Note: Descriptions are shown in the official language in which they were submitted.
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1593P07CA01
MARINE SEISMIC STREAMER AND METHOD
FOR MANUFACTURE THEREOF
Background of Invention
Field of the Invention
The invention relates generally to the field of marine seismic data
acquisition equipment.
More specifically, the invention relates to structures for a marine seismic
streamer, and
methods for making such streamers.
Background Art
Marine seismic surveying is typically performed using "streamers" towed near
the surface of
a body of water. A streamer is in the most general sense a cable towed by a
seismic vessel
having a plurality of seismic sensors disposed thereon at spaced apart
locations. The sensors
are typically hydrophones, but can also be any type of sensor that is
responsive to the
pressure in the water, or in changes therein with respect to time. The sensors
may also be any
type of particle motion sensor or acceleration sensor known in the art.
Irrespective of the
type of such sensors, the sensors generate an electrical or optical signal
that is related to the
parameter being measured by the sensors. The electrical or optical signals are
conducted
along electrical conductors or optical fibers carned by the streamer to a
recording system.
The recording system is typically disposed on the seismic vessel, but may be
disposed
elsewhere.
In a typical marine seismic survey, a seismic energy source is actuated at
selected times, and
a record, with respect to time, of the signals detected by the one or more
sensors is made in
the recording system. The recorded signals are later used for interpretation
to infer structure
of, fluid content of, and composition of rock formations in the Earth's
subsurface.
A typical marine seismic streamer can be up to several kilometers in length,
and can include
hundreds of individual seismic sensors. Because of the weight of all of the
materials used in
a typical marine seismic sensor, because of the friction (drag) caused by the
streamer as it is
moved through the water, and because of the need to protect the sensors,
electrical and/or
optical conductors and associated equipment from water intrusion, a typical
seismic streamer
includes certain features. First, the streamer includes one or more strength
members to
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transmit axial force along the length of the streamer. The strength member is
operatively
coupled to the seismic vessel and thus bears all the loading caused by drag
(friction) of the
streamer in the water. The streamer also includes, as previously explained,
electrical and/or
optical conductors to carry electrical power and/or signals to the various
sensors and (in
certain streamers) signal conditioning equipment disposed in the streamer and
to carry signals
from the various sensors to a recording station. The streamer typically
includes an exterior
jacket that surrounds the other components in the streamer. The jacket is
typically made from
a strong, flexible plastic such as polyurethane, such that water is excluded
from the interior
thereof, and seismic energy can pass essentially unimpeded through the jacket
to the sensors.
A typical streamer also includes buoyancy devices at spaced apart locations
therealong, so
that the streamer so that the cable is substantially neutrally buoyant in the
water. The interior
of the jacket is typically filed with oil or similar electrically insulating
fluid that is
substantially transparent to seismic energy.
Another device that is typically affixed to a streamer at spaced apart
locations therealong is
known as a "compass bird." A compass bird includes a directional sensor,
typically a
magnetometer, to determine the orientation of the streamer at the position of
the compass
bird. The compass bird may include an electromagnetic transducer to
communicate its
measurements through the streamer jacket to a detector inside the jacket.
Direction
measurements are used to infer the position of the streamer along its length,
because currents
in the body of water can cause the streamer to move transversely with respect
to the direction
of motion of the seismic vessel.
A seismic streamer including the various components described above is
typically made by
inserting the various components inside the jacket, and filling the interior
space within the
jacket with oil or other electrically insulating material. During manufacture,
axial stress may
be applied to the strength member, and during handling and storage,
essentially no axial
stress is applied. As a result, the various components within the jacket may
move laterally
and/or axially with respect to the jacket. Thus, the geometry of the typical
streamer may
change between handling, storage, deployment and actual operation, where
substantial axial
force is applied to the strength member. Compass bird orientation with respect
to the
streamer jacket and internal components is particularly susceptible to error
due to changes in
streamer component geometry.
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There is a need for a marine seismic streamer that has precisely controlled
geometry during
manufacture, and which geometry substantially does not change between
manufacture,
handling, storage and use.
Summary of Invention
One aspect of the invention is a seismic streamer, including a jacket covering
an exterior of
the streamer. At least one strength member extends along the length of the
jacket. The
strength member is disposed inside the jacket. Seismic sensors are disposed at
spaced apart
locations along the interior of the jacket. A flexible, acoustically
transparent material fills the
space inside the jacket. The material is introduced into the inside of the
jacket in liquid form
and undergoes a state change thereafter. The strength member is maintained at
least near a
position along the jacket to which a device is to be attached externally,
during the state
change in substantially axial alignment with the jacket.
Another aspect of the invention is a method for making a seismic streamer. A
method
according to this aspect includes inserting at least one strength member and
seismic sensors
into a jacket. The jacket is then filled with a liquid having a composition
adapted to undergo
a change in state from liquid to substantially solid after the filling. The
strength member is
held, during the state change, in substantially axial alignment with the
jacket. The holding is
performed at least at a location along the jacket at which a device is to be
externally affixed.
In one embodiment, a selected tension is applied to the at least one strength
member to effect
the holding. In one embodiment, the tension is an amount selected to maintain
the strength
member and the sensors in essentially the desired position of the strength
member with
respect to the jacket when the streamer is towed by a seismic vessel in a body
of water.
Other aspects and advantages of the invention will be apparent from the
following description
and the appended claims.
Brief Description of Drawings
Figure 1 shows typical marine seismic data acquisition using a streamer
according to one
embodiment of the invention.
Figure 2 shows a cut away view of one embodiment of a streamer segment
according to the
invention.
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Detailed Description
An example marine seismic data acquisition system as it is typically used is
shown in Figure
1. A seismic vessel 14 moves along the surface of a body of water 12 such as a
lake or the
ocean. The marine seismic survey is intended to detect and record seismic
signals related to
structure and composition of various subsurface Earth formations 21, 23 below
the water
bottom 20. The seismic vessel 14 includes source actuation, data recording and
navigation
equipment, shown generally at 16, referred to for convenience as a "recording
system." The
seismic vessel 14, or a different vessel (not shown), can tow one or more
seismic energy
sources 18, or arrays of such sources) in the water 12. The system includes at
least one
seismic streamer 10, which includes a strength member 26 operatively coupled
to the seismic
vessel 14, and a plurality of sensors 24 or arrays of such sensors, disposed
at spaced apart
locations along the streamer 10. During operation, equipment (not shown
separately) in the
recording system 16 causes the source 18 to actuate at selected times. When
actuated, the
source 18 produces seismic energy 19 that emanates generally outwardly from
the source 18.
The energy 19 travels downwardly, through the water 12, and passes, at least
in part, through
the water bottom 20 into the formations 21, 23 below. Seismic energy 19 is at
least partially
reflected from one or more acoustic impedance boundaries 22 below the water
bottom 20,
and travels upwardly whereupon it may be detected by the sensors 24. Structure
of the
formations 21, 23 can be inferred by travel time of the energy 19 and by
characteristics of the
detected energy such as its amplitude and phase.
An important aspect of inferring the structure of the formations 21, 23 is
precise knowledge
of the geographic position of the sensors 24 during the survey, so that the
geographic position
of the boundaries 22 may be correctly inferred and so that the geographic
position of various
compositions of the formations 21, 23 may be estimated accurately.
Having explained the general method of operation of a marine seismic streamer,
an example
embodiment of a streamer according to the invention will be explained with
reference to
Figure 2. Figure 2 is a cut away view of a portion (segment) 10A of a marine
seismic
streamer (10 in Figure 1). A streamer as shown in Figure 1 may extend behind
the seismic
vessel (14 in Figure 1) for several kilometers, and is typically made from a
plurality of
streamer segments as shown in Figure 2 connected end to end behind the vessel
(14 in Figure
1 ).
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The streamer segment 10A in the present embodiment may be about 75 meters
overall length.
A streamer such as shown at 10 in Figure 1 may be formed by connecting a
selected number
of such segments 10A end to end. The segment 10A includes a jacket 30, which
in the
present embodiment is made from 3.5 mm thick transparent polyurethane, having
a nominal
external diameter of about 62 millimeters. In some embodiments, the jacket 30
may be
externally banded in selected places with an alloy number 304 stainless steel,
copper flashed
band (not shown).
In each segment 10A, each axial end of the jacket 30 may be terminated by a
coupling/termination plate 36. The termination plate 36 may include elements
36A on a
surface inserted into the end of the jacket 30 to seal against the inner
surface of the jacket 30,
and to grip the termination plate 36 to the jacket 30 when clamped externally
(not shown). In
the present embodiment, two strength members 42 are coupled to the interior of
each
termination plate 36 and extend the length of the segment 10A. In a particular
implementation of the invention, the strength members 42 may be made from a
fiber rope,
using a fiber sold under the mark VECTRAN, which is a registered trademark of
Hoechst
Celanese Corp., New York, NY. The strength members 42 transmit axial force
along the
length of the segment 10A. When one segment 10A is coupled end to end to
another segment
(not shown in Figure 2), mating termination plates 36 are coupled together
using any suitable
connector, so that the axial force is transmitted through the termination
plates 36 from the
strength members 42 in one segment 10A to the strength member in the adjoining
segment.
The segment 10A includes buoyancy spacers 32 disposed in the jacket 30 at
spaced apart
locations along its length. The buoyancy spacers 32 may be made from foamed
polypropylene. The buoyancy spacers 32 have a density selected to provide the
segment 10A
with approximately the same overall density as water (12 in Figure 1), so that
the streamer
(10 in Figure 1) will be substantially neutrally buoyant in the water. As a
practical matter,
the buoyancy spacers 32 provide the segment 10A with an overall density very
slightly less
than that of fresh water. Appropriate overall density may then be adjusted in
actual use by
adding selected amounts of dense ballast (not shown) to the exterior of the
jacket, thus
providing adjustment in the buoyancy for changes in water temperature and
salinity.
The segment 10A includes a generally centrally located conductor cable 40
which includes a
plurality of insulated electrical conductors (not shown separately), and may
include one or
more optical fibers (not shown). The cable conducts electrical and/or optical
signals from the
sensors (which will be further explained below) to the recording system (16 in
Figure 1). The
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cable may also carry electrical power to various signal processing circuits
(not shown
separately) disposed in one or more segments 10A or disposed elsewhere along
the streamer
(10 in Figure 1). The length of conductor cable 40 within a cable segment 10A
is generally
longer than the axial length of the segment 10A under the largest expected
axial stress, so that
the electrical conductors and optical fibers will not experience any
substantial axial stress
when cable 10 is towed through the water by a vessel. The conductors and
optical fibers may
be terminated in a connector 38 disposed in each termination plate 36 so that
when the
segments 10A are connected end to end, corresponding electrical and/or optical
connections
may be made between the electrical conductors and optical fibers in the
conductor cable 40 in
adjoining segments 10A.
Sensors, which in the present embodiment may be hydrophones, can be disposed
in selected
ones of the buoyancy spacers, shown in Figure 2 generally at 34. The
hydrophones in the
present embodiment can be or a type known to those of ordinary skill in the
art, including but
not limited to those sold under model number T-2BX by Teledyne Geophysical
Instruments,
Houston, TX. In the present embodiment, each segment 10A may include 96 such
hydrophones, disposed in arrays of sixteen individual hydrophones connected in
electrical
series. In a particular implementation of the invention, there are thus six
such arrays, spaced
apart from each other at about 12.5 meters. The spacing between individual
hydrophones in
each array should be selected so that the axial span of the array is at most
equal to about one
half the wavelength of the highest frequency seismic energy intended to be
detected by the
streamer (10 in Figure 1). It should be clearly understood that the types of
sensors used, the
electrical and/or optical connections used, the number of such sensors, and
the spacing
between such sensors are only used to illustrate one particular embodiment of
the invention,
and are not intended to limit the scope of this invention. In other
embodiments, the sensors
may be particle motion sensors such as geophones or accelerometers. A marine
seismic
streamer having particle motion sensors is described in U.S. Patent
Application No.
10/233,266, filed on August 30, 2002, entitled, "Apparatus and Method for
Multicomponent
Marine Geophysical Data Gathering", assigned to an affiliated company of the
assignee of
the present invention and incorporated herein by reference.
At selected positions along the streamer (10 in Figure 1) a compass bird 44
may be affixed to
the outer surface of the jacket 30. The compass bird 44 includes a directional
sensor (not
shown separately) for determining the geographic orientation of the segment
10A at the
location of the compass bird 44. The compass bird 44 may include an
electromagnetic signal
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transducer 44A for communicating signals to a corresponding transducer 44B
inside the
jacket 30 for communication along the conductor cable 40 to the recording
system (16 in
Figure 1). Measurements of direction are used, as known in the art, to infer
the position of
the various sensors 34 in the segment 10A, and thus along the entire length of
the streamer
( 10 in Figure 1 ). Typically, a compass bird will be affixed to the streamer
( 10 in Figure 1 )
about every 300 meters (every four segments 10A). One type of compass bird is
described in
U.S. Patent No. 4,481,611 issued to Burrage and incorporated herein by
reference.
In the present embodiment, the interior space of the jacket 30 may be filled
with a material 46
such as a gel, which may be a curable, synthetic urethane-based polymer. The
gel 46 serves
to exclude fluid (water) from the interior of the jacket 30, to electrically
insulate the various
components inside the jacket 30, and to transmit seismic energy freely through
the jacket 30
to the sensors 34. The gel 46 in its uncured state is essentially in liquid
form. Upon cure, the
gel 46 no longer flows as a liquid, but instead becomes substantially solid.
However, the gel
upon cure retains some flexibility to bending stress, some elasticity, and
freely transmits
seismic energy to the sensors 34. For purposes of defining the scope of the
invention, it
should be understood that the gel used in the present embodiment only is one
example of a
substance which would perform according to the invention. Chemical and/or
evaporative
curing of a urethane compound is a convenient method for forming a streamer
segment
according to the invention, however other methods could be used with other
materials. For
example, heating a selected substance, such as a thermoplastic, above its
melting point, and
introducing the melted plastic into the interior of the jacket 30, and
subsequent cooling, may
also be used in a streamer according to the invention. It is preferable that
the material used
has similar acoustic properties, density and electrical properties as the
disclosed BVF-25
urethane so that the streamer will have similar mechanical and acoustic
response
characteristics to the disclosed streamer. All that is required for the
invention to work is that
the material undergo a state change from liquid at the time of filling the
interior of the jacket
to substantially solid thereafter.
In making a streamer according to the invention, first, the components
described above
including the sensors 34, buoyancy spacers 32, strength members 42 and
conductor cable 40
are inserted into the jacket 30. In the present embodiment, the strength
members 42 are then
stretched to approximately the same degree as would be the case when the
streamer is in use
towed by the seismic vessel (10 in Figure 1). By applying the appropriate
amount of axial
tension to the strength members 42, the spacers 32 and the strength members 42
may be
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maintained in essentially the same geometry with respect to the jacket 30 that
they will
assume during operation of the streamer as towed by the seismic vessel. Then,
the uncured
urethane compound (gel 46) is inserted into the interior of the jacket 30 to
fill the space
therein. During the time needed for the urethane compound to cure, which may
be on the
order of two weeks for the present embodiment, the axial tension is maintained
on the
strength members 42. When the urethane compound is cured, the streamer may be
made
ready for storage and transportation, such as on a reel (not shown). For the
segment
embodiment shown in Figure 2, during assembly of the segment 10A, the
termination plates
36 are coupled to the strength member 42, and inserted into the jacket 30.
Tension may be
applied to the strength members 42 during cure by way of the termination
plates 36, thus
making a completed segment 10A. Made according to this embodiment, the
streamer will
maintain essentially the same geometry of the various internal components,
including the
spacers 32, the sensors 34 and the strength members 42 irrespective of the
amount the tension
applied to the strength member 42.
In other embodiments, the stretching of the strength members may be made only
at the
position along the jacket 30 at which the compass bird 44 is to be affixed to
the exterior of
the jacket.
It should be understood that stretching the strength members is only one
convenient way to
cause the strength members to remain in their ordinary operating position
during cure of the
gel 46. For purposes of defining the scope of the invention, it is only
necessary to maintain
the strength members 42 in their desired position during operation of the
streamer, during
cure of the gel 46.
Having a curable gel or similar filling the jacket 30, rather than liquid as
in prior art
streamers, can also reduce the possibility of streamer failure in the event of
breach of the
jacket 30. In the event of such breach, the substantially solid nature of the
cured gel 46 will
provide some mechanism to continue to exclude water from the active components
of the
streamer, including the sensors 34 and the cable conductor 40, similar to the
action of a
potting compound.
Streamers and streamer segments made according to the various aspects of the
invention may
have improved control over relative geometry of the internal components as
compared with
prior art streamers, and may provide more accurate placement of navigational
devices thereon
for increased accuracy in seismic surveying.
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While the invention has been described with respect to a limited number of
embodiments,
those skilled in the art, having benefit of this disclosure, will appreciate
that other
embodiments can be devised which do not depart from the scope of the invention
as disclosed
herein. Accordingly, the scope of the invention should be limited only by the
attached
claims.
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