A NUMERICAL EVALUATION OF ELECTROMAGNETIC METHODS IN GEOTHERMAL-EXPLORATION

The size and low resistivity of the clay cap associated with a geother mal system create a target well suited for electromagnetic (EM) method s and also make electrical detection of the underlying geothermal rese rvoir a challenge. Using 3-D numerical models, we evaluate four EM tec hniques for use in geothermal exploration: magnetotellurics (MT), cont rolled-source audio magnetotellurics (CSAMT), long-offset time-domain EM (LOTEM), and short-offset time-domain EM (TEM). Our results show th at all of these techniques can delineate the clay cap, but none can be said to unequivocally detect the reservoir. We 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 c harge 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 wher e only the magnetic field is measured. The anomaly produced by boundar y 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 e fficient multidimensional tools are developed for LOTEM interpretation . Although CSAMT employs electric field measurements, this method is n ot recommended for reservoir detection because the anomaly caused by a deep reservoir is obscured by transmitter effects that cannot be isol ated reliably. A combination of CSAMT and TEM measurements appears mos t appropriate for delineation of the clay cap.