Transient electromagnetic and magnetotelluric geophysical methods in the Hengill area, SW-Iceland

Abstract

Geophysical methods, particularly electrical resistivity methods, have been the most powerful tools in geothermal exploration for decades. Of these, TEM and MT methods are important electrical methods in outlining geothermal resources. TEM soundings have proven to be more downward focussed than the traditional DC-soundings and have a better resolution at depth. The resistivity structures of high-temperature geothermal systems in volcanic areas are generally characterized by a high-resistivity core below a low-resistivity cap. In this project, existing TEM and MT data from the Hellisheidi geothermal field of the Hengill volcanic area were interpreted. An interpretation program called TEMTD was used to perform 1D inversion on the TEM and MT data and the results obtained used to make iso-resistivity maps and cross-sections to describe the resistivity distribution. The results from TEM show that there exists a high-resistivity core below a low-resistivity cap from about 500 m b.s.l. down to about 900 m b.s.l., and at as shallow a depth as ~ 100 m b.s.l. along the fissure swarm. The low-resistivity cap extends up to sea level and in the northern part of the study area, up to 100 m a.s.l. and even to the surface at some points. Moreover, results from MT data show that there is a low-resistivity structure at depth, at about 5000 m b.s.l. down to about 10,000 m b.s.l. that is interpreted as a heat source. The results from these resistivity surveys indicate that this area is potentially a good high-temperature area.