCI (or ISA) compatible electronics.
Laser Sources
In one preferred embodiment of the invention, the lasers L1-L6 produce optical
radiation at a nominal wavelength of 1319 nanometers (nm), corresponding to an
optical
frequency of approximately 227 terahertz (THz) in optical fiber. The
frequencies may
be separated by approximately 0.5 to 3 gigahertz (GHz) and are modulated by
respective carriers between approximately 2 (megahertz) MHz and 7 MHz.
The lasers L1-L6 may comprise Nd:YAG lasers that are all identical except for
the optical frequency at which they are operated. The temperatures of the
lasers Ll-L6
are preferably adjusted so that each laser has a unique operating optical
frequency/wavelength. Operating at different optical frequencies avoids
optical
interference between the optical signals from different sources in the same
fiber.
Although Nd:YAG lasers operating at a nominal wavelength of 1319 nm are
described above as being appropriate for use as lasers L1-L6, the invention is
not so
limited. Rather, other lasers and other wavelengths can be employed in
accordance with
the present invention. Additionally, other modulation frequencies can be
employed.
The selection of appropriate modulation frequencies is discussed more fully
below.
Similarly, although six laser sources modulated at six modulation frequencies
are shown in FIGURE 3, the number of laser sources that can be employed is not
restricted to six. The number of laser sources employed can range from one to
more
than twelve.
More, generally, instead of employing laser sources LS1-LS6 to couple light
into the acoustic sensors S 1-S 192, other optical sources can be used. The
optical source
can be a coherent source, such as a laser diode, or an incoherent source, such
as a light
emitting diode (LED) or a fiber source.
Frequency Synthesizer Card
-34-
CA 02315438 2000-07-27
The frequency synthesize card 632 provides waveforms to the laser sources LS1-
LS6 to establish the frequencies at which the outputs of the lasers L1-L6 are
modulated.
The frequency synthesizer card 632 also provides clock, synchronization, and
timing to
the fiber receivers 604 for synchronizing the system 100 and phase locking the
demodulators 604 to the modulators M 1-M6.
In the embodiment shown in FIGURES 6, 9A, and 9B, the frequency synthesizer
produces six periodic waveforms at six different frequencies w,, w2, w3, c04,
ws, u06= The
frequency synthesizer card sends the waveforms at the six frequencies coõ co2,
w3, w49 w5~
(06, to the laser modulation control driver card 636 in the laser drawer 638
via electrical
line 914. The frequency synthesizer card 630 also sends the critical timing
and
synchronization signals to each of the fiber receiver cards 604. The frequency
synthesizer card 630 sends these signals to the fiber receiver cards 604 via a
plurality of
shielded signal lines 916.
As discussed above, the frequency synthesizer card 630 sends the sync signal
and clock signal to the fiber receiver cards 604 and, from these two signals,
the fiber
receiver cards 604 generate digital carriers at the six modulation frequencies
c,)õ wZ, w3,
(04, ws, w6 and at twice the six modulation frequencies 2wõ 2co2, 2c,03, 2(04,
2w5, 2w6 for
mixing and demodulation.
Although six frequencies are generated by the frequency synthesizer card 630
shown in FIGURES 6, 9A, and 9B, the number of frequencies produced is not
restricted
to six. The number of frequencies employed can range from two to more than
twelve.
Preferably, the number of frequencies will correspond to the number of laser
sources
LS 1-LS6.
Selection of Modulation Frequencies
As noted above, the signals from six sensors, e.g. S 1-S6, may be multiplexed
within a single return fiber, e.g., RFI, using frequency division
multiplexing. Due to
the nonlinear nature of the interferometer, this modulation results in signal
output from
the interferometer modulated not just at the six modulation frequencies, ~, (=
(o,f2n),
where n = 1, ..., 6, but also at 2f., 3f,, , 4f,,, etc. The f,, frequencies
will be referred to as
the "modulation frequencies" or "fundamental frequencies," and the higher
multiples of
-35-
CA 02315438 2000-07-27
fõ will be referred to as "harmonics," such that the 2fo signals are the
"first harmonics,"
or "harmonics of the first order," the 3fn signals are the "second harmonics,"
or
"harmonics of the second order," etc. The group of N fundamental frequencies
will be
referred to as the "fundamental set." Similarly, the group of N first harmonic
frequencies will be referred to as the "first harmonic set," and so on for the
higher
harmonics.
As noted above, the multiplexed intensity signal received by a given detector
may be demultiplexed by detection of the signals at f. and 2f. For the
foregoing
demodulation technique to work, however, each of the fõ and 2~ components of
the
multiplexed signal must be isolated in frequency space. That is, the set of ~
modulation
frequencies must be selected so that no fn or 2~ components (i.e., the
"information
containing components") overlaps with any other frequency component, including
any
of the higher harmonics. Any information containing component that is
overlapped in
frequency space cannot be unambiguously demodulated. As will become more clear
below, this limitation complicates the selection of modulation frequencies.
Each frequency component in the multiplexed output contains signal over a
bandwidth centered about the frequency. The size of the bandwidth depends upon
the
frequency characteristics of the signal received by the sensor and possibly
upon the
frequency response of the sensor itself. Once the operating bandwidth of the
frequency
components is known, the various fn values must be selected with sufficient
spacing to
ensure that no overlapping results. The minimum spacing needed to avoid
overlap
between neighboring components will be referred to as M.
FIGURES l0A and lOB illustrate one approach to selecting frequencies so as to
avoid interfering with information carrying components. The plot depicts the
multiplexed signal frequency spectrum containing acoustical information
received
simultaneously by a single detector from a plurality of acoustical sensors.
The numbers
represent frequency values in multiples of Af. Thus, if Af = 0.5 MHz, the
positions
indicated as 9, 10, 11, 12, and 13 correspond to actual frequencies of 4.5
MHz, 5.0
MHz, 5.5 MHz, 6.0 MHz, and 6.5 MHz, respectively. The larger the selection of
Af, the
greater the possible dynamic range of the system. Thus, in practice, Af is
selected to be
as large as possible.
-36-
CA 02315438 2000-07-27
The multiplexed signal is depicted as a series of bullet-shaped components
distributed along the spectrum. The width of each component depicts the
frequency
bandwidth for that component of the signal. The frequency value associated
with a
particular component indicates the frequency at the center of the component.
Components containing the letter "F" represent fundamental frequencies.
Components
containing a number represent harmonic frequencies, with the number
representing the
order of the harmonic. Thus, the first order harmonics contain a"1," the
second order
harmonics contain a "2," etc. Harmonics higher than second order are omitted
from
FIGURES l0A and 10B in the interest of clarity.
FIGURES l 0A and l OB show multiplexed signal spectra for two systems in
which the fundamental, first harmonic, and second harmonic sets do not
overlap. The
five-light-source system of FIGURE l0A utilizes evenly spaced modulation
frequencies
at 9Af through 130f. The spacing between neighboring fundamental frequencies
is
selected to equal Of, the smallest spacing allowed. FIGURE 10B illustrates the
analogous six-light-source system using modulation frequencies at 11 Af
through 16M
This approach ensures that the fundamental components will not be interfered
with by
any of the harmonics, and that the first harmonics will not be interfered with
by the
fundamentals or by the second or higher harmonics. Since there is no
overlapping of
any of the information carrying signals, complete demodulation of the
transmitted
signal is possible. This approach, however, fails to use large portions of the
frequency
spectrum. For example, FIGURE l0A demonstrates that the five-light-source
system
makes no use of the frequencies at Of multiples of 0 to 8, 14 to 17, 19, 21,
23, or 25.
The highest information-containing frequency is depicted in FIGURES l0A and
l OB as a dashed vertical line. In order to simplify the electronics needed
for processing
the received signal, it is preferable to select this frequency to be as low as
possible.
FIGURES l0A and lOB illustrate that, in the absence of overlapping sets, the
processing for five-light-source and six-light-source systems must be designed
to handle
frequencies of at least 26Af and 320f, respectively.
The problem of unused frequency space associated with the approach of
FIGURES l0A and lOB is aggravated as the number of light sources increases.
For an
N-light-source system, the lowest fundamental frequency, f,, may not be chosen
below
-37-
CA 02315438 2000-07-27
(2N-1)Af, and the processing system must handle the largest first harmonic
frequency,
2fN, of (6N-4)Af. For example, a twelve-light-source system could not do
better than f,
= 23Af and 2fõ = 68M.
FIGURES 11A and 11B illustrate two embodiments in accordance with one
aspect of the present invention. The embodiments maintain an equally spaced
set of
fundamental frequencies starting at a lower frequency than allowed in the non-
overlapping approach of FIGURES l0A and l OB.
Comparison of FIGURES 10A and 11 A indicates that for the five-light-source
system the embodiment of FIGURE 11 A reduces the lowest fundamental frequency
from 9Af to 7M, while the highest first harmonic frequency is reduced from
260f to
22M. This lowering of frequencies causes the beginning of the second harmonic
set (at
210f) to be at a lower frequency than the maximum frequency of the first
harmonic set
(at 22M). The overlapping of sets interleaves the individual frequency
components in
such a manner that none of the information carrying components is interfered
with. In
particular, the non-information carrying component 3f,, at 21 Of, is
harmlessly nestled
between the information carrying components 2f4 and 2f5, at 20Af and 220f,
respectively.
Similarly, a comparison of FIGURES lOB and 11B indicates that for the six-
light-source system the embodiment of FIGURE 11 B reduces the lowest
fundamental
frequency from 110f to 90f, while the highest first harmonic frequency is
lowered from
320f, to 280f. As with the five-light-source system, the lowest second
harmonic
frequency is interleaved between the two highest first harmonic frequencies,
such that
no information carrying components is interfered with.
The embodiments illustrated in FIGURES 11 A and 11 B may be generalized to
any multiplexed system utilizing three or more light sources. For an N-light-
source
system, where N> 3, an embodiment includes equally spaced fundamental
frequencies
starting at f, =(2N-3)Af. For the remaining modulation frequencies, this
gives, for
1 >n>N, fn = fõ_, + Of.
This class of embodiments results in a highest first harmonic frequency at 2fN
=
(6N-8)Of. Comparing these values with the corresponding values above indicates
that
these embodiments reduce the lowest fundamental frequency by 2Af and the
highest
-38-
CA 02315438 2000-07-27
first harmonic frequency by 4Af relative to the best non-overlapping approach.
TABLE
I illustrates the selection of modulation frequencies associated with these
embodiments
for values of N ranging from 3 to 9.
TABLE I
N Modulation Frequencies (multiples of Af)
3 3, 4, 5
4 5,6,7,8
5 7, 8, 9, 10, 11
6 9,10,11,12,13,14
7 11,12,13,14,15,16,17
8 13, 14, 15, 16, 17, 18, 19, 20
9 15, 16, 17, 18, 19, 20, 21, 22, 23
FIGURES 12 and 13 illustrate two embodiments that utilize a 2Af gap in an
otherwise equally spaced (at Of intervals) set of fundamental frequencies.
FIGURE 12 shows a five-light-source embodiment with fundamental
frequencies ranging from 6Af to 110f, skipping an intermediate position at
90f. This
selection of fundamental frequencies allows the first harmonic set to shift
down near the
fundamental set. It also allows the second harmonic set to substantially
overlap the first
harmonic set. The second harmonic components are interleaved, however, so as
not to
interfere with any of the first harmonic components.
Comparison of FIGURES l0A and 12 indicates that this five-light-source
embodiment reduces the lowest fundamental frequency from 90f to 6Af relative
to the
best non-overlapping approach, while the highest first harmonic frequency is
lowered
from 26M, to 220f.
The embodiment illustrated in FIGURE 12 may be generalized to any
multiplexed system utilizing five or more light sources. For an N-light-source
system,
where N> 5, an embodiment includes equally spaced fundamental frequencies
starting
at f, =(2N-4)Af, except for skipping the frequency at 3(N-2)Af. This gives the
following modulation frequencies: f, _(2N-4)Of; fN_, = fN_2+20f, fN =(3N-4)Of,
and, for
1 <n<N-1, f,; + Of.
-39-
CA 02315438 2000-07-27
This class of embodiments results in a highest first harmonic frequency at 2fN
=
(6N-8)Of. TABLE II illustrates the selection of modulation frequencies
associated with
this embodiment for N ranging from 5 to 11.
TABLE II
N Modulation Frequencies (multiples of Af)
5 6, 7, 8, 10, 11
6 8, 9, 10, 11, 13, 14
7 10,11,12,13,14,16,17
8 12, 13, 14, 15, 16, 17, 19, 20
9 14, 15, 16, 17, 18, 19, 20, 22, 23
16, 17, 18, 19, 20, 21, 22, 23, 25, 26
11 18, 19, 20, 21, 22, 23, 24, 25, 26, 28, 29
FIGURE 13 shows a six-light-source embodiment with fundamental frequencies
ranging from 7Af to 130f, skipping an intermediate position at 12M. This
selection of
fundamental frequencies allows the first harmonic set to shift down until it
abuts up
10 against the fundamental set. The second harmonic components substantially
overlap the
first harmonic components, but are interleaved so as not to interfere with any
of the
information carrying components.
Comparison of FIGLJRES lOB and 13 indicates that this six-light-source
embodiment reduces the lowest fundamental frequency from 110f to 7Af relative
to the
best non-overlapping approach, while the highest first harmonic frequency is
lowered
from 32Af to 26Af.
The embodiment illustrated in FIGURE 13 may be generalized to any
multiplexed system utilizing four light sources or six or more light sources.
For an N-
light-source system, where N> 4, N:~ 5, an embodiment includes equally spaced
fundamental frequencies starting at f, =(2N-5)Of, except for skipping the
position at
3(N-2)Of.. This gives the following modulation frequencies: f, =(2N-5)Of; fN =
fN_,+20f, and, for 1<n<N, fõ = fn_, + Af.
-40-
CA 02315438 2000-07-27
This class of embodiments results in a highest first harmonic frequency at 2fN
=
(6N-10) Af. TABLE III illustrates the selection of modulation frequencies
associated
with this embodiment for N ranging from 4 to 11.
TABLE III
N Modulation Frequencies (multiples of Af)
4 3, 4, 5, 7
6 7, 8, 9, 10, 11, 13
7 9,10,11,12,13,14,16
8 11, 12, 13, 14, 15, 16, 17, 19
9 13, 14, 15, 16, 17, 18, 19, 20, 22
15, 16,,17, 18, 19, 20, 21, 22, 23, 25
11 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 28
FIGURES 14A and 14B illustrate a six-light-source embodiment that utilizes
two gaps of unequal size. The embodiment uses fundamental frequencies, shown
isolated in FIGURE 14A for clarity, at Af multiples of 5z/3, 7, 8, 9, 10, and
12'/z. As
10 shown in FIGURE 14B, this embodiment results in an overlap between the
fundamental
and first harmonic sets, with the lowest first harmonic frequency (at 11'/30f)
interleaved
between the two highest fundamental frequencies (at 1OOf and 12%ZM). The third
harmonic set joins the second harmonic set in overlapping the first harmonic
set. As
required, the interleaving of the higher harmonics avoids interfering with any
of the
information carrying components.
FIGURES 15A and 15B illustrate a six-light-source embodiment that utilizes
three gaps. This embodiment uses fundamental frequencies, shown isolated in
FIGURE
15A for clarity, at Af multiples of 3, 4, 5, 7, 11, and 13. As shown in FIGURE
15B, this
embodiment results in the first, second and third harmonic sets all
overlapping the
fundamental set. The first harmonic set is overlapped by higher harmonics
extending
out to the seventh harmonic set. Although FIGURE 15B indicates that there is
substantial overlapping between different signal components (depicted by the
bands on
top of other bands), none of the overlapping interferes with the information
carrying
components.
-41-
CA 02315438 2000-07-27
The embodiment illustrated in FIGURES 15A and 15B may be generalized to
any multiplexed system utilizing four or more light sources. For an N-light-
source
system, where N> 4, an embodiment includes fundamental frequencies at
multiples of
Of equaling 3 and 4, followed by the next N-2 consecutive prime numbers
beginning at
5. Thus, the modulation frequencies may be written as: f, = 30f; f2 = 40f;
and, for
2<n<N, fõ = XAf, where Xõ is the (n-2)th consecutive prime number starting at
5.
TABLE IV illustrates the selection of modulation frequencies associated with
this
embodiment for different values of N.
TABLE IV
N Modulation Frequencies (multiples of Af)
4 3, 4, 5, 7
5 3, 4, 5, 7, 11
6 3, 4, 5, 7, 11, 13
7 3, 4, 5, 7, 11, 13, 17
8 3, 4, 5, 7, 11, 13, 17, 19
9 3,4,5,7, 11, 13, 17, 19,23
10 3, 4, 5, 7, 11, 13, 17, 19, 23, 29
Although the embodiments illustrated above usually present the modulation
frequencies as integer values of the minimum spacing parameter, Of, it will be
recognized by one skilled in the art that the invention could be practiced by
choosing
frequencies varying slightly from these integer values. The amount of
variation allowed
depends upon the relative sizes of the component bandwidths and M.
Furthermore,
FIGURES l0A through 15B depict systems with component bandwidths exactly equal
to Of. This aspect of the figures is stylistic. The embodiments presented
above include
systems for which the component bandwidths are narrower than Of.
Hi-gh Pressure, High Temperature Hydrophone
In preferred embodiments of the present invention, the hydrophone sensor array
operates at pressures of at least 5,000 psi and at temperatures of at least
130 C. More
preferably, the hydrophone sensor array operates at pressures of at least
5,000 psi and at
-42-
CA 02315438 2000-07-27
temperatures of at least 150 C. Most preferably, the hydrophone sensor array
operates
at pressures of at least 5,000 psi and at temperatures of at least 180 C.
In particularly preferred embodiments of the present invention, the hydrophone
sensor array operates at pressures of at least 8,000 psi and at temperatures
of at least
110 C. More preferably, the hydrophone sensor array operates at pressures of
at least
8,000 psi and at temperatures of at least 150 C. Most preferably, the
hydrophone sensor
array operates at pressures of at least 8,000 psi and at temperatures of at
least 180 C.
The small outer diameter of the hydrophone sensor array of the present
invention
is particularly advantageous. In preferred embodiments of the present
invention, the
outside diameter of the sensor array is no more than 1.5 inches. In
particularly preferred
embodiments, the outside diameter of the sensor array is no more than
approximately
1.375 inches. In other preferred embodiments, the outside diameter of the
sensor array
is between approximately 1.375 inches and approximately 1.5 inches. In still
other
preferred embodiments, the outside diameter of the sensor array is no more
than
approximately 1.0 inch. The small outside diameter of the sensor array allows
the
hydrophone to be inserted into the downhole casing of a well without removing
the
production tubing. The sensor array may also be inserted into a length of
production
tubing.
The outside diameter of the hydrophone sensor array of the present invention
is
substantially uniform ( 0.020 inch) over the length of the array. The uniform
outside
diameter permits the array to be inserted into a conventional grease injection
head of an
oil well under pressure so that pressure control of the oil well may be
maintained. The
outer covering of the array fits snugly in the injection head and is
lubricated by grease
under pressure so that the array may be lowered into the well without
releasing the
pressure in the well. One skilled in the art will appreciate that a stacked
fitting is
advantageously applied to the wellhead to accommodate the smaller uniform
outside
diameter of the downlead cable.
The general layout of a preferred hydrophone embodiment 1000 is shown in
FIGURE 16, which is capable of operating under extreme conditions such as
temperatures of up to about 220 C and pressures of 10,000 or even 15,000-
20,000
pounds per square inch (psi). The hydrophone may also operate satisfactorily
under less
-43-
CA 02315438 2000-07-27
extreme conditions such as temperatures of at least 150 C and pressures of
8000 psi, or
temperatures of at least 130 C and pressures of at least 6000 psi. Sensors
1002 are
inserted at periodic intervals along a 1.0 inch to 1.5 inch diameter (e.g.,
1.25 inch
diameter) cable 1004, with one such sensor 1002 being shown in FIGURE 16.
Alternatively, the cable 1004 may have a diameter between 0.9 inch and 2.0
inches. In
one preferred embodiment, the sensors are spaced almost exactly 5 feet from
each other,
within a tolerance of 1/4 inch. The cable 1004 includes an outer sheath 1008
which
surrounds a filler member 1012 that extends around the sensors 1002. In the
portions of
the cable 1004 away from the sensors 1002, the outer sheath 1008 surrounds a
core
member 1016 which surrounds a plurality of tubular strands 1020 disposed
around a
central strength member 1024. These relationships are seen more clearly in the
cross
sectional view of the cable 1004 shown in FIGURE 17.
The central strength member 1024 is located along the center of the cable 1004
and provides strength to the cable 1004 except at those locations where the
sensors 1002
are located. The strength member 1024 includes a plastic sheath 1028 that
surrounds 6-
8 bundles 1030, with each bundle having 15-20 steel strands 1034 of a diameter
of
approximately 0.015 inch. The overall diameter of the strength member 1024 may
be
7/32 inch. The tubular strands 1020 may be, for example, 0.084 inch diameter
HytrelTM
5556, HytrelTM 7246, or HytrelTM 8238 from DuPont (which have melting points
and
Vicat softening points of 203 C, 180 C; 218 C, 207 C; and 223 C, 212 C,
respectively). The tubular strands 1020 surround conductors or optical fibers,
or the
tubular strands may just be empty (filler strands) to lend structural
integrity to the
hydrophone 1000. In one particular embodiment, twelve tubular strands 1020 are
used,
in which two strands carry copper conductors, four strands each carry six
optical fibers,
and the six remaining strands are filler strands. Such an embodiment is
suitable for use
in a 6 x 16 array in which two optical fibers are designated as spares. The
copper
conductors may be used to provide electrical power to a device at the distal
end of the
cable 1004, e.g., a gamma tool for sensing purposes.
The core member 1016 extends along the length of the cable 1004 except in and
around the sensors 1002. The core member 1016 may advantageously be Furon
(0611-
950 from Furon Company). In the area of each sensor 1002, the filler member
1012 is
-44-
CA 02315438 2000-07-27
advantageously polyurethane (e.g., PRC 1547 from Courtaulds Aerospace) which
extends out to a diameter of 1.0 inch to hold together the components making
up the
sensors 1002. As such, the filler member 1012 is formed around the sensors
1002 after
the sensors have been positioned within the cable 1004. The outer sheath 1008
may be
0.1 inch thick HytrelTM 5556, HytrelTM 7246, or HytrelTM 8238 and extends
along the
entire length of the cable 1004. (A high temperature, Teflon-based material
such as
Tefzel may be substituted for the HytrelTM materials herein.) The outer sheath
1008, the
filler member 1012, and the core member 1016 function as protective layers to
protect
the hydrophone 1000 (including its reference mandrel and its sensing mandrel,
discussed below) from a corrosive environment. The outside diameter of the
hydrophone 1000 is preferably less than approximately 1.5 inches, and more
preferably
is less than approximately 1 inch.
As seen in FIGURE 16, the strength member 1024 is joined to a flange 1040
which transfers axial load from the strength member 1024 to a stress relief
mechanism
such as a plurality of stress relief wires 1050 (discussed below in connection
with
FIGURE 18) and then to a second flange 1040. In this manner, the hydrophone
1000
(and in particular, the reference mandrel, the sensing mandrel, the reference
fiber, and
the sensing fiber, which are discussed below) are substantially isolated from
the axial
load. The strength member 1024 is advantageously surrounded by a spring 1060
near
that point where the strength member 1024 is joined to the flange 1040 by a
conventional high-pressure swaging process. The tubular strands 1020 also
advantageously pass through the spring 1060, although the strands 1020 are not
shown
in this portion of FIGURE 16 for the sake of clarity.
As seen in FIGURE 18, the flanges 1040 are located near respective ends of the
hydrophone 1000. The flanges 1040 may include a plurality of raised areas 1064
around which the stress relief wires 1050 are wrapped and between which there
are
grooves (not shown in FIGURE 18) that receive the tubular strands 1020. A
plurality of
1-inch long spring members 1080 (discussed below) support the stress relief
wires 1050.
The stress relief wires 1050 advantageously cross over each other as shown in
FIGURE
18 to form a "cage" that prevents the cable 1004 from being twisted
excessively, which
could damage the sensors 1002. The stress relief wires 1050 preferably wrap at
least
-45-
CA 02315438 2008-09-10
2/3 of the way around the sensor 1002 in the radial sense as they extend from
one flange
1040 to the other flange. With this arrangement, the stress relief wires 1050
cross over
each other between the spring members 1080 rather than on top of the spring
members 1080. The flanges 1040 themselves preferably have no sharp edges or
features,
in order to reduce the risk of damage to the tubular strands 1020, or to the
conductors
or optical fibers therein. For the same reason, the stress relief wires 1050
may be Teflon
coated. The hydrophone 1000 is advantageously constructed to be flexible
enough that it
can be bent to a radius of curvature of less than approximately four feet.
As illustrated in FIGURE 19, the sensor 1002 includes a telemetry can 1104, a
reference mandrel 1110, and at least one, but preferably two, sensing mandrels
1120, 1122, all of which are aligned end-to-end (coaxially) to reduce the
profile of the
cable 1004. This is to be contrasted with the common prior art configuration
of placing the
reference mandrel within the sensing mandrel. Using two sensing mandrels 1120,
1122
instead of just one may result in improved sensitivity, since all other things
being equal,
using two sensing mandrels permits more sensing fiber to be used. The
telemetry
can 1104 has a hole 1128 therein for receiving a distribution fiber (bus) 1130
that carries an
input optical signal 1132 generated by an optical source. Together, the
sensors 1002
along the cable 1004 may advantageously form a sensor array such as the 6 x 16
optical
array described in the copending U.S. Patent No. 6,249,622 entitled
"Architecture for
large optical fiber array using standard 1 x 2 couplers,". The distribution
fiber 1130 is
spliced to an input telemetry coupler 1150 (see FIGURE 20A), which is
advantageously
located within the telemetry can 1104. A second hole 1134 in the telemetry can
1104
permits passage of the distribution fiber 1130 out of the telemetry can 1104
after a portion
of the input optical signal has been tapped off by the coupler 1150. When the
sensor 1002 forms part of an array, the distribution fiber 1130 may be
advantageously coupled to other sensors at further locations along the array
cable 1004.
The telemetry can 1104 likewise houses an output telemetry coupler
1154 coupled to a return fiber (bus) 1160. The return fiber 1160 enters the
telemetry can
1104 through a third hole 1164. As the return fiber 1160 enters the telemetry
can
1104, the fiber 1160 already carries output optical signals from sensors
located distal of the
-46-
CA 02315438 2000-07-27
sensor 1002, unless the sensor 1002 is the most distal sensor on a return
fiber. A
perturbed, output optical signal 1168 from the sensor 1002 is coupled by the
output
telemetry coupler 1154 onto the return fiber 1160. The return fiber 1160 then
passes
through a fourth hole 1172 in the telemetry can 1104 and may be coupled to
other
sensors along the cable 1004 before being directed towards signal processing
electronics.
The optical architecture related to the reference mandrel 1110 and sensing
mandrels 1120, 1122 is now described. The input optical signal tapped off by
the input
telemetry coupler 1150 is directed along an input optical fiber 1180 that
passes through
a hole 1184 in the telemetry can 1104 and a hole 1188 in the reference mandrel
1110.
As shown in FIGURE 20A, the input optical fiber 1180 is joined to an input
hydrophone coupler 1192. The input hydrophone coupler 1192 is located within
the
reference mandrel 1110 and directs a fraction of the input optical signal onto
a reference
fiber 1196. Another fraction of the input optical signal is directed onto a
sensing fiber
1198.
The reference fiber 1196 and the sensing fiber 1198 act as a reference arm and
a
sensing arm of an interferometer, respectively, which in FIGURE 20A is
illustrated as
being a Mach-Zehnder interferometer. The reference fiber 1196 exits a hole
1202 in the
reference mandrel I110 and forms 8 "layers" around the reference manual (i.e.,
the
reference fiber is wrapped 8 times in a close packed fashion around the
reference
mandrel 1110 such that each loop of the reference fiber on the mandrel is in
contact
with an adjacent loop of the reference fiber) before reentering the reference
mandrel
1110 through another hole 1206. The sensing fiber 1198 passes out of a hole
1210 in
the reference mandrel 1110 and forms one layer around the sensing mandrel 1120
before being directed to the sensing mandrel 1122, where the sensing fiber
forms 4
layers. The sensing fiber 1198 is then directed back onto the sensing mandrel
1120
where the sensing fiber forms 3 additional layers, so that the sensing fiber
forms a total
of 4 layers on the sensing mandrel 1120. At this point, the sensing fiber 1198
enters a
hole 1214 in the reference mandrel 1110. The reference fiber 1196 and the
sensing fiber
1198 are spliced to an output hydrophone coupler 1218 (see FIGURE 20A) located
within the reference mandrel 1110. Light propagating to the coupler 1218 from
the two
-47-
CA 02315438 2000-07-27
arms interferes at the coupler 1218. Specifically, the output hydrophone
coupler 1218
receives an optical signal from the reference arm (reference fiber 1196) and
an optical
signal from the sensing arm (sensing fiber 1 l98), and produces an output
optical signal
which is directed onto an output optical fiber 1222. The output optical fiber
1222
passes out of a hole 1226 in the reference mandrel 1110 and into a hole 1230
in the
telemetry can 1104. The output optical fiber 1222 carries the perturbed,
optical output
signal and is spliced to the output telemetry coupler 1154 as described above.
The sensing fiber 1198 is wound in tension around the sensing mandrels 1120,
1122. The sensing mandrels 1120, 1122 deform (expand and contract) in response
to
acoustic signals, such that the tension in the sensing fiber 1198 that
surrounds the
sensing mandrels is modified, thus changing the overall length of the sensing
fiber
1198. The length of the sensing fiber 1198 and thus the optical path length
for optical
radiation passing through the sensing fiber 1198 is altered, which in turn
affects the
phase difference between the optical radiation propagating in the reference
fiber 1196
and the optical radiation propagating in the sensing fiber 1198. In this way,
the sensor
1002 acts as a Mach-Zehnder interferometer that records variations in acoustic
pressure.
Although a preferred sensor architecture has been described with respect to 8
layers of
fiber around the reference inandrel 1110 and 4 layers of fiber around each of
the sensing
mandrels 1120 and 1122, utilizing a different number of layers is possible.
Increasing
the number of layers and sensing mandrels leads to greater sensitivity, but
also increases
the cost. The sensor 1002 herein advantageously has a high scale factor of -
140 dB
relative to radians/micropascal.
A different interferometer configuration, e.g., Michelson or Fabry-Perot is
also
possible. FIGURE 20B illustrates an alternative configuration, which functions
as a
Michelson interferometer. The input hydrophone coupler 1192 and the output
hydrophone coupler 1218 are replaced by a single hydrophone coupler 1199 which
performs both functions. At the end of the reference fiber 1196 and at the end
of the
sensing fiber 1198 are placed respective reflectors 1200a and 1200b, thereby
permitting
optical interference in the hydrophone coupler 1199. The hydrophone coupler
1199 of
this Michelson configuration is advantageously placed within the reference
mandrel
1110.
-48-
CA 02315438 2000-07-27
FIGURE 20C illustrates yet another alternative configuration, which functions
as a Fabry-Perot interferometer. In this design, there is no reference fiber
1196 or
reference mandrel 1110. At the output side of the input telemetry coupler 1150
there is
a partial reflector 1201a. Similarly, a partial reflector 1201b is at the
input side of the
output telemetry coupler 1154. The partial reflectors 1201a, 1201b form the
Fabry-
Perot interferometer and are preferably fiber Bragg gratings. In this
configuration, the
input telemetry coupler 1150, the output telemetry coupler 1154, and the
partial
reflectors 1201a, 1201b are advantageously housed within the telemetry can
1104.
The telemetry can 1104, the reference mandrel 1110, and the sensing mandrels
1120, 1122 preferably include respective main bodies 1260a, 1260b, 1260c,
1260d of
length 3.9 inches and diameter of approximately 0.48 inch as well as
respective pairs of
endcaps 1264a, 1266a; 1264b, 1266b; 1264c, 1266c; 1264d, 1266d (discussed in
more
detail below), as illustrated in FIGURE 19. FIGURE 21 illustrates the
reference
mandrel 1110 in more detail. As indicated in FIGURE 19, the various fibers
enter and
exit through holes located in the endcaps 1264a, 1266a; 1264b, 1266b. Fibers
do not
pass through any of the endcaps in the sensing mandrels 1120 and 1122. The
endcaps
1264a, 1266a; 1264b, 1266b; 1264c, 1266c; 1264d, 1266d (discussed in more
detail
below) preferably have a convex-shaped, hemispherical contour to help
withstand high
pressure and advantageously have diameters which are slightly larger than the
diameter
of their respective main bodies 1260a, 1260b, 1260c, 1260d, so that the layers
of fiber
are confined to wrap around the main body. The telemetry can 1104 is
preferably of
metallic construction, such as steel, and preferably has metallic endcaps
1264a, 1266a.
The reference mandrel 1110 provides a stable reference against which optical
path length changes in the sensing ann can be determined, i.e., the reference
mandrel is
substantially insensitive to acoustic signals to reduce the effect of the
acoustic signals
on the reference fiber 1196. To reduce deformation of the reference mandrel
1110 in
response to changes in pressure, the reference mandrel, including its endcaps
1264b,
1266b, is advantageously made of metal, such as steel. On the other hand, the
walls of
the reference mandrel 1110 are preferably kept thin, e.g., to about 0.05 inch,
to reduce
the profile of the device, which tends to allow some pressure response from
the
reference mandrel 1110 (i.e., some flexing of the reference mandrel) in
response to
-49-
CA 02315438 2000-07-27
acoustic signals. To compensate for this and reduce the sensitivity of the
reference
mandrel 1110 to acoustic signals, a cover 1270 may be advantageously placed
over the
reference fiber 1196 (shown in cutaway in FIGURE 21), in which the cover 1270
advantageously extends between and is sealed to the endcaps 1264b, 1266b. An
air
cavity at, for example, 1 atmosphere may be formed between the cover 1270 and
the
reference fiber 1196 to act as a pressure buffer. The outside diameter (O.D.)
of the
cover 1270 may be about 0.53 inches. An adhesive such as TorrsealTM may be
used to
seal the cover 1270, in which the adhesive is allowed to flow over the endcaps
1264b,
1266b as well as those portions of the reference fiber 1196 extending
approximately 6
mm from either end of the main body 1260b. The cover 1270 thus isolates the
reference
fiber 1196 from ambient pressure, thereby improving the stability of the
reference
mandrel 1110 as an interferometric reference source. The reference mandrel
1110 may
be partially potted to hold the input and output hydrophone couplers 1192,
1218 in
place, or alternatively, glue may be used.
The sensing mandrels 1120, 1122 are made of a high temperature material
which, when it is subjected to high pressure, is stiff enough that the
mandrels do not
crack due to deformation. On the other hand; the mandrels 1120, 1122 are
flexible
enough that they bend (undergo strain) in response to acoustic pressure,
without
buckling under hydrostatic pressure. Further, this high temperature material
has a
stiffness that remains relatively stable at temperatures over 200 C. Two
plastics that
are suitable for this purpose are TorlonTM (specifically TorlonTM 5030) and
CelazoleTM.
Of the two, CelazoleTM is preferred because it is stable up to higher
temperatures, and
because its slightly lower stiffness results in greater sensor sensitivity.
Further,
CelazoleTM exhibits lower creep under hydrostatic loading. This latter feature
is
important in the context of interferometers, since changes as small as a few
tenths of a
percent in the length of the sensing fiber 1198 can significantly diminish the
noise
performance of the hydrophone sensor 1002. Both TorlonTM and CelazoleTM are
advantageous over the prior art materials, which include thin wall aluminum
and
polycarbonate. Polycarbonate, for example, is in general not suitable for work
at
temperatures above about 105 C. TorlonTM and CelazoleTM, however, are
suitable for
-50-
CA 02315438 2000-07-27
work at pressures of at least 10,000 or even 15,000-20,000 pounds per square
inch and
temperatures of at least 220 C.
TorlonTM 5030 is a polyamideimide and has a tensile strength of 24,000 psi, a
tensile modulus of elasticity of 1,200,000 psi, an elongation of 4 %, a
flexural strength
of 36,000 psi, a flexural modulus of elasticity of 1,000,000 psi, a
compressive strength
(10% deformation) of 38,000 psi, a compressive modulus of elasticity of
600,000 psi,
all of which are measured at 73 F. Further, TorlonTM 5030 has a coefficient
of linear
expansion of 1.0 x 10-5 in/in/ F, a heat deflection temperature at 264 psi of
539 F, and a
maximum continuous service temperature in air of 500 F. (All values are
approximate.)
CelazoleTM PBI (polybenzimidazole) has a tensile strength of 23,000 psi, a
tensile modulus of elasticity of 850,000 psi, an elongation of 3%, a flexural
strength of
32,000 psi, a flexural modulus of elasticity of 950,000 psi, a compressive
strength (10%
deformation) of 50,000 psi, a compressive modulus of elasticity of 900,000
psi, all of
which are measured at 73 F. Further, CelazoleTM 5030 has a coefficient of
linear
expansion of 1.3 x 10"5 in/in/ F, a heat deflection temperature at 264 psi of
800 F, and a
maximum continuous service temperature in air of 750 F. (All values are
approximate.)
The endcaps 1264a, 1266a; 1264b, 1266b; 1264c, 1266c; 1264d, 1266d are
advantageously hemispherical so that the telemetry can 1104, the reference
mandrel
1110, and the sensing mandrels 1120, 1122 flex more uniformly when subjected
to
pressure and can thereby withstand the higher pressures that may be
encountered in the
down hole applications disclosed herein, which may easily exceed 3000-4000
psi. This
hemispherical design avoids stress being concentrated in small areas and is to
be
contrasted with the prior art design of cylindrical endcaps which can fail
under
hydrostatic pressure.
The endcaps 1264a, 1266a; 1264b, 1266b; 1264c, 1266c; 1264d, 1266d (shown
in their assembled configuration in FIGURES 19 and 21) are advantageously all
the
same shape, which is illustrated by the cross sectional representation of a
preferred
endcap 1264a shown in FIGURE 22. The outside diameter (O.D.) of the endcap
1264a
(designated as "C" in FIGURE 22) is advantageously approximately 0.477 inches.
The
endcap 1264a has a lip 1280 that has an O.D. of about 0.206 inches (designated
as "B"
in FIGURE 22) and an I.D. of about 0.206 inches (designated as. "A" in FIGURE
22).
-51-
CA 02315438 2000-07-27
The lips 1280 of the endcaps 1264a, 1264b, 1264c, 1264d are designed to slip
within
and mate with their respective main bodies 1260a, 1260b, 1260c, 1260d. Each of
the
endcaps 1264a, 1266a; 1264b, 1266b; 1264c, 1266c; 1264d, 1266d is preferably
of the
same material as its corresponding main body 1260a, 1260b, 1260c, 1260d. Thus,
the
endcaps 1264a, 1266a, 1264b, 1266b are preferably metallic. The endcaps 1264c,
1266c, 1264d, 1266d are preferably either TorlonTM or CelazoleTM to match the
construction of their respective main bodies 1260c and 1260d.
FIGURE 19 shows that there are three pairs of oppositely facing endcaps:
1266a, 1264b; 1266b, 1264c; and 1266c, 1264d. Each of these pairs of endcaps
is
advantageously surrounded with a resilient, pliable material (not shown in
FIGURES
16, 18, 19, 21, 22 for the sake of clarity) such as polyurethane (PRC 1547 is
preferred)
which forms a flexible interlink. For example, polyurethane forms a flexible
interlink
1296 (see FIGURE 23) that joins the endcap 1266a of the telemetry can 1104 to
the
endcap 1264b of the reference mandrel 1110. The interlink 1296 includes
grooves
1300, 1304 therein for accepting the optical fibers 1180 and 1222. Likewise,
another
flexible interlink (not shown) joins the reference mandrel 1110 to the sensing
mandrel
1120, and yet another flexible interlink (not shown) joins the sensing
mandrels 1120,
1122 to each other. Each of these additional interlinks has grooves therein
for accepting
the sensing fiber 1198, thereby protecting the sensing fiber 1198 from damage.
In the case of the telemetry can 1104 and the reference mandrel 1110, the
interlink grooves 1300, 1304 are aligned at both ends of the flexible
interlink 1296 with
a hole in an endcap, e.g., the groove 1300 may be used to route the input
optical fiber
1180 from the hole 1184 in the telemetry can 1104 to the hole 1188 in the
reference
mandrel 1110. Similarly, the groove 1304 may be used to route the output
optical fiber
1222 from the hole 1226 in the reference mandrel 1110 to the hole 1230 in the
telemetry
can 1104. (The endcaps 1264c, 1266c, 1264d of the sensing mandrels 1120, 1122
advantageously use grooves (not shown) rather than holes for receiving the
sensing fiber
1198.) The interlink 1296 is thicker between the endcaps 1266a, 1264b than it
is near
the endcaps as a result of the hemispherical shapes of the endcaps, which
helps reduce
any localized stresses that might break the fibers 1180, 1222. Further, the
grooves 1300
and 1304 are advantageously cut to different depths so that the fibers 1180
and 1222 lie
-52-
CA 02315438 2000-07-27
in different planes, i.e., the fibers 1180 and 1222 cross over and are
adjacent each other
without "pinching" each other. Specifically, the respective depths of the two
grooves
1300, 1304 may be selected to differ by at least the width of one of the
fibers 1180,
1222. For example, the groove 1300 may be cut one fiber width deeper than
groove
1304, with the input optical fiber 1180 (which carries the input optical
signal) being laid
down first during assembly. With the input optical fiber 1180 in place, the
output
optical fiber 1222 (which carries the perturbed, output optical signal) may
then be
placed down in the groove 1304 so that the output optical fiber 1222 crosses
over the
input optical fiber 1180.
The flexible interlinks, such as the interlink 1296, permit the cable 1004 to
be
bent and flexed in the normal course of operations, e.g., while the cable 1004
is being
reeled in or out, without breakage or damage to any of the fibers. Likewise,
the grooves
1300, 1304, as well as the grooves in the other interlinks (not shown), are
multi-layered
so that when the cable 1004 is bent, the fibers will not damage each other.
The grooves
1300, 1304 allow the fibers 1180, 1222 to be routed with a well controlled
pitch across a
flexible portion of the hydrophone 1000, namely, the interlink 1296. The
grooves 1300,
1304 also ensure that the fibers 1180, 1222 maintain this pitch while entering
and
exiting the interlink 1296. In one preferred embodiment, this pitch is
approximately 1/3
inch, i.e., the fiber 1180 (1222, 1198) makes one complete revolution around
the
interlink 1296 for every 1/3 inch along the length of the interlink. The fiber
1180 (1222,
1198) preferably forms an angle of at least about 72 degrees with the axis of
the cable
1004 (or hydrophone 1000) if the interlink 1296 has a diameter of 0.48 inch
(or a
smaller angle for a smaller diameter interlink, and a larger angle for a
larger diameter
interlink). Thus, the fiber 1180 (1222, 1198) preferably forms an angle 0 with
the
longitudinal axis of the hydrophone 1000 such that cos6 times the diameter of
the
hydrophone (or interlink 1296) is less than about 0.18. The interlinks 1296
may
advantageously be 1 inch long, corresponding to 3 complete revolutions of the
fiber
1180 (1222).
The interlinks may be constructed by taking a pair of endcaps (e.g., 1266a,
1264b) and aligning them so that they are oppositely facing each other, in
accordance
with FIGURES 19 and 23. Short segments of wire (not shown) such as copper
wires
-53-
CA 02315438 2000-07-27
may then be inserted into each of the holes 1184, 1230 of the endcap 1266a and
the
holes 1188, 1226 of the endcap 1264b. With the wire segments in place, a mold
(not
shown) may be used to form polyurethane around the pair of oppositely facing
endcaps
1266a, 1264b, during which time the wire segments keep polyurethane out of the
holes
1184, 1230, 1188, 1226. The wire segments may then be removed and the grooves
1300, 1304 cut in the polyurethane, so that the grooves 1300, 1304 are
properly aligned
with their respective holes in the endcaps 1266a, 1264b.
The telemetry can 1104 is preferably assembled by beginning with two pieces
(not shown) corresponding to the two halves of a main body that would be
formed when
the main body is cut lengthwise. Next, the fibers 1130, 1160 are cut, passed
through
their corresponding pairs of holes (1128, 1134 and 1164, 1172, respectively)
in the
endcap 1264a and spliced to the couplers 1150, 1154. The couplers 1150, 1154
along
with their corresponding splices, as well as the fibers 1130, 1160 may then be
placed
into one of the halves. The fibers 1180 and 1222, in turn, may then be passed
through
their respective holes 1184, 1230 in the interlink 1296, specifically through
the endcap
1266a (see FIGURE 23). The interlink 1296 and the endcap 1264a are then be
glued to
the main body 1260a with epoxy, and the fibers 1130, 1168 are glued into their
respective holes using epoxy. (The epoxy herein may be a high temperature
aluminum
filled epoxy such as Cotronics 454B.) The interlink 1296 is then dipped in
polyurethane
to form a thin layer 1308 that encapsulates the fibers 1130, 1160 to keep the
fibers in a
fixed position (i.e., the fibers are "hard potted"). The telemetry can 1104
may then be
partially potted using epoxy (or glue may be used) to keep the couplers 1150,
1154 and
their corresponding splices from being jostled and damaged during operation.
The two
halves may then be sealed together at ambient pressure with epoxy to form the
telemetry
can 1104, which is capable of withstanding hydrostatic pressure and protecting
the
couplers 1150, 1154 which are positioned therein.
The reference mandrel 1110 and the sensing mandrels 1120, 1122 are
advantageously assembled in a similar fashion, except that it is not necessary
to begin
the assembly procedure with halves of a main body. (In the case of the
reference
mandrel 1110, the hydrophone couplers 1192, 1218 may be inserted into the
reference
mandrel through one of its ends before the reference mandrel is sealed with
its endcaps.
-54-
CA 02315438 2000-07-27
The sensing mandrels 1120, 1122, on the other hand, do not house optical
components.)
The reference fiber 1196 and the sensing fiber 1198 are wrapped around the
reference
mandrel and the sensing mandrel, respectively. The reference mandrel 1110 and
the
sensing mandrels 1120, 1122 are likewise sealed at ambient pressure and can
withstand
very large hydrostatic pressures. In the case of the reference mandrel 1110,
the cover
1270 may be placed over the reference mandrel 1110 to act as a pressure
buffer, as
discussed above.
Once the assembly of the sensor 1002 (see FIGURES 16 and 19) is complete,
the interlinks 1296 of the sensor 1002 are advantageously surrounded by the
spring
members 1080 (see FIGURE 18) for additional protection against the strains and
stresses that may be encountered during deployment and operation of the
hydrophone
1000. Following assembly of the flanges 1040 and their associated stress
relief wires
1050 around the sensor 1002, a material such as polyurethane (e.g., the PRC
1547 from
Courtaulds Aerospace, discussed above) may be molded around the sensor 1002,
the
spring members 1080, the spring 1060, the flanges 1040, and the stress relief
wires 1050
to form the filler member 1012 so that the hydrophone 1000 is well shielded
from the
harsh chemical and mechanical conditions associated with down hole
applications. As a
result of this molding procedure, the interlinks 1296 are well surrounded by
polyurethane, since polyurethane is also advantageously used to construct the
interlinks,
as discussed above. In this manner, the fibers 1180, 1230, 1196, 1198 are
embedded in
flexible interlinks 1296 which have the pitch and tension necessary to survive
the
bending encountered during deployment and handling of the cable 1004.
The molding procedures disclosed herein (in connection with, for example, the
interlink 1296 or the hydrophone 1000) may be performed by placing a mold
around the
object to be encased and then pulling a vacuum on that object. The object may
be
heated to 140 F for 10-15 minutes before polyurethane is injected around it.
After
injecting polyurethane around the object, the vacuum may be maintained for 15-
20
minutes to degas the polyurethane. The polyurethane may then be cured for 14
hours at
40-70 psi and 140 F before the mold is removed.
The use of polyurethane in the various components disclosed herein (e.g., the
filler member 1012 and the interlink 1296) limits use of the hydrophone 1000
to
-55-
CA 02315438 2000-07-27
temperatures up to about 150 C. Teflon or Viton may be substituted for
polyurethane,
however, and these materials may be used up to about 220 C. The optical
couplers and
adhesives herein may function up to temperatures of 200 C or even 220 C.
System Performance
The acoustic sensing system 100 of the present invention may include numerous
acoustic sensors S 1-S 192. The embodiments described above include 96 and 192
sensors S 1-S 192, respectively, as well as 96 and 192 channels in the
processing
electronics 304 for processing the output of the 96 or 192 sensors. Having a
large
number of sensors S 1-S 192 offers a significant improvement over prior art
systems. For
example, having a large number of sensors S 1-S 192 increases the potential
resolution of
measurements such as cross-well tomography and also dramatically reduces the
time
required to complete a geological survey.
The acoustic sensing system 100 of the present invention offers other
advantages
over the prior art. TABLE V provides a summary of the performance and
specifications
of the acoustic sensing system 100 described above comprising 96 fiber optic
sensors
S 1-S96. The acoustic sensing array 602 of the present invention, however, is
not
limited to 96 or 192 sensors S 1-S 192 but may include as many as 400 sensors.
As discussed above, the acoustic array 602 is small enough to fit into
production
tubing. The cable 202 shown in FIGURE 2 can be inserted in production tubing
having
an inner diameter of two inches and even in production tubing having an inner
diameter
of 1.25 inches. The cable 202 in the embodiment described above with 96
sensors has
an outer diameter of 1.22 inches and includes armoring. Thus, the acoustic
array 602
can be inserted in the production tubing in the casing of a well 118 rather
than requiring
removal of the production tubing to fit the cable in the casing.
The acoustic sensing system 100 of the present invention is rugged enough to
operate in the harsh downhole environment. The cable 202 can be inserted in a
well 118
to a depth of over 10,000 feet where the temperature is over 150 C and the
pressure is
over 5,500 pounds per square inch.
The acoustic sensing system 100 of the present invention has a large enough
bandwidth to perform real time sensing of the acoustic wave, including
processing the
-56-
CA 02315438 2000-07-27
output of the acoustic sensors S 1-S 192 and outputting data in conventional
seismic
format. Since the acoustic sensors are optical sensors, they do not limit the
bandwidth
of the system. Rather, the bandwidth is limited by the bandwidth of the
processing
electronics 304. However, the processing electronics 304 is fast enough to
measure the
acoustic vibration produced by an acoustic source 130 and permit viewing of
the results
soon thereafter. Consequently, if data are to be acquired, processed, and
outputted in
real time and in a format that the surveyor can read, the surveyor can modify
the survey
based on the results being generated. For example, if the data appears to
indicate the
possible presence of an in-place reserve, the acoustic source 130 and/or
acoustic sensor
array 602 could be repositioned for further investigation.
In contrast, limitations on speed and bandwidth prevent conventional acoustic
sensor arrays from achieving real time processing. Rather, once measurements
are
taken, data is recorded on magnetic tape and is transported to a location away
from the
well 118 or drill site for processing.
In addition to being fast, the acoustic sensor system 100 of the present
invention
.has a low acoustic noise floor. In particular, the integrated RMS acoustic
noise over the
detection bandwidth is 0.1 microbar RMS.
The acoustic sensor system 100 of the present invention also has a wide
dynamic
range. Large voltage outputs for small acoustic signals enable the system to
sense and
record small amplitude acoustic waves 102. At the same time, the system is
able to
sense and record large amplitude acoustic waves 102. Specifically, the
embodiment
described above having 96 sensors S1-S96 has an instantaneous dynamic range of
132
decibels (dB) for the acoustic band ranging from 1 Hz to 400 Hz and has an
instantaneous dynamic range of 120 dB for the acoustic band ranging from 401
Hz to
1000 Hz.
-57-
CA 02315438 2000-07-27
TABLE V
PERFORMANCE CHARACTERiSTICS CAPABILITY
Number of Acoustic Channels 96 Expandable to 192
Lead Cable Length - 10,000 feet
Array Cable Length 500 feet
Array Cable Diameter 1.22 inches Includes armoring
Operating Pressure in excess of 5500 p.s.i.
Operating Temperature in excess of 150 C
Noise Floor 0.1 mbar RMS
Instantaneous Dynamic Range 132 dB Minimum from 1 Hz to
400 Hz
120 dB Minimum from 401 Hz to
1000 Hz
Distortion -80 dB
Crosstalk -85 dB
Acoustic Passband 1 Hz to 1440 Hz
Ripple +/-0.2 dB
Channel-to-channel +/-0.34 dB
Output Data Sample Rate 4 kHz, 2 kHz, I kHz, and Selectable
500 Hz
Output Data Format SEG-D Rev. 2
Output Data Resolution 24 bits Fixed point
Auxiliary Channels 16
Input Signal Amplitude 10 VDC (0 to peak)
Maximum Input Frequency 1.5 kHz
Sample Rate 4 kHz
Resolution 16 bits
External Sync 10 msec Bi-directional TTL or
switch closure
Electronics Cabinet 48" x 19" x 17"; AC powered
less than 250 lbs.
GPS Capability Included 1575 MHz antenna
Ganuna Tool Included
-58-
CA 02315438 2000-07-27
The acoustic sensor system 100 minimizes crosstalk between signals of a
different wavelength. The crosstalk of the system having 96 sensors S 1-S96 is
-85 dB.
The acoustic sensor system 100 also minimizes distortion. The distortion of
the
system having 96 serisors S 1-S96 is -80 dB.
The acoustic sensor system 100 has an acoustic bandpass between 1 Hz and
1440 Hz. Accordingly, frequency components between 1 Hz to 1440 Hz of the
acoustic
wave are sensed by the system 100. The acoustic sensor system 100 outputs data
in
SEG-D REV.2 format, a conventional format complying with standards set by the
Society of Exploration Geophysicists that is well know in the art. The
acoustic sensor
system 100 also can output data at a sample rate of 500 Hz, 1 kHz, 2 kHz, and
4 kHz
upon the user's selection. The output data resolution is 24 bits.
As described above, the system 100 can accept auxiliary channels. The
embodiment described above having 96 sensors S 1-S96 can accept sixteen single-
ended
auxiliary channels or eight differential auxiliary channels. These auxiliary
channels
have a maximum input frequency of 1.5 kHz. These channels are sampled at a
rate of 4
kHz and with a resolution of sixteen bits.
The system 100 also accepts an external sync pulse. The embodiment described
above having 96 sensors S1-S96 accepts a 10-millisecond long external sync
pulse.
This sync pulse can be generated using bi-directional TTL (i.e., with active
pull-up and
active pull-down) or switch closure (i.e., active pull-down with resistive
pull-up).
As described above, the acoustic sensing system 100 preferably comprises a
GPS system 628. The acoustic sensing system 100 additionally may comprise a
gamma
tool. Gannna tools, which are well known in the art, are used to measure the
depth of
the cable by counting markers on the casing as discussed above.
One additional advantage provided by the acoustic sensing system 100 of the
present invention is that this system is significantly less sensitive to tube
waves than
conventional systems. A tube wave, as is well known in the art, corresponds to
acoustic
waves traveling up and down the borehole 124, either through the metal casing
or
through water in the bore hole. During data acquisition, the acoustic sensing
system
100 of the present invention advantageously is less affected by tube waves
than
conventional acoustic sensing systems.
-59-
CA 02315438 2000-07-27
Although the acoustic sensing system 100 of the present invention has been
described in the downhole environment for the purpose of geophysical surveys
designed
to locate oil reserves, its use is not so limited. This acoustic sensing
system 100 of the
present invention may be otherwise employed to measure acoustic vibrations at
a series
s of remote locations.
More generally, the present invention may be embodied in other specific forms
without departing from the essential characteristics described herein. The
embodiments
described above are to be considered in all respects as illustrative only and
not
restrictive in any manner. The scope of the invention is, therefore, indicated
by the
following claims rather than the foregoing description. Any and all changes
which
come within the meaning and range of equivalency of the claims are to be
considered in
their scope.
-60-
