Comparison of borehole and surface resistivity at Theistareykir, NE-Iceland

Abstract

Resistivity logs from five boreholes in the Theistareykir high-temperature geothermal field and resistivity from surface exploration are compared in this report. The resistivity is also compared to the first appearance of alteration minerals. The available resistivity logs from each section of drilling are combined for each well and inserted into Petrel, a 3D modelling software, for viewing geological and geophysical data. A 3D model of the resistivity which had previously been found by 3D inversion of MT soundings from field work performed by ÍSOR had previously been inserted into Petrel. The resistivity well logs are averaged to the same resolution as the TEM and MT model. The previous study of TEM and MT resistivity from Theistareykir shows a conventional resistivity structure for a high-temperature geothermal system in Iceland, consisting of a low resistivity cap underlain by a highresistivity core in the uppermost 2 km and a deep seated low-resistivity layer. The resistivity in boreholes and resistivity from the inversion of TEM and MT have slight differences but, in general, show a very good correlation in all of the studied wells. The study also confirmed what had previously been shown that lower resistivity was linked with smectite-zeolite zonation and the higher resistivity with chlorite and epidote-amphibole deeper down in the wells, which show alteration with increasing temperature, respectively.