3D MT Characterization of Two Geothermal Fields in Iceland

Abstract

Three-dimensional (3D) magnetotelluric (MT) inversions leading to the characterization of the electrical structure of geothermal reservoirs in a single self-consistent manner and presumably optimal accuracy and resolution are now feasible. Our work focused on two large geothermal fields – the Hengill and Krafla volcanic complexes, 200 km apart, and both known as high-temperature systems located within neo-volcanic zones of Iceland. This is the first full 3D MT inversion of Krafla MT dataset. The inverted model of electrical resistivity reveals the presence of highly resistive near surface layer, identified as unaltered porous basalt, which covers a low resistivity cap corresponding to the smectite-zeolite zone. Below this cap a more resistive zone is identified as the epidote-chlorite zone or also called the resistive core. Resistivity in the upper 1-2 km does not to correlate with lithology but with alteration mineralogy. At the site of the IDDP well, which encountered magma at 2.1 km depth, the resistivity image shows high resistivity most likely due to epidote-chlorite geology. Just to the northwest of the well, however, an intrusive electrically conductive feature has been imaged rising from depth, and has been interpreted as a magma reservoir. A possible explanation for the magma encounter at the IDDP well is the existence of pathways or fissures connecting the magma chamber to the well. The MT response to magma pathways is not to be discernible in the data. Hengill geothermal area can be divided into two major complexes, one in the southwest and one in the northeast. The inverted model identified two low-resistivity layers. The nature of the uppermost low-resistivity layer and the increasing resistivity below is due to hydrothermal mineral alteration while the nature of the deep low-resistivity layer is not yet well understood. 3D MT inversions of Krafla and Hengill data sets showed that this approach is very promising in imaging geothermal reservoirs and that knowledge of the subsurface electrical resistivity can contribute to a better understanding of complex geothermal systems.