Note: Descriptions are shown in the official language in which they were submitted.
BACKGROUND OF T~-IE INVENTION
This invention relate~ to methods and systems ~r
measuring acoustic wave travel times in earth formations in
the vicinity of the well borehole. More particularly, the
present invention relates to techniques for mea~urin~
multiple acoustic wave componen~ (or wave propagation mode)
travel times in earth formations in ~he vicinity of a well
borehole. The measurement method~ use swept frequency
transmitting techniques and cross correlation CompariSOn
techniques between the transmitted signal and received
signal.
Sonic or acoustic well logging has become an important
method or dete~mining the physical characteristics of ear~h
ormations in the vicinity o a well borehole. Mea~uremen~
lS of the acoustic compressional wave velocity or travel kime
between a transmitter and a receiver in a well bor~hole can
define physical characteristics of the ~arth formations
which are indicative of the capability of these formations
to produce oil or gas. For example, a mea~urement of the
compressional wave travel time or velocity gives a direct
indication of the porosity o~ the formation in the vicinity
o the well borehole. guch acoustic veloci~y or acou3~ic
travel time measurements have therefore become practically
standard for all new wells which are drilled.
In the prior art, acoustic pulse or pulsed 50nic
~logging techniques have been used to measure the travel time
or velocity o~ acoustic waves in the earth formations in the
vicinity of a borehole. Such me~hods of the prior art havP
typically used impulse driven acoustic transmitters. An
acoustic transmitter is fired impulsively or pulsed and the
~3~
length of time necessary for the acoustic wave pulse generated
by the transmitter to propagate from the tran~mj.tter through
the ear~h formations in the vicini~y of the borehole and
back to an acoustic receiver located a ~paced di~tance away
from the txansmitter is measured. By appropriately combining
the measuremen~s of acoustic wave ~ravel time at several
- acousti.c receivers, spaced different di~tance3 from either a
single tor multiple) acoustic transmitter, then the acoustic
wave travel time or sonic compressional wave velocity of
propagation of the earth formation may be determined. Quite
elaborate schemes and geome~rical considerations for eliminating
the effect on the travel time measuremen~ of the borehole
and borehole fluids have also been developed.
In more recent years, it has become desired to measure
other acoustic wave mode travel time~ than merely compres-
~ional wave velocity. For example, in U.S. Patent 4,131,875
issued December 26, 1978, techniques are described for
measuring the so called l'late arrival" waves or Stonely
waves. Similarly, other prior art techniques such a~ tha~
shown in U.S. Patent 3,354,983 is~ued November 28, 1967,
describes techniques for measuring acoustic shear wave
velocities. In all of these techniques, an acoustic pulse i~
generated by the transmitter and the waveform of the acoustic
signal at one or more receivers i~ analyzed in order to
determine the velocity of compressional, ~hear, or Stonely
~waves in the vicinity of the borehole.
Pulsed acoustic techniques depend upon the amplitude
detection of ~he arrival of acoustic waves at a receiver.
Such techniques are prone to errors generated by random
noise which occurs as a well logging instrument is moved
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through the borehole. Acoustic noise maybe generated by the
lnstrument body, or cen-tralizers on the instrument body,
scraping along the sides of the borehole as the tool is
moved therekhrough.
Similarly, pulsed acous~ic techniques involving pulsed
acoustic transmitters for mea~uring sheax waves or 5tonely
waves depend upon an elaborate interpre~a~ion of the wave-
form of the arriving wave at the receiver. Such interpre-
tations are generally based on theoretical calculations made
with simplified mathematic~l models of the earth form~tions
in the vicinity of the borehole. If the simpli~ied mathe-
matical model proves ~o be in error, then the interpretation
of the arriving waveform at the receiver may be in error and
its relationship to more complicated real life geometries
and conditions than taken into account in the model can lead
to false interpretations of the waveorm of the arriving
acoustic signal.
It would be highly desirable to provide a method for
measuring the travel time of various components of acousti~
energy (compressional or primary wave r shear wave, Rayleigh
or pseudo Rayleigh, direct (fluid) wave, extentional, and
Stonely wave) in earth formation~ in the vicinity of a well
borehole which was not dependent upon a theoretical inter-
pretation of an arriving acou~tic pulse waveform in terms of
a model. The system of the present invention provides a
~direct measurement of the travel time of several components
of acoustic energy from a transmitter to a receiver in earth
formations in the vicinity of a well borehole.
BRIEF DESCRIPTION OF THE INV~NT:I:ON
In the present invention, a downhole well logging
instrument is provided with an acoustic transmitter and
$~
at least one acoustic receiver ~hat i~ spaced a longitudinal
dis~ance from the transmitter. I~ desired, multiple trans-
mitters and receivers could be used. ~rhe output signal from
the acoustic transmitter in the present invention is repeti~ively
swept over a predetermined frequency range. The ~requency
swept output of the transmitter i~ propagated in all the
various modes of propagakion of acous~ic energy through the
earth formations and borehole and i~ detected at the spaced
receiver. A synchronization signal i8 also generated at the-
commencement of each repetitive sweep of the transmitterthrough its predetermined frequency xange. The ~ynchroniza-
tion signal and the received signal from the receiver are
transmitted to the surface of the earth via conductors of
the well logging cabl~. At the surface, the received signal
is converted from analog to digital form and 3tored in a
memory. ~he transmitter sweep signal i5 stored in a surace
located sweep signal memory storage in digital form. Upon
completion of a sweep of the transmitter and after receiving
digitizing and storing the received signal for a predeter-
mined le~gth of time the sweep signal from the tran~mitteris cross-correlated with the received signal. Because of
the characteristic swept frequency pattern applied to the
tra~smitter signal, indications are derived from the cro~s-
correlation of the arrivals of various modes of acou~tic
energy propagation at the receiver. The timing differences
between the synchronization pulse and the arrival of the
various modes of acoustic propagation at the receiver may
then be interpreted in terms of the travel time o~ the
variou~ modes of acoustic propagation at the receiver.
These signals may then be recorded as a function of borehole
depth as the well logging instrument is moved through the
borehole. The entire sweep, transmit, and receive process
is repetitively performed during such movement of the borehole
instrument.
In one aspect of the present invention, there
is provided a well logging system for measuring and recording
the acoustic energy propagation characteristics of earth
formations penetrated by a well borehole comprising a fluid
tight hollow body member sized and adapted for passage through
a well borehole, means in the body member for repetitively
generating swept frequency acoustic energy outputs ha~ing a
linearly varying range of frequencies from a lowes-t frequency
of approximately two kilohertz to a highest frequency of
approximately twelve kilohertz in a characteristic pattern,
the pattern having a duration of approximately four milli-
seconds, means for digitizing the characteristic swept fre-
quency signal and for providing a digital signal representative
of the characteristic signal at the generating means'
receiver means longitudinally spaced from the generating means
by a distance of from eigh~ to twelve feet in the body member,
for detecting acoustic energy propagated from the generating
means through the borehole and earth formations~in the vicinity
of the borehole and for generating digital signals representative
of the detected acoustic energy, means for cross correlating
the digital signal representative of the characteristic signal
at the generating means and the digital signal representative
of the detected acoustic energy and for providing a correlator
output signal representative of the arrival at the receiver means
of different modes of propagation of acoustic energy in the
borehole and earth formations in the vicinity of the borehole,
computer means responsi.ve to said correlator output signal for
deriving therefrorn measurements of the speed of propagation of
the different modes of pro~agation of acou~tic energy in the
~ 5 -
earth formations in the vicinity of the well borehole, and
means for recording the measurements of the speed o~ propagation
of different modes of acoustic energy as a function of borehole
depth, thereby providing on a record medium a well log of speed
of propagation of different modes of acoustic energy.
The invention may be best understood by the
following detailed description thereof, when taken in conjunction
with the appended drawings in which:
B~IEF DESCRIPTION OF THE DRAr~INGS
Figure 1 is an overall block diagram illustrating
schematically a well logging system in
accordance with the conce~ts of the
present invention.
Figure 2 is a schematic diagram illustrating an
acoustic waveform received at a spaced
receiver from a pulsed acoustic trans-
mitter as utilized in the prior art.
Figure 3 is a graphical representation illustrating
a typical swept frequency waveform applied
to the acoustic transmitting transducer in
the present invention.
Figure 4 is a graphical representation illustrating
a swept frequency signal applied to an
acoustic transmitter in accordance with the
concepts of the invention, a composite or
mixed mode arrival signal which arrives
at the acoustic receiver of the pres~nt
invention, and the output of a cross--
correlation ~etween the sweep an~ the
composite arrival signal in accordance
with the present invention, and
- 5a -
Figure 5 is an illustration schematically showing
a well log as a func-tion o~ depth of the
acoustic compressional wave velocity and
the correlator output showing compressional
and shear wave arrivals in accordance with
the concepts of the present lnvention
5b -
D~SCRIPTION OF THE PREFERRED EMBO~I~ENT
Referring initially to Fig. 1, a sys~em or generating
and receiving acoustic signals and for logging a well bore-
hole in accordance with the concepts of the present in~en-
tion is illustrated schema~ically. A well ~orehole 10penetrates earth formations 15 and is filled with a borehol~
fluid 12. A downhole well loggi~g sonde 11 is suspended,
via a well logging cable 13, which pas3es over a heave wheel
14, in the borehole 10. The sheave whe~1 14 i~ electrically
or mechanically coupled to a well logging recorder 2~ of
conventional design as illustrated by dotted line 16 so
that measurements made by the down hole sonde 11 may be
recorded as a function of borehole depth.
The downhole sonde 11 comprises a ~luid tight, hollow,
body member sized and adapted for pas~age through a well
borehole. Housed inside the fluid tight sonde 11 i8 an
acoustic transmitter 32 and an acoustic receiver 33. Cir-
cuitry for drivin~ the acoustic transmitter 32 comprise~ a
~weep signal storage memory 29, which may comprise a read
only memory (ROM) or the like, a digital to analog convertex
30, and a ~ilter 31.
The acoustic re~eiving transducer 33 iB shown longi-
tudinally spaced from the transmitting ~ransducer 32.
~ypical spacing dis~ances of from 3 to 10 feet may be used
as desired. It will be appreciated that acoustic trans-
'`mitting transducer 32 and acoustic receiving transducer 33
are acoustically coupled to the borehole by acoustic impe-
dance matching material such as oil or oil-filled bellows
or the like (not shown) in a manner known in the art. The
transmitting and receiving transducer3 may comprise plezo-
electric transducers. The transmitting and receiving
transducers are sized and arranged to have a linear or
"flat" response over the swept frequency range used in the
technique o~ the present invention.
While only one acoustic transmitter and one acoustic
receiver are illus~rated in the system of Fig. 1~ It will
be apprec1ated by those skilled in the art that the number
of acoustic receivers could be varied and the number o~
acoustic ~ransmitters could be varied, if desixed. In such
an instance different weep patterns could be u~ed for each
acoustic ~ransmitter to characterize it~ output acoustic
energy from that of any other acou~tic transmitter which is
utilized in the logging instrument.
The sweep signal storage memory 29 contains digital
numbers representative of the amplitude of sweep pattern to
be applied to the transmitting transducer 32 as a function
of time at a preselected sampling inter~al time or rate.
For example, a typical sweep frequency pattern could be that
given by Equation 1.
I t=T2
f(t) = sin [~1 + (~2 ~l)t]t (1)
t=T~ 2L
In Equation 1 a sine wave whose frequency changes in a
linear fashion from ~1 at Tl to ~2 at T2 i8 described. Such
sweep function amplitudes can be generated by computer as a
~function of time and the results then stored in a read only
memory or ROM device for subsequent use in the subsurface
tool and surface equipment as desired.
Digital signals from the sweep signal storage ROM 29
are read out sequentially and converted to analog signals ~y
a digital to analog converter 30. The output o the digital
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to analog converter 30 is filtered by low pass filter 31 to
remove ~he small sample -to sample skep introduced by the
digital to analog converter (i.e. to remo~e high frequency
components) and the output voltage signals from the filter
31 dxive ~he transmi~ter transducer.
A typical sweep pattexn ~uch a3 that described by
Equation l is illustrated in Fig. 3. A synchronization
pulse i8 generated at the beginning of a sweep cycle and is
labelled as "sync pulse" in Fig. 4. A ~wept ~requency
acoustic signal having a linearly increa~ing ~requency and
starting at a ~ime approximately 0.1 millisecond after the
synchronization pulse is illustrated. The frequency of the
tran~mitter drive signal increases un~il a tim~ approxi-
mately 5 milliseconds following the sync pulse, thus gene
rating a swept frequency acoustic signal having approxi
mately constant amplitude and linearly varying frequency of
from, for example, 2 to 12 kilohertz and having a duration
of approximately 4 milliseconds. It will be appreciated
that other durations or ther swept frequency ranges could
be used if desired.
The acoustic signals detected by receiving transducer
33 are filtered by a band pass filter 34 to remove any noise
signals which are far removed from the pass band of the
original swept frequency signal. After filtering, the
signals are amplified by an amplifier 35 and applied to
'`a telemetry system 36 which transmit~ the received acoustic
signal waveform to the surface via conductors of well
logging cable 13.
Timing for the transmitter sweep event and the synchroni-
zation pulse is controlled by the telemetry sys~em 36 whichcontains a precise frequency clock ~uch as crystal controlled
oscillator therein. The synchronization signal illustrated
in Fig. 3 is transmitted ~o ~he ~ur~ace so thak the 5urface
electronics may be exactly synchronized or each time o~
starting o the transmitter ~weep cycle. For a 4 milli~
second sweep rate and an approximately 10 millisecond
receiver recording time, such as that illu~trated in Fig. ~,
the entire cycle of transmitter sweep and receiver reception
transmisto ~he surface may be repeated at a repetition
rate of from lO to 20 cycles per second. It will be appre-
ciated by those skilled in the art that the duration of
recep~ion by the receiver and the transmission of recei~ed
signals is a function o~ the spacing between the transmitter
and receiver. For typical pacings on the ordex of four to
six feet, the 10 millisecond receive Yignal transmis~ion
cycle illustrated in Fig. 4 is appropriateO
lS At the surfaceS a synchronization detector and timing
circuit 18 detects the synchronization signal and generates
outputs to an analog to digital converter 21, a signal memory
22, a correlator memory 24 and a sweep ~ignal memory storage
l9. The receiver Yignal from the downhole telemetry 3ystem
is amplified in an amplifier 20 and con~erted to digital
~ormat by analog to digital converter 21, which is tim2d by
the signal from the sync detector and timing circuit 18.
The digitized form of the received signal i~ th~n ~tored in
a signal memory 22. At an appropriate time which allows for
the complete receiver signal waveform to be digitized and
` stored in signal memory 22, the synchronization detector in
~iming circuit 18 supplies a strobe or outpu~ signal pulse
: to the sweep signal torage memory 19 and to the ~ignal
memory 22 which cause these two ~ignals to be supplied a~
input in digital form to a correlator 23.
The corxelator 23 per~orms a cross coxrelation ~u~ction ,
on the two input siynal~ which is defined by Equation 2.
K=N
~xy(~ ) ~ N (Xk)( ~ (2~
In Equation 2, Xk and Yk ara discreet functions o~ timeO
Hence the cross-correlation function ~xy i8 al30 a di~creet
function of time. If Xk and Yk each contain N points and
the shift amount T iS equal to the ~ampling interval of Xk
and Y~ then the total number of points produced by the
cross-correlator 23 will be 2N-l. The number of products
formed by the cross-correlations for an example o N points
is N~.
The digital output of the correlator 23 is supplied to
a ~orrelator memory 24 which is al~o supplied wi~h ~iming
pulses from the synchronization detectox and timing circuit
18 as previously described. The digital output from the
correlator memory, upon receipt of an appropriate timing
pulse rom circuit 18, is supplied to a digital to analog
converter 25 where it is reconverted to analog fonm for
display as illustrated in Fig. 5. The output from the
digital to analog correlator 25 is then filter~d via band-
pass filter 27 and supplied to the recorder 28 for recording
as a variable density display as illustrated in the right-
hand half of the well log as a function of depth illustrated
in Fig. 5.
The output from the coxrelator memory 24 is also
supplied to a travel time computer 26 which computes the
travel time from the transmitter to the receiver for
-10
s~
selected arrivals at the receiver such as the compres~ional
wave travel time and the ~hear wave traveL time. The
compressional wave travel time or shear wave travel time is
then supplied to the recorder 28 for recording a~ a function
of depth as illus~rated i~ the left-hand half of ~he well
log of Fig. 5.
Referring now ~o Fig. 4 ~he sweep ~ignal, the composlte
receiver signal and the cross-correlation of the ~weep
signal and compo~ite receiver signals ar~ illustrated as a
function of time. It will be noted that the cro~s~correla-
tion output formed illustrates peaks which may be inter-
preted in terms of the compressional wave arrival, the shear
wave arrival, the direct wave arrival, a~d the Stonely wave
arrival. Travel times for these various acoustic modes may
thus be computed by the travel time comput r 26 by comparing
these arrivals with the sync pul~e and deriving the time
diference from it to the~e arrivals.
It will be appreciated by those skilled in the art that
power for the operation of the downhole electronics as well
as the surface electronic~ may be supplied from a ~urface
located supply 17 via conductors of the well logging cable
13. Appropriate downhole power converters (not shown) may
be housed in the downhole ~onde 11 in ordex to provide
operational voltages for the downhole electronic sy~tems in
a manner well known in the art.
` Referring now to Fig. 2, an acoustic waveform from a
pulsed transducer such as that used in the prior art is
illustrated. The typical acou~tic waveform may be inter-
preted according to propagation velocities of variou~ modes
of acoustic energy propagation in the borehole. Thus the
~6~L~5~
initial arrival is generally lnterpreted as that from the
compressional wave which is usually propagated faster
through the earth formations in the vicinity o~ a well
borehole. Appearing later on in arriving waveform are
energy peaks which may be interpreted a~ the shear wave, the
fluid wave and the Stonely wave portions of tha acoustic
wave form. Depending upon the tran~mitter to receiver
spacing and the amount of reflection o curring within the
borehole, interference between the different modes of
propagation can occur in prior art pulsed acoustic travel
time measurements for the different modes of acoustic
propagation. The present inver.tion by utili~ing a unique or
characteristic variable frequency swept siynal and correla-
tion of this signal with the entire acoustic w~ve train
lS arriving at the receiving tran~ducer can produce more
readily identifiable output pulses on the cross-correlator
output as illustrated in Fig~ 4 to separate the various
arrivals of acoustic mode propagation in a manner superior
to that known in ~he prior art. Thus improved acoustic
travel time measurements of compressional, shear, Stonely
and other modes of acoustic propagation are pro~ided by the
present invention which were subject to ambigious inter-
pretation in the prior art.
It will be recognized by those skilled in the art
that the acoustic transmitting transducer and asoustic
~receiving transducer of the present invention may be mounted
on pad arms (not shown) and urged against the wali of the
borehole if desired, rathex than being housed in the body of
the sonde as illustrated in Fig. 1. Similarly, a back-up
arm (not shown) could be used if desired, to urge the body
of the sonde of Fig, 1 against one wall o~ ~he borehole.
Because of the statis~ical nature o the cro~s-correlation
in detecting the arriving signal~ at the receiving trans-
ducers in the present inven~ion so called "road noise~ or
S noise generated by the motion of ~he logginy tool through
the borehole is minimized.
Other changes and modification~ which fall within
the true spirit and scope of the present invention may
be suggested by the foregoing descriptions to those skilled
in the art. Accordingly, it is the aim of the appended
claims to cover all such changes and modifications as
may be made apparent to those skilled in the art~
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