Seismic streamer cable

A streamer-cable assembly is provided having a tubular outer jacket enclosing an open-cell and closed-cell foam core. The foam core provides an axial hole for retaining a plurality of pressure sensors and a plurality of off-axial holes providing passages for the transmission bundle. Longitudinal channels along the exterior of the foam inserts receive substantially rigid stress members which provide longitudinally strength to the streamer. Cable noise associated with the cable jerk and cable whip/vibration is radially reduced by the open-cell foam core and rigid stress members while the closed-cell foam core provides buoyancy to the streamer cable at the connectors between sections.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to apparatus used to detect pressure waves 
propagating through a fluid and particularly to a low-noise streamer cable 
having improved buoyancy compensation. 
2. Description of the Related Art 
A conventional streamer cable used in seismic exploration may consist of a 
water-tight jacket enclosing a plurality of pressure sensors disposed at 
known intervals, depth transducers, and electrical and optical conductors 
extending through a series of bulkheads. The bulkheads also receive and 
are anchored to a plurality of steel or nylon stress members extending 
therethrough. The stress members absorb the strain of towing, thus 
preventing the cable jacket from stretching and eventually rupturing. The 
bulkheads provide a cylindrical shape to the streamer cable jacket, 
separate the stress members from each other, and may be used to protect 
the enclosed pressure sensors. The streamer cable often is filled with a 
noncorrosive light kerosene to provide neutral buoyancy while in the 
water. A few streamer cable designs have employed a closed-cell 
isocyanurate foam to provide buoyancy and to protect the enclosed sensors 
from crushing impacts, but closed-cell structure streamer cables are 
restricted to limited depths of operation. 
Conventional streamer cables suffer from substantially high amounts of 
background noise while under tow. Movement of the streamer cable through 
the water may result in cable noise on the order of 5 to 10 microvolt. A 
percentage of this noise may be attributed to sources away from the cable 
such as the propellers of the ship and the breaking waves in a rough sea. 
The remainder of the noise is generated by weights and depth controllers 
attached to the outside of the cable used to attain and maintain a common 
depth for the length of the cable. 
Two mechanisms are generally believed to be responsible for cable-generated 
noise. In the first mechanism, the cable is subject to irregular towing 
tension resulting in cable jerk. Shock from cable jerk propagates along 
the stress members to the bulkheads which in turn generate pressure waves 
in the cable oil. The pressure waves propagate along the length of the 
cable section with the cable jacket acting as a wave guide. Rarefractional 
waves also develop within the cable. The multiplicity of pressure waves 
and/or rarefractional waves (hereafter called tube waves) are detected by 
the enclosed pressure sensors as background noise. The second mechanism 
for generating cable noise may result from objects outside the cable or 
from the cable moving laterally to the towing direction. That is to say 
the cable may whip or vibrate; the motion having a range of amplitudes and 
wave lengths. Cable whip causes the cable jacket to shift about the 
bulkheads causing vibration to be transferred to the cable oil. 
Additionally, the cable jacket may impact one or more of the stress 
members thus generating a pulse which is detected by the sensors. In 
addition to cable whip, cable noise is generated by vibration of the 
stress members. Cable jerk pulls the stress members taught from the 
relaxed position, setting them to vibrate and generate noise. 
It is preferred that a streamer cable have substantially zero background 
noise while in tow, but if this is not readily attainable, it is most 
desirable to reduce background noise as much as possible. 
SUMMARY OF THE INVENTION 
It is a primary object of this invention to provide a lownoise streamer 
cable. 
It is another object of this invention to provide a streamer cable having a 
greater longitudinal rigidity to reduce cable whip and vibration. 
It is yet another object of this invention to provide a streamer cable 
which is substantially incapable of having tube waves propagate therein. 
It is yet another object of this invention to substantially reduce the need 
for external ballast or depth controlling devices to control the tow depth 
of the streamer cable. 
The instant invention may have a flexible tubular jacket enclosing a 
plurality of pressure sensors disposed at known intervals along its 
length, all interconnected to a recording and control unit aboard a towing 
vehicle by a transmission bundle. A unitary, open cell foam insert may be 
enclosed within the jacket and enclose the plurality of pressure sensors 
so as to physically locate each along the axis of the cable. A plurality 
of off-axial holes each extending the length of the inset contain a 
transmission bundle. Located at each end of the streamer cable within the 
cable jacket, and proximate the end connectors, may be a closed cell foam 
core insert. The closed cell insert contains the same holes as extending 
through the open cell insert described above. 
A plurality of rigid stress members are longitudinally disposed within the 
jacket and exterior the insert, each containing a steel or nylon cable 
extending the length thereof. Disposed around the inserts and inside of 
the stress members at known intervals may be an annular locator for 
maintaining the radial position of each stress member and assist in 
maintaining the cylindrical shape of the cable. 
The pressure sensors may each have a polyvinylchloride (PVC) housing which 
may support the sensor within the foam insert. The polyurethane inserts, 
stress members and PVC cable jacket and sensor housing all act in concert 
to dampen tube waves and spurious pressure pulses resulting from cable 
movement, as well as provide a more stable cable profile in the water as 
it is towed. The low cable profile improves the laminar fluid flow 
adjacent the cable exterior and reduces cable noise.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
In the following detailed description and corresponding figures, like 
reference numerals will indicate like components, wherein FIG. 1 generally 
illustrates a ship 10 towing a streamer cable 12 through a body of water 
14. Streamer cable 12 may be of any desired length up to 3 kilometers or 
more. The length of the streamer cable 12 may be changed by adding or 
subtracting cable sections 16 interconnected to each other by connectors 
18. The distal end of the streamer cable 12 with respect to the ship 10, 
may have a positioning buoy coupled thereto by a length of cable 22 so 
that the location of the cable end may be determined visually, by radar or 
other positioning system known in the art. The proximal or near end of the 
streamer cable may be connected to the ship 10 by a lead-in cable section 
24, which in turn is operably connected through a reel to a recording and 
control unit 26. 
FIGS. 2 and 3 are an isometric view of a streamer cable segment 28 
representative of any one of the cable sections 16. Segment 28 is shown in 
quarter section to reveal the interior components. In one embodiment of 
the inventive streamer cable, each cable section 16 may be comprised of a 
tubular outer jacket 30 enclosing a foam-core 32 and 33. It is preferred 
that core 32 have an open-cell construction with approximately 30 cells 
per linear inch and extruded as a single piece. The core 32 may be made 
from a polyurethane having a specific gravity substantially equal to 0.9 
grams per cubic centimeter (gm/cm.sup.3), but other plastics may be used 
having essentially the same properties. The transverse shape of the core 
may vary but FIG. 2 illustrates generally a cylindrical shape having an 
axial hole 34 and three off-axial holes 36 running the length of the 
member. The holes are preferably cut into the core 32 during the extrusion 
process. A cylindrical plug 38 cut from the axial hole 34 remains therein 
while the off-axial cylindrical pieces may be removed. Defined along the 
exterior of core 32 may be a plurality of longitudinal channels 40 
equidistant from each other. FIG. 2 illustrates three such channels, each 
located along the exterior at approximately 120 degrees from each other 
and positioned between the off-axial holes 36. Core 32 and plug 38 are 
easily cut to the desired length by scissors or a hot knife. The open-cell 
foam core 32 extends substantially the length of a cable section 16. 
Refer to FIG. 3. Located at either end of the cable section 16 between the 
cable connector 18 and the open cell core 32 is a closed-cell foam 33. The 
length of the core insert 33 may vary, depending upon the weight of the 
cable at the connectors. The purpose of the closed-cell foam is to 
provided the proper amount of buoyancy to that portion of the cable 
proximate the connectors. The shape of the core 33 and the location of the 
holes in the closed cell foam are the same as in the open cell foam. 
Extending the length of each cable section 16 and received within each 
channel 40 of core 32 and 33 are stress members 42. Each stress member may 
be comprised of a twisted steel or nylon cable 44 within an extruded 
thermoplastic rubber shell 46. The shell 46 is preferably extruded 
directly over cable 44 in a shape so as to fit snugly within channel 40 
inside jacket 30. For the purposes of example only the shape of the stress 
member may be oval or a curved rectangle. The stress members 42 are 
maintained in radial relationship to each other by a locator 48 shown in 
FIG. 4. Each locator 48 may be a plastic ring 50 molded from a 
polycarbonate blend to conform to the interior of jacket 30. The exterior 
of the locator may have channels 52 which align with channels 40 in the 
foam inserts. 
Disposed within axial hole 34 in the foam 32 at predetermined intervals 
along the cable section 16 may be a plurality of pressure sensors such as 
54 shown in FIG. 2. It is preferred that a predetermined portion of the 
foam plug 38 be removed at these intervals and the sensors inserted. Each 
sensor 54 may be operably coupled to a predetermined set of conductors 56 
and 58 comprising a portion of one of the transmission bundles 59 
extending the length of the cable section through the off-axial holes 36. 
It is preferred that no sensors be located within the closed cell foam 
insert, however other electronic packages may be contained therein. 
FIG. 5 is an isometric view in partial cross section of a preferred 
pressure sensor 54. In the figure, the pressure sensor 54 may by comprised 
of a piezoelectric transducer 60 such as a WM2-036 hydrophone manufactured 
by the assignee of this invention, however other hydrophones may also be 
employed. The hydrophone is preferably disposed within an axial hole 62 of 
a cylindrical housing 64 having an outside diameter substantially greater 
than the diameter of axial hole 34. Housing 64 also is preferably molded 
from polyvinylchloride (PVC) to form a resilient capsule. The hydrophone 
may be centrally located and retained within the hole 62 by a plurality of 
lugs or mounts 66 mounted to the hydrophone 60. Although piezoelectric 
hydrophones are disclosed other types of sensors such as fiber optic 
transducers or accelerometers may be used. 
After all the components have been physically located with the polyurethane 
cable jacket 30 and sealed by the connectors 18 at each end, a cable oil 
such as light kerosene is added (not . The streamer cable 12 may be 
deployed and towed behind the ship 10 in the normal manner. Acoustic waves 
generated by a nearby source and reflected from subsurface layers of the 
earth impinge upon the streamer cable and are detected by the sensors 54 . 
The sensors 54 convert the received pressure pulse or pulses to electrical 
or optical signals which are transmitted through the bundles 59 to the 
recording and control unit 26. 
Cable noise typically associated with conventional cables is radically 
reduced. Tube waves generated by cable jerk within the cable are dispersed 
and dissipated by the polyurethane foam interior. The tube-wave energy is 
greatly reduced or absorbed by the many small scale interferences of the 
tube wave with itself resulting from the many collisions with the open 
cell network. Each cell in the core 32 acts to redirect a fraction of each 
tube wave so as to collide with another fraction thus preventing the tube 
wave to propagate and be detected by the sensors 54. Any tube wave noise 
that may reach sensors 54 is again reduced because of the absorbing 
tendency of the PVC housing 64 surrounding the transducer 60 Tube-wave 
noise originating near a sensor 54 may be reduced by being directed around 
the transducer by the conical ends 68 and 70 of housing as well as being 
absorbed by the PVC from which it is made. As mentioned earlier, it is 
preferred that the cylindrical housing 64 be made of PVC. This is because 
we found PVC has a greater characteristic for absorbing noise within the 
frequency spectrum of interest than any other materials tested; however, 
this is not to say that better materials do not exist. Noise associated 
with cable whip and/or vibration is also radically reduced because the 
stress members provide flexural rigidity to the length of the cable 
reducing the transverse whipping motion. Vibration of the cable stress 
member is additionally damped by the thermoplastic rubber shell extruded 
thereabout, adding flexural rigidity to the cable section. Noise generated 
from vibration or cable whip may develop is also absorbed by the 
polyurethane open cell foam interior. 
The closed cell foam disposed at the ends of the cable sections provide 
buoyancy at the cable connections. Without the closed cell foam, the 
weight of the connectors and electronic modules depress that portion of 
the streamer cable. The middle of the cable section, being more buoyant 
than the cable ends, has a tendency to float. Thus the profile of the 
streamer cable would have a wave or undular profile. To correct this 
profile, lead weights and dive planes are used to cause the streamer cable 
to be towed at a common depth. These external devices produce external 
cable noise which is undesirable. 
For illustrative purposes, our invention has been described with a certain 
degree of specificity. Variations will occur to those skilled in the art 
but which may be included within the scope and spirit of this invention 
which is limited only by the appended claims.