Electromagnetic Mapping of Electrical Conductivity Beneath the Columbia Basalts

Journal Article · Thu Jan 02 04:00:00 EST 1992 · Geophysical Prospecting

OSTI ID:822967

Morrison, H Frank ^[1]; Shoham, Yoram; Hoversten, G Michael; Torres-Verdin, Carlos

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1. LBNL Library

Sedimentary rocks beneath the Columbia River Basalt Group are recognized as having potential for oil and gas production, but the overlying layered basalts effectively mask seismic reflections from the underlying sediments. Four electromagnetic (EM) methods have been applied on profiles crossing Boylston Ridge, a typical east-west trending anticline of the Yakima Fold Belt, in an attempt to map the resistivity interface between the basalts and the sediments and to map variations in structure and resistivity within the sediments. The EM surveys detected strong variations in resistivity within the basalts, and in particular the continuous magnetotelluric array profiling (EMAP) revealed resistivity lows beneath the surface anticlines. These low resistivity zones probably coincide with fracturing in the core of the anticlines and they appear to correlate well with similar zones of low seismic velocity observed on a nearby seismic profile. The controlled-source EM surveys (in-loop transient, long-offset transient, and variable-offset frequency-domain) were designed in anticipation of relatively uniform high resistivity basalts, and were found to have been seriously distorted by the intrabasalt conductors discovered in the field. In particular, the resistivity sections derived from 1D inversions were found to be inconsistent and misleading. The EMAP survey provided the most information about the subsurface resistivity distribution, and was certainly the most cost-effective. However, both controlled-source and EMAP surveys call for accurate 2D or 3D inversion to accommodate the geological objectives of this project. [References: 18]

• Research Organization: Ernest Orlando Lawrence Berkeley National Laboratory, Berkeley, CA (US)
• Sponsoring Organization: USDOE Director, Office of Science (US)
• DOE Contract Number: AC03-76SF00098
• OSTI ID: 822967
• Report Number(s): LBNL--54735
• Journal Information: Geophysical Prospecting, Journal Name: Geophysical Prospecting Journal Issue: 6 Vol. 44; ISSN GPPRAR; ISSN 0016-8025
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
ANTICLINES
ARRAY PROFILING EMAP WASHINGTON SOUNDINGS PLATEAU BASIN EARTH SCIENCES
BASALT
COLUMBIA RIVER
DISTRIBUTION
ELECTRIC CONDUCTIVITY
FRACTURING
PRODUCTION
SEDIMENTARY ROCKS
SEDIMENTS
VELOCITY