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Structure Mapping at Trap Spring Oilfield, Nevada, Using Controlled-source Magnetotellurics

This article gives a particularly good description of the CSAMT method, a type of CSEM surveying in which the transmitter is placed far field from the receiver. At the time of this article, this field practice was necessary to simplify the mathematics involved in modeling the data but is no longer necessary with modern inversion software. Thus the method predominantly known as “CSEM” by the petroleum industry is referred to as “CSAMT” in early research and today the terms are interchangeable.

The advantages of using CSEM geophysics exploration techniques for oil-field reconnaissance revealed in this Zonge case study are as relevant today as they were in the 1980s. It was then that Zonge first began recommending an integrated, stepped methodology combining EM with seismic investigations.

Scott Urquhart, president and managing geophysicist, Zonge International

Publisher –
48th  Meeting of the European Association of Exploration Geophysicists 1986; First Break, 1987; Vol. 5; pp. 403-418.

Authors –
Norman R. Carlson*, Larry J. Hughes, Zonge Engineering & Research Organization, Inc., Tucson, Arizona, U.S.A.

Paper –  [pdf]  O&G_CSAMT_Oilfield-FirstBreak1987

The seismic reflection method has been a highly successful tool in oil and gas exploration and presently accounts for about 98% of all geo-physical expenditures worldwide. However, the relatively high cost of seismic exploration and its limitations in certain geologic environments are continuing problems. The controlled-source audio-frequency magnetotellurics (CSAMT) technique is a shallower-penetrating variation of MT which uses an artificial signal source and provides fast, 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; and (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 polari-zation 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 recon-naissance mapping to develop seismic prospects, at approximately one sixth the cost of seismic.