Interpretation of resistivity soundings in the Krýsuvík high-temperature geothermal area, SW-Iceland, using joint inversion of TEM and MT data

Abstract

The resistivity structure of the Krýsuvík high-temperature geothermal field, SW-Iceland, has been outlined by using a joint inversion of Transient Electromagnetic (TEM) and Magnetotelluric (MT) data. The TEM method delineates resistivity structures at a depth ranging from tens of metres down to almost 1,000 m whereas MT signals delineate resistivity structures down to a depth of several tens of kilometres. Joint inversion of TEM and MT data is useful in correcting for the static shift of MT data, which can cause large errors in the interpretation. Both the TEM and MT data reveal a low-resistivity cone shaped zone. The low resistivity is closest to the surface in the fracture zone. Below the conductive zone, there is a resistive core. The MT and TEM electrical methods resolve resistivity layers with the resistivity differences mainly based on alteration. The layers consist of different clay minerals e.g. smectite and zeolite, mixed-layer clay, chlorite and epidote. The zone where the smectite-zeolite clay minerals are dominant is located above the mixed-layer clay zone. The chlorite-epidote zone is located below it and formed at higher temperatures than the clay minerals above. The top layers presumably consist of unaltered volcanic lava flows while the bottom part consists of a core body with resistive high-temperature minerals and intrusive dykes....