ELECTROMAGNETIC MAPPING OF ELECTRICAL-CONDUCTIVITY BENEATH THE COLUMBIA BASALTS

Sedimentary rocks beneath the Columbia River Basalt Group are recogniz ed as having potential for oil and gas production, but the overlying l ayered basalts effectively mask seismic reflections from the underlyin g sediments. Four electromagnetic (EM) methods have been applied on pr ofiles crossing Boylston Ridge, a typical east-west trending anticline of the Yakima Fold Belt, in an attempt to map the resistivity interfa ce between the basalts and the sediments and to map variations in stru cture and resistivity within the sediments. The EM surveys detected st rong variations in resistivity within the basalts, and in particular t he continuous magnetotelluric array profiling (EMAP) revealed resistiv ity lows beneath the surface anticlines. These low resistivity zones p robably coincide with fracturing in the core of the anticlines and the y 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 freque ncy-domain) were designed in anticipation of relatively uniform high r esistivity basalts, and were found to have been seriously distorted by the intrabasalt conductors discovered in the field. In particular, th e resistivity sections derived from 1D inversions were found to be inc onsistent and misleading. The EMAP survey provided the most informatio n 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 ob jectives of this project.