Marine geophysical prospecting system

Abstract

A marine geophysical prospecting system employs a hydrophone streamer cable containing electromagnetic field sensors, modified to be towed at a preselected distance above the sea floor by a first marine vessel senses electromagnetic energy from selected substrata beneath bodies of water. The voltage between sensors may be amplified by amplifiers in the cable or by amplifiers aboard the towing vessel. Optionally, a second similarly modified cable is preferably located above the near-bottom cable. Both cables may also contain hydrophones and/or accelerometers, as well as depth and position sensors. Optionally, a second vessel tows at least one conventional seismic source to create compressional energy which propagates downwardly through the water into the substrata beneath the body of water. At appropriate porous subsurface formations, the acoustic energy is converted to electromagnetic energy. This electromagnetic energy propagates upwardly and is detected by the electromagnetic sensors in the near-bottom cable; the second cable provides a reference array to suppress electromagnetic noise. Alternatively, the electromagnetic sensors in the near-bottom cable and a power source on the towing vessel may create an alternating or pulsed electromagnetic field in the sea floor. This electromagnetic field travels into the substrata beneath the water body and via conversion of the electromagnetic waves into seismic pressure waves at other appropriate porous subsea earth formations, generates a seismic wave. The resulting seismic wave is detected by the hydrophones and/or accelerometers in the cables.

What is claimed is: 
     
       1. A marine geophysical prospecting system, comprising: a streamer cable modified to operate near the bottom of a body of water,   a plurality of electromagnetic energy transceivers mounted to said cable and disposed so as to contact said body of water, and   at least one acoustic energy source.   
     
     
       2. The marine geophysical prospecting system of claim 1, further comprising, a second streamer cable modified to operate above said streamer cable and having electromagnetic energy transceivers. 
     
     
       3. The marine geophysical prospecting system of claim 1, further comprising: a plurality of acoustic energy detectors mounted to said cable.   
     
     
       4. The marine geophysical prospecting system of claim 1, wherein said electromagnetic energy transceivers are adapted to detect electromagnetic energy resulting from subsurface conversion of acoustic energy from said acoustic energy source into electromagnetic energy. 
     
     
       5. The marine geophysical prospecting system of claim 1, wherein said electromagnetic energy transceivers are adapted to transmit electromagnetic energy downwardly into the formations below said body of water, and wherein said system further comprises at least one acoustic energy detector adapted to detect acoustic energy resulting from subsurface conversion of said electromagnetic energy into acoustic energy. 
     
     
       6. A marine geophysical prospecting system, comprising: a streamer cable modified to operate near the bottom of a body of water,   a plurality. of conductive electrodes disposed to contact said body of water mounted on said cable,   a plurality of acoustic energy detectors mounted in said cable, and   at least one acoustic energy source.   
     
     
       7. The marine geophysical prospecting system of claim 6, wherein said electrodes are spaced between 1 and 1000 feet apart. 
     
     
       8. The marine geophysical prospecting system of claim 6, further comprising, amplifier means for amplifying the electromagnetic waves detected by said electrodes. 
     
     
       9. A marine streamer cable comprising: a plurality of weights affixed to the exterior or said cable for maintaining said cable at a preselected height above the bottom of a body of water and   a plurality of electromagnetic energy transceivers mounted to said cable and disposed so as to contact said body of water.   
     
     
       10. The marine streamer cable of claim 9, further comprising, depth measuring means for ascertaining the depth at which said cable is operating above the bottom of said body of water. 
     
     
       11. The marine streamer cable of claim 9, further comprising: a plurality of acoustic energy detectors mounted to said cable.   
     
     
       12. The marine streamer cable of claim 9, wherein said electromagnetic energy transceivers are adapted to detect electromagnetic energy. 
     
     
       13. The marine streamer cable of claim 9, wherein said electromagnetic energy transceivers are adapted to transmit electromagnetic energy downwardly into the formations below said body of water.