Structure mapping at Trap Spring Oilfield, Nevada, using controlled-source magnetotellurics

The seismic reflection method has been a highly successful tool in oil and gas exploration for half a century, and it presently accounts for about 98% of all geophysical expenditures world-wide. However, the relatively high cost of seismic exploration and its limitations in certain geologic environments are continuing problems. Some help has been provided by the magnetotelluric (MT) sounding technique, but the cost of MT is also quite high due to the low natural signal strengths being measured.

The controlled-source audio-frequency magnetotellurics (CSAMT) technique is a shallower-penetrating variation of MT which uses an artificial signal source. This permits faster and more economical data acquisition. CSAMT has a penetration of about 2 km in typical petroliferous environments. CSAMT does not replace seismic but functions in three specific roles: (1) as a reconnaissance tool to help focus seismic coverage, or to help avoid 'no-record' zones; (2) to assist in static corrections and in interactive seismic interpretation; (3) as a primary tool in certain environments (volcanics, complex thrust areas) where seismic data acquisition is limited.

An example of the application of CSAMT to structure mapping comes from data taken over Trap Spring Field, located in the frontier Great Basin of the western United States. The field produces oil from fractured volcanics at the edge of a major graben fault. The CSAMT data delineate the major subsurface faulting and stratigraphic relationships in the area. The resolution of the CSAMT survey is significantly better than previously obtained induced polarization (IP) data. Detailed comparisons with electric log, drill hole, and air-photo data show an excellent correlation between the CSAMT features and known geology. The work suggests that CSAMT could be used in this area for reconnaissance mapping to develop seismic prospects, at approximately one sixth the cost of seismic.