A numerical evaluation of electromagnetic methods in geothermal exploration
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文摘
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, four EM techniques are evaluated 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). The results show that all of these techniques can delineate the clay cap, but none can be said to unequivocally direct the reservoir. However, 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 of 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.