MARINE MAGNETOTELLURICS FOR PETROLEUM-EXPLORATION PART I - A SEA-FLOOR EQUIPMENT SYSTEM

Induction in electrically conductive seawater attenuates the magnetote lluric (MT) fields and, coupled with a minimum around 1 Hz in the natu ral magnetic field spectrum, leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1000 s, For this reason the marine MT method traditionally has been used only at periods of 10(3) to 10(5) s to probe deep mantle structure; rarely does a sea-floor MT response extend to a 100-s period. To be useful fo r mapping continental shelf structure at depths relevant to petroleum exploration, however, MT measurements need to be made at periods betwe en 1 and 1000 s. This can be accomplished using ac-coupled sensors, in duction coils for the magnetic field, and an electric field amplifier developed for marine controlled-source applications. The electrically quiet sea floor allows the attenuated electric field to be amplified g reatly before recording; in deep (l-km) water, motional noise in magne tic field sensors appears not to be a problem. In shallower water, mot ional noise does degrade the magnetic measurement, but sea-floor magne tic records can be replaced by land recordings, producing an effective sea-surface MT response. Field trials of such equipment in l-km-deep water produced good-quality MT responses at periods of 3 to 1000 s: in shallower water, responses to a few hertz can be obtained. Using an a utonomous sea-floor data logger developed at Scripps Institution of Oc eanography, marine surveys of 50 to 100 sites are feasible.