Magnetotelluric Method and Instrumentation for Geothermal Prospecting

Aquifers are usually characterized by low electrical resistivity due to conductive fluid presence in the interconnected pores. In geothermal regions this resistivity is still lower because the ions of dissolved minerals in the hot water possess higher mobility which increases the conductivity of solution. All inductive electromagnetic (EM) methods of geophysics reliably resolve highly conductive bodies or layers in the subsurface. Among other, the magnetotelluric (MT) one has several advantages. The most important one is that it is basically non destructive for environment, since it utilize the natural geomagnetic field variations as a source. MT allows recording of the EM field variations in a wide frequency (periods) band what directly relates to different depth of exploration. To this, with recent progress in instrumentation parameters and modern approaches development to data processing and interpretation the application of MT method became very useful and cost efficient for geothermal study. It has to be stressed that the successful application of MT method greatly depends on the used instrumentation parameters. The wide band MT station (audio MT or AMT) may be considered as the most suitable instrument for the geothermal research. The frequency band from 0.001 to 10000 s allow us to study the subsurface conductivity distribution from a few hundred (sometimes even from a few tens) of meters to tens of kilometers. The most important parameters of corresponding MT instruments, their recent upgrade in modern wide band AMT instruments are presented and compared. It is necessary also to mention that increasing data processing quality postulates, following from the recent practice of the MT results, the application of remote reference technique with simultaneous using as a minimum of two synchronously operating MT instruments. The details of such processing procedures with corresponding examples are discussed in our report also.