Marine magnetotellurics for petroleum exploration, Part 1: Asea-floor equipment system
Induction in electrically conductive seawater attenuates themagnetotelluric (MT) fields and, coupled with a minimum around 1 Hz inthe natural magnetic field spectrum, leads to a dramatic loss of electricand 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 atperiods of 10(3) to 10(5) s to probe deep mantle structure; rarely does asea-floor MT response extend to a 100-s period. To be useful for mappingcontinental shelf structure at depths relevant to petroleum exploration,however, MT measurements need to be made at periods between 1 and 1000 s.This can be accomplished using ac-coupled sensors, induction coils forthe magnetic field, and an electric field amplifier developed for marinecontrolled-source applications. The electrically quiet sea floor allowsthe attenuated electric field to be amplified greatly before recording;in deep (l-km) water, motional noise in magnetic field sensors appearsnot to be a problem. In shallower water, motional noise does degrade themagnetic measurement, but sea-floor magnetic records can be replaced byland recordings, producing an effective sea-surface MT response. Fieldtrials of such equipment in l-km-deep water produced good-quality MTresponses at periods of 3 to 1000 s: in shallower water, responses to afew hertz can be obtained. Using an autonomous sea-floor data loggerdeveloped at Scripps Institution of Oceanography, marine surveys of 50 to100 sites are feasible.
- Research Organization:
- COLLABORATION - Scripps Institution ofOceanography
- DOE Contract Number:
- AC02-05CH11231
- OSTI ID:
- 902136
- Report Number(s):
- LBNL--45071; Accession #: ZP843-0004
- Journal Information:
- Geophysics, Journal Name: Geophysics Journal Issue: 3 Vol. 63; ISSN GPYSA7; ISSN 0016-8033
- Country of Publication:
- United States
- Language:
- English
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