Magnetotelluric Method and Instrumentation for Geothermal Prospecting

Abstract

Aquifers are often characterized by low electrical resistivity due to conductive fluid in the interconnected pore. In geothermal regions, this resistivity is usually even lower because the ions of dissolved minerals in the hot water possess higher mobility which increases the conductivity of the solution. Most of the inductive electromagnetic methods of geophysics reliably resolve high conductive bodies or layers in the subsurface. Among other, the magnetotelluric (MT) one has several advantages. Since it utilizes the natural geomagnetic field variations as a source it is basically nondestructive for the environment. Different types of instrument allow us to record the variations in a wide frequency (period) band which directly relates to different depth of interest. Also with the recent progress of instrumentation parameters, and modern approaches to data processing and interpretation the application of MT method became very useful and cost-effective for geothermal study. It is further stressed that the successful application of the MT method greatly depends on the field instrumentation parameters. The wideband MT station (Audio MT or AMT) can 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 popular parameters, recent upgrade and further perspective of them for modern wideband AMT instruments are presented, compared and discussed. The modern requirements followed from the recent practice to the MT results postulate the application of remote reference technique to the data processing and simultaneous using as a minimum of two MT instruments respectively. The details of such processing procedures with corresponding examples are discussed in our report also.