1D joint inversion of TEM and MT resistivity data, with an application of soundings from the Námafjall high-temperature geothermal area, NE-Iceland

Abstract

Geophysical methods are most useful in extracting subsurface information. Which geophysical method should be used to characterize a site depends on what information one needs. Resistivity has a direct relationship with the subsurface rock temperature. TEM and MT are very cost effective methods in a subsurface resistivity study. Although the MT method is very efficient at getting information down to the mantle, it suffers a static shift problem. Joint interpretation of TEM and MT data removes the static shift from MT data. Geologically, Námafjall is a very important area in Iceland. Tectonically it is related to Krafla volcano. It was formed in subglacial eruptions during the last glaciation and has undergone cooling. In this study sixteen TEM and MT soundings were used to evaluate the subsurface resistivity. At shallow depth there is a low-resistivity layer having resistivity less than 10 Ωm below Námafjall ridge. On the surface this area coincides with surface geothermal manifestations. It signifies the presence of low-temperature alteration minerals, mainly zeolites and smectite. Resistivity values above this low-resistivity cap indicate the presence of unaltered fresh rock. Below 200 m b.s.l., high resistivity values below the low resistivity signify high-temperature alteration minerals, mostly chlorite and epidote. At about 800 m depth, there is again a low-resistivity layer at the northernmost site, indicative of a fracture zone which might be connected to the Krafla volcano. A low-resistivity zone with resistivity of less than 5 Ωm is also found from 2 km to 10 km b.s.l. This low-resistivity zone signifies a probable heat source for Námafjall area. Electrical strike analysis and Tipper study suggest a conductive zone at less than 1 km depth and the presence of fractures at different depths that are not always parallel to the major geological structures.