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.