Electromagnetic mapping of electrical conductivity beneath the Columbia basalts
- Lawrence Berkeley Lab., CA (United States)
- Shell Development Corp., Houston, TX (United States)
- Schlumberger-Doll Research, Ridgefield, CT (United States)
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.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 438987
- Report Number(s):
- CONF-9206464-; ISSN 0016-8025; TRN: IM9711%%451
- Journal Information:
- Geophysical Prospecting, Vol. 44, Issue 6; Conference: 54. European Association of Exploration Geophysicists (EAEG) meeting, Paris (France), Jun 1992; Other Information: PBD: Nov 1996
- Country of Publication:
- United States
- Language:
- English
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