Electromagnetic induction in geothermal fields and volcanic belts

This review covers electromagnetic studies in geothermal and volcanic regions presented in the literature since 1983. It has been arranged by geographical areas, emphasizing where possible the data gathering, the interpretation techniques and the results of each study. The main conclusions of this review are: In all the surveys, people are measuring the complete MT impedance tensor. However, in general, this information is not being used in the interpretation mainly because of the poor quality of the data. This unfortunate situation originates by the presence of strong noise in the surveyed area and generally, by the lack of use of the remote reference technique. Crews with equipment and techniques that can gather data of very good quality, generally perform very detailed interpretations using most of the gathered information. Other groups that collect noisy data oversimplify the interpretation by using only one mode or averaging the resistivity of both modes and interpreting the results using simplified 1-D interpretations. At the interpretation stage, most of the mid-crustal conductors identified are being associated to the presence of trapped water of magmatic origin. In general, magma chambers are not being detected, probably because either they are absent or because there is a lack or resolution of the electromagnetic methods to detect them.