A numerical evaluation of electromagnetic methods in geothermal exploration
- Geological Survey, Denver, CO (United States)
- ABnormal Geophysics, Golden, CO (United States)
- Univ. of Utah, Salt Lake City, UT (United States). Dept. of Geology and Geophysics
The size and low resistivity of the clay cap associated with a geothermal system create a target well suited for electromagnetic (EM) methods and also make electrical detection of the underlying geothermal reservoir a challenge. Using 3-D numerical models, the authors evaluate four EM techniques for use in geothermal exploration: magnetotellurics (MT), controlled-source audio magnetotellurics (CSAMT), long-offset time-domain EM (LOTEM), and short-offset time-domain EM (TEM). Their results show that all of these techniques can delineate the clay cap, but non can be said to unequivocally detect the reservoir. They do find, however, that the EM anomaly from a deep, conductive reservoir overlain by a larger, more conductive clay cap is caused by the presence of the electric charge at conductivity boundaries rather than electromagnetic induction. This means that, for detection of the reservoir, methods such as MT, which rely on electric field measurements, are superior to those where only the magnetic field is measured. The anomaly produced by boundary charges at the reservoir is subtle and will be evident only if high-quality data are collected at closely spaced measurement sites. LOTEM electric field measurements look promising and should be useful when efficient multidimensional tools are developed for LOTEM interpretation. Although CSAMT employs electric field measurements, this method is not recommended for reservoir detection because the anomaly caused by a deep reservoir is obscured by transmitter effects that cannot be isolated reliably. A combination of CSAMT and TEM measurements appears most appropriate for delineation of the clay cap.
- OSTI ID:
- 207838
- Journal Information:
- Geophysics, Journal Name: Geophysics Journal Issue: 1 Vol. 61; ISSN GPYSA7; ISSN 0016-8033
- Country of Publication:
- United States
- Language:
- English
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