Marine streamer cable

A marine seismic cable comprising a core, a body of flotation material positioned around the core, an acoustic energy transducer positioned proximate the core, and a body of acoustically transparent material positioned around the acoustic energy transducer.

BACKGROUND OF THE INVENTION 
This invention relates generally to seismic exploration of substrata 
beneath bodies of water and, more particularly, to a marine seismic cable 
for sensing reflected seismic waves from such substrata. 
Marine seismic exploration is often conducted by towing a seismic streamer 
at a given depth through the ocean or other body of water. The streamer is 
provided with a plurality of acoustic sensitive transducers, that is 
hydrophones, disposed at appropriate intervals along the length thereof. 
Acoustic wave energy is provided in the vicinity of the cable by an air 
gun or other suitable means; this wave energy travels downwardly through 
the earth with a portion of it being reflected upwardly at levels where 
there is a contrast in the acoustic impedance of the strata. The 
hydrophones sense the acoustic pressure waves produced in the water by the 
upwardly traveling seismic reflections and provide electrical signals 
indicative thereof to suitable processing and recording equipment located 
on the seismic vessel that is towing the streamer. 
The magnitude of the reflected signals is extremely small, thus making it 
essential to minimize extraneous noise detected by the hydrophones and to 
maximize the signal-to-noise ratio. One source of such noise is boundary 
layer or flow noise which is generated by the water flowing past the 
surface of the cable in a turbulent fashion. It has been found that flow 
noise is essentially a localized pressure disturbance which is normal to 
the surface of the cable. 
Therefore, it is an object of the present invention to provide a marine 
seismic cable that reduces the flow noise sensed by a hydrophone mounted 
in the cable. 
SUMMARY OF THE INVENTION 
The present invention relates to a marine seismic cable of layered solid 
construction having a core and a body of flotation material positioned 
around the core. Acoustic energy transducers, such as cylindrical 
hydrophones or electret hydrophones, are positioned proximate the core. A 
body of acoustically transparent material is positioned around the 
hydrophones so that the acoustic pressure waves produced in the water by 
the seismic reflections are transmitted therethrough to the hydrophones. 
The wavelengths of the seismic pressure waves are long compared to the 
length of a hydrophone so that essentially the entire length of the 
hydrophone is subjected to the same pressure. However, the flow noise 
consists essentially of a plurality of uncorrelated point sources exterior 
of the cable. The effective aperture of a hydrophone positioned near the 
cable core, that is, the area on the cable surface in which signals can be 
sensed, provides a relatively large sampling area compared to the prior 
art teachings of a hydrophone positioned near the surface of the cable. It 
has been found that when the uncorrelated point sources of flow noise are 
averaged over this larger area the point sources tend to add 
non-coherently; whereas, the desired seismic signal adds coherently thus 
resulting in an improved signal-to-noise ratio. Moreover, positioning the 
hydrophone near the core removes the hydrophone from the immediate 
location of the noise source so that the noise signal is attenuated when 
it reaches the hydrophone. 
In the preferred embodiment of the invention, the body of acoustically 
transparent material that is positioned over the hydrophone is the same 
material as the flotation body. In addition, a body of elastic material is 
positioned between the core and the hydrophone to decouple or insulate the 
hydrophone from noises propagated along the core; this elastic body also 
centers the cylindrical hydrophones on the cable core. 
Other objectives, advantages and applications of the present invention will 
be made apparent by the following detailed description of the preferred 
embodiments of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a seismic exploration vessel 10 is shown towing a 
marine streamer cable 12 through a body of water located above the 
substrata that is to be seismically explored. Cable 12 can be quite 
lengthy, for example, about 2 miles, and is normally composed of a number 
of individual sections 14 connected end to end. The forward section of 
cable 12 is connected to vessel 10 by a typical lead-in section 16. Each 
section 14 contains a number of hydrophones (not shown) that are 
positioned and electrically connected to form an array as is known in the 
art. Acoustic wave energy is provided in the vicinity of cable 12 by an 
air gun 18 or other suitable means. This wave energy travels downwardly 
through the earth with a portion of it being reflected upwardly at levels 
where there is a contrast in the acoustic impedance between layers of the 
strata, for example at point 20. The hydrophones sense the acoustic 
pressure waves produced in the water by the upwardly traveling seismic 
reflections. 
Referring to FIGS. 2 and 3, a section of a marine seismic cable according 
to the present invention is indicated generally by the numeral 22. Cable 
22 has a core 24 which consists of a stress member 26, which can be a 
flexible steel cable or a synthetic fiber rope, and a plurality of 
electrical conductors 28 disposed around stress member 26. The voids 
between conductors 28 are filled with a suitable filler compound 30, and a 
jacket 32 of urethane or other suitable material is extruded over the 
outer portion of conductors 28 and filler 30. Core 24 is disposed axially 
within a cylindrical body of flotation material 34, such as soft urethane 
having embedded glass or plastic microspheres or balloons, to provide the 
desired buoyancy. 
At discrete locations along cable 22 a cylindrical hydrophone 36 is 
positioned around core 24. O-rings 38 are positioned at each end of 
hydrophone 36 between core 24 and hydrophone 36. If desired, elastic 
material, such as soft urethane, can be positioned in space 40 between 
core 24, hydrophone 36 and O-rings 38. Alternatively, O-rings 38 can be 
replaced by a layer of elastic material, such as soft urethane, on core 24 
that covers substantially the entire length of hydrophone 36. 
Electrical leads 42 from hydrophone 36 are connected in a suitable manner 
to a pair of electrical conductors 44 from the set of electrical 
conductors 46 provided by the termination (not shown) of the cable 
section. Preferably, the portion of flotation material 34 that is 
positioned around hydrophone 36 has the same outside diameter as the 
portion of flotation material 34 that is positioned around core 24 to 
provide a smooth exterior surface. A sheath 48 of, for example, 
polyurethane plastic, is extruded over flotation material 34 to provide a 
relatively smooth and damage resistant outer surface. 
FIGS. 4 and 5 disclose alternative embodiments of the present invention in 
which electret hydrophones are employed. The portions of the cable in 
FIGS. 4 and 5 that are the same as those shown in FIGS. 2 and 3 are 
indicated by the same numerals. Referring to FIG. 4, a plurality of 
electret sensors or hydrophones 50 are positioned longitudinally along 
core 24. A body of elastic material 52 having low bulk compressibility, 
such as soft urethane having embedded glass or plastic microspheres or 
balloons, is positioned between each of electret hydrophones 50 to isolate 
them from noises transmitted along core 24. Electret hydrophones 50 are 
electrically connected together by means of their wires 54 and 56 which, 
in turn, are connected to the pair of wires 44 from the set of wires 46 
provided by the termination (not shown) of the cable section. FIG. 5 
discloses an embodiment in which a plurality of electret sensors or 
hydrophones 58 and 60 are helically wrapped around core 24 such that they 
overlap. A body of elastic material 61 is positioned between electret 
hydrophones 58 and 60 and core 24. Electret hydrophones 58 and 60 are 
electrically connected together by means of their wires 62 and 64 which, 
in turn, are connected to the pair of wires 44 from the set of wires 46 
provided by the termination (not shown) of the cable section. 
It is to be understood that variations and modifications of the present 
invention can be made without departing from the scope of the invention. 
It is also to be understood that the scope of the invention is not to be 
interpreted as limited to the specific embodiments disclosed herein, but 
only in accordance with the appended claims when read in light of the 
foregoing disclosure.