| Abstract |
A 3D inversion was performed of static shift corrected off-diagonal impedance tensor elements of 102 MagnetoTelluric (MT) resistivity soundings from the Greater Krýsuvík geothermal area, SW Iceland. In the inversion, 21 periods were used, evenly distributed from 0.01 to 100 s on a logarithmic scale. The robustness of the inversion was tested by using three different initial models: A model compiled from a joint 1D inversion of TEM and MT soundings, a homogeneous earth of resistivity 100 Ωm and a homogeneous earth of resistivity 10 Ωm. The final RMS was around 1.5 for all three models. The resulting models were elevation corrected and are presented here as smoothed resistivity maps at different elevations and as resistivity cross sections. It is striking how the results of the 1D inversion and results of the 3D inversion based on the three initial models are similar. As one would expect, the 1D inversion reproduces the main resistivity structures while the 3D inversion shows considerably more details. The subsurface resistivity structure in Krýsuvík has the same main features as other high temperature areas in Iceland and in general where the host rocks are basaltic. Above a shallow conductive cap, a high resistivity zone is seen, reflecting unaltered rock. The conductive cap reflects smectite-zeolites hydrothermal mineral alteration. Below the low-resistivity cap, a resistive core is found, reflecting chlorite-epidote alteration. If the alteration mineralogy and the rock temperature are in equilibrium, resistivity reveals the rock temperature as well. Good correlation is seen between subsurface resistivity structure and hydrothermal alteration as revealed in cuttings from boreholes. Parts of the field have cooled down and, therefore, the resistivity structure indicates alteration mineralogy but not necessarily present rock temperature. The geothermal up-flow zones are most likely where the hydrothermal alteration and the resistive core reach highest elevation. The inversion shows an indication of a relatively deep seated conductive body below the central part of the high temperature area. This body coincides horizontally approximately with an inflation source at an estimated depth of 4–5 km suggested by GPS recordings and InSAR data. The deformation was accompanied by a seismic swarm. There are no signs of S-wave attenuation and therefore, it is not likely that the body is composed of partially molten material. It has been suggested that the inflation is due to emission of gas. The deep seated conductive body is probably connected to the heat source of the geothermal field. An analysis of the electrical strike directions based on the Tipper as well as the induction arrows are in a fairly good agreement with the final resistivity model and support it. |