3D Resistivity Model (MT) of Reykjanes High Temperature Field in SW Iceland

Abstract

The resistivity structure of The Reykjanes high-temperature system is presented, derived from a three dimensional (3D) model developed in 2016. The model is the final results of a 3D inversion of MT data, corrected for static shift by joint inversion with TEM soundings at the same location as the MT soundings. In the model presented here, a total of 136 sounding pairs are used for the inversion. The model covers Reykjanes and Sandvík geothermal areas. The resistivity structure of the former high enthalpy system reflects dominantly the thermal alteration of the rock. The Reykjanes model reveals a conventional resistivity structure for a high enthalpy geothermal system, i.e. a low resistivity cap underlain by a high resistivity core. The low resistivity cap reaches surface in the steaming ground around the Gunnuhver steam crater, and dips down in all directions forming an elongated area of 2.5 km x 3 km striking in a north-easterly direction down to 1 km depth bounded by the low resistivity. The resistivity of the low resistivity cap is less than 3 Ωm and is underlain by a higher resistivity core of 10–30 Ωm down to 3 km depth. At greater depth, still higher resistivity anomalies become prominent there apparently cut by NE-SW elongated lower resistivity structure below the central anomaly. This structure coincides with an aseismic zone at depth and may indicate a fracture zone with higher temperature and/or permeability within the geothermal system. Comparison with data from the IDDP-2 drilling as well as seismic monitoring confirms that suggestion. Hydrothermal alteration follows closely the shape of the resitivity anomalies, with smectite-zeolites and mixed layer zones being synonymous with the low-reistivity cap and then the increasing resistivity as chlorite, epidote and amphibole alteration is reached. The increasing resistivity to greater depths conforms also to the decreasing hydrous alteration, increasing proportion of low-porosity and less intensely altered intrusive rocks. The low resistivity anomaly cutting through that conforms to a zone of better permeability and hydrous content.