Marine magnetotellurics for petroleum exploration. Part 1: A sea-floor equipment system
- Scripps Institution of Oceanography, La Jolla, CA (United States)
- Arnold Orange Associates, Austin, TX (United States)
- Lawrence Berkeley National Lab., CA (United States)
Induction in electrically conductive seawater attenuates the magnetotelluric (MT) fields and, coupled with a minimum around 1 Hz in the natural magnetic field spectrum, leads to a dramatic loss of electric and magnetic field power on the sea floor at periods shorter than 1,000 s. For this reason the marine MT method traditionally has been used only at periods of 10{sup 3} to 10{sup 5} s to probe deep mantle structure; rarely does a sea-floor MT response extend to a 100-s period. To be useful for mapping continental shelf structure at depths relevant to petroleum exploration, however, MT measurements need to be made at periods between 1 and 1,000 s. This can be accomplished using ac-coupled sensors, induction coils for the magnetic field, and an electric field amplifier developed for maine controlled-source applications. The electrically quiet sea floor allows the attenuated electric field to be amplified greatly before recording; in deep (1-km) water, motional noise in magnetic field sensors appears not to be a problem. In shallower water, motional noise does degrade the magnetic measurement, but sea-floor magnetic records can be replaced by land recordings, producing an effective sea-surface MT response. Field trials of such equipment in 1-km-deep water produced good-quality MT responses at periods of 3 to 1,000 s; in shallower water, responses to a few hertz can be obtained. Using an autonomous sea-floor data logger developed at Scripps Institution of Oceanography, marine surveys of 50 to 100 sites are feasible.
- Sponsoring Organization:
- USDOE, Washington, DC (United States)
- DOE Contract Number:
- AC03-76SF00098
- OSTI ID:
- 619621
- 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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