Marine shear cable

A marine cable for detecting converted shear waves reflected from the strata beneath a body of water in response to a compressional wave generated in the body of water. The cable comprises a jacket, a mount attached to the stress member, a geophone positioned in the mount, a first weight assembly attached to the stress member and positioned proximate a first end of the mount and a second weight assembly positioned proximate a second end of the mount. The mount is sized and made of a material so that its weight is sufficient to cause at least the portion of the cable jacket that is adjacent the mount to contact the seafloor. The first and second weight assemblies are sized and made of material so that their respective weights cause at least the portion of the cable jacket that is adjacent to the respective weight assemblies to contact the seafloor.

BACKGROUND OF THE INVENTION 
This invention relates generally to seismic exploration of substrata 
beneath bodies of water and, more particularly, to marine seismic 
exploration conducted by sensing converted shear waves reflected from such 
substrata in response to a downwardly travelling compressional wave. 
Marine seismic exploration is generally 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 pressure sensors, such as 
hydrophones, disposed at appropriate intervals along its length. 
Compressional wave energy is provided in the vicinity of the cable by an 
air gun or other suitable means; this compressional 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 
characteristics of the strata. The pressure sensors detect the 
compressional waves produced in the water by the upwardly travelling 
seismic reflections and provide electrical signals indicative thereof to 
suitable processing and recording equipment located on the seismic vessel 
that is towing the streamer. It has been found that shear waves are 
generated from the compressional waves at interfaces in the strata; these 
shear waves contain additional information on the nature of the strata. 
However, this data is not considered since the reflected shear waves are 
not sensed by the marine seismic systems of the prior art. 
Therefore, it is an object of the present invention to provide a marine 
cable for detecting waves reflected from the strata during seismic 
exploration with a compressional source. 
SUMMARY OF THE INVENTION 
In accordance with the present invention there is provided a marine cable 
for detecting converted shear waves reflected from the strata beneath a 
body of water in response to a compressional wave generated in the body of 
water. The cable comprises a jacket, a mount attached to the stress 
member, a geophone positioned in the mount, a first weight assembly 
attached to the stress member and positioned proximate a first end of the 
mount and a second weight assembly positioned proximate a second end of 
the mount. The mount is sized and made of a material so that its weight is 
sufficient to cause at least the portion of the cable jacket that is 
adjacent the mount to contact the seafloor. The first and second weight 
assemblies are sized and made of material so that their respective weights 
cause at least the portion of the cable jacket that is adjacent to the 
respective weight assemblies to contact the seafloor. The cable can also 
comprise pressure transducers for sensing the compressional waves 
reflected from the strata in response to the compressional wave generated 
in the body of water. The pressure transducers and geophones are 
interspersed along the length of the cable. Preferably, weight assemblies 
are positioned equidistantly on each side of the geophones and pressure 
transducers to ensure that the cable settles to the ocean bottom so that 
the geophones are properly coupled thereto. 
In the preferred embodiment the weight assemblies consist of three sections 
that are fastened together to form a cylinder. Each of the sections has a 
groove in each end such that when the sections are fastened together the 
grooves form apertures that are sized and positioned to accommodate the 
three stress members in the cable. The weight assemblies also have a 
central aperture which is sized to accommodate the group of electrical 
wires from the geophones and pressure transducers.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, a seismic exploration vessel 10 is shown deploying a 
marine cable 12 to seismically explore the substrata that is beneath body 
of water 14. Cable 12 can be quite lengthy, for example, a mile or more, 
and is normally composed of a number of individual active sections 16 
connected end to end. Each section 16 contains a plurality of geophones 
(not shown) and is positioned adjacent bottom 18. Cable 12 can be 
positioned on bottom 18 in the desired location by dragging it to the 
desired location or by reeling it in and then unreeling it at the desired 
location as vessel 10 moves forward. Section 20 of cable 12 which is 
connected to the first section 16 is a weighted section containing, for 
example, lead or other suitable material. Lead-in section 22, which can be 
an armored cable, connects section 20 to vessel 10. Section 20 should 
contain sufficient weight so that the waves in body of water 14 acting on 
vessel 10 and lead-in section 22 do not tend to decouple sections 16 from 
bottom 18. If desired, the tail end of cable 12 can also be provided with 
a weighted section 20 and a suitable location buoy, as is known in the 
art. Compressional wave energy is provided in the vicinity of cable 12 by 
an air gun 24 or other suitable means; air gun 24 can be deployed from 
vessel 10 or a second vessel which can move in the vicinity of the 
geophones without moving cable 12. Compressional wave 26, which is 
generated by air gun 24 and is indicated by a straight line, travels 
downwardly through body of water 14 and the earth with a portion of it 
being reflected upwardly at points where there is a contrast in the 
acoustic impedance between layers of the strata, for example, points 28 
and 30, where a portion of compressional wave 26 is reflected upwardly as 
indicated by reflected compressional waves 32 and 34. In addition, 
converted shear waves 36 and 38 are reflected at points 28 and 30 
respectively. Reflected shear waves 36 and 38 travel upwardly through the 
strata and are detected by the geophones located in sections 16 of cable 
12. The electric signals produced by the geophones in response to the 
reflected shear waves are transmitted along wires in cable 12 to suitable 
recording and/or processing equipment located on vessel 10. In addition, 
if desired, hydrophones or other compressional wave transducers can be 
positioned in active sections 16 to detect reflected compressional waves 
32 and 34. It should be noted that cable 12 should be allowed to settle 
for a predetermined period of time, for example, 10-12 seconds has been 
found to be a suitable length of time after the cable has been towed into 
position at a speed of three knots, before air gun 24 is activated to 
ensure that cable 12 is properly coupled to bottom 18 and to ensure that 
the noise transients generated during the positioning of cable 12 have 
been attenuated. 
Referring to FIGS. 2, 3 and 4, cable 12 has three stress members 42 which 
are maintained in the shape of an equilateral triangle by a plastic spacer 
(not shown), as is known in the art. At predetermined locations along 
cable 12 geophone assemblies 44 are positioned such that geophones 45 are 
in line with the axis of cable 12. Each geophone assembly 44 includes a 
conventional geophone 45, which is used to detect horizontal motion, and a 
mount 46 adapted for securing geophone 45 at a predetermined location 
along cable 12. A damping resistor 47 is connected across terminals 51 of 
geophone 45, and wires 49 are connected to terminals 51. Preferably, 
geophone 45 and damping resistor 47 are positioned in protective housing 
48, such as PVC tubing, and the ends of housing 48 are sealed by epoxy 50 
or the like to protect geophone 45 from corrosion and pressure damage. 
Mount 46 comprises three similarly shaped segments 52 which form a 
cylindrical housing or mount when assembled by screws 53 in apertures 54. 
Both ends of each segment 52 have a groove 55 which mates with grooves 55 
in the adjacent segments 52 to form apertures 56 which are sized and 
positioned to accommodate the three stress members 42. Central cavity 60 
is sized such that housing 48 is held securely therein when screws 53 are 
tightened. Mount 46 can be provided with a further smaller cavity 62 
adjacent to central cavity 60 for epoxy or the like to further ensure 
proper bonding between mount 46 and hosuing 48. Each of sections 52 has a 
groove 58 in its outer surface, and approximately one-third of the wires 
from group of wires 66 are wrapped in a protective covering 68, such as 
polyurethane, and positioned in each of grooves 58. Wires 49 from 
terminals 51 are connected to a pair of wires from group of wires 66 by 
conventional means. Mount 46 should be made of a relatively heavy material 
that resists deformation and corrosion, such as brass, to ensure that 
geophone 45 is properly coupled to the ocean bottom. 
Weight assembly 72 can be made of, for example, lead or other suitable 
material and preferably are positioned equidistantly on each side of 
geophone assembly 44 to ensure that cable 12 settles and is properly 
coupled to the ocean bottom. Weight assembly 72 consists of three sections 
74 that are held together to form a cylinder by metal strap 76 or other 
suitable means. Each of sections 74 has a groove 75 in each end such that 
when sections 74 are held together by strap 76 grooves 75 form apertures 
78 which are sized to accommodate stress members 42. Weight assembly 72 
has a central aperture 80 which is sized to accommodate group of wires 66. 
A protective covering 82, such as polyurethane, can be inserted in central 
aperture 80 to prevent chafing of group of wires 66. Cable 12 is provided 
with a jacket 70 of, for example, polyurethane plastic, which provides a 
relatively smooth and damage resistant outer surface and is filled with a 
suitable liquid, such as kerosene. 
FIG. 5 illustrates an alternative embodiment of the cable of the present 
invention which includes both geophones and pressure transducers. Geophone 
assemblies 44 and hydrophones 90 or other suitable pressure transducers 
are positioned at predetermined locations along cable 92 so that geophone 
assemblies 44 and hydrophones 90 are interspersed. Hydrophones 90 can be 
mounted by conventional means as is known in the art. Preferably, weight 
assemblies 72 are positioned equidistantly on each side of geophone 
assemblies 44 and hydrophones 90. Stress members 42 can be maintained in 
proper orientation by conventional spacers 94 located between geophone 
assemblies 44 and hydrophones 90. 
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.