Restistivity surveying and electromagnetic methods

The task involved in geothermal exploration is the detection and delineation of geothermal resources and the understanding of their characteristics, the location of exploitable reservoirs, and the siting of boreholes through which hot fluids at depth can be extracted. Geological and geochemical mapping are usually limited to direct observations on the surface and conclusions and extrapolation that can be drawn about the system and possible underlying structures. Geophysical surface exploration methods are different. They utilize equipment that measures directly some physical parameters on the surface that are directly created by physical properties or processes at depth. Geophysical exploration is a young scientific discipline that developed slowly in the first half of the twentieth Century. Geophysical exploration methods can be classified into several groups like seismic methods, electrical resistivity methods, potential methods (gravity and magnetics), heat flow measurements, and surface deformation measurements. Resistivity methods are the most important geophysical methods in geothermal exploration. The reason is that the resistivity is highly sensitive to temperature and geothermal alteration processes and is directly related to parameters characterizing the reservoir. Here, a review will be given on resistivity surveying and electromagnetic methods; the transient electromagnetic (TEM) and magnetotelluric (MT) method as well as the DC method: Schlumberger soundings.