Magnetotelluric and transient electromagnetic methods in geothermal exploration with examples from the Krýsuvík area, SW-Iceland

Abstract

Geophysics plays a great role in subsurface exploration for geothermal resources. Several geophysical exploration methods are applicable, divided into direct and indirect (structural) methods. One of the direct geophysical methods for geothermal exploration is the resistivity method. Electrical resistivity methods like magnetotellurics (MT) and transient electromagnetics (TEM) are powerful tools in mapping subsurface conductivity variations. In this report the main geophysical methods are discussed in general while electromagnetic methods are discussed in detail. MT and TEM data collected along a profile from Krýsuvík hightemperature geothermal area were processed and interpreted and results are presented as resistivity cross-sections. The MT soundings on the profile were aligned in a N-S direction and the corresponding nearby TEM data were analysed for each MT station. 1-D inversion for the TEM data and 1-D joint inversion of MT and TEM data were performed along the profile. The results are presented as resistivity cross-sections. The uppermost layer was found to be resistive, unaltered volcanic rocks while the second corresponds to the conductive smectite-zeolite and mixed layer clay zone. Below these, a relatively resistive layer, corresponding to the chlorite-epidote zone was clearly mapped. The correlation between resistivity values and alteration mineralogy and temperature in wells is also discussed.