Transient electromagnetic resistivity survey at the Geysir geothermal field, S-Iceland

Abstract

A general description of the resistivity method is given, and the results of a TEM resistivity survey at the Geysir geothermal field, S-Iceland are presented. The effects of temperature, porosity, salinity, and water-rock interaction on resistivity of rocks are briefly described. Theoretical aspects of DC resistivity measurements are presented and the general principles and application of Transient Electromagnetic (TEM) soundings are discussed. Comparison of DC and TEM resistivity soundings is dealt with briefly. The famous Geysir high-temperature area is in a shallow valley elongated N-S in the upper southern lowlands of Iceland. A total of 9 TEM stations were interpreted with the one-dimensional inversion program, CLTINV. Results reveal the resistivity distribution of the rock formation in the uppermost kilometre. The resistivity in the uppermost 100-400 m is quite high, ranging between 200 and 3000 ohm-m. Below that, the resistivity is low, and at some level it is obviously influenced by the vicinity of high-temperature geothermal activity, indicated by resistivity values of 1-10 ohm-m. The resistivity structure within the Geysir field shows the characteristics of a geothermal system with very low resistivity at depth, 1-5 ohm-m, around the Geysir area, but without confirmed evidence of high resistivity in the central part of the geothermal system. Provided there is an equilibrium between thermal alteration of the rock and present temperature, the resistivity survey indicates temperatures as high as 200-220°C in the uppermost kilometre of the geothermal system, but not exceeding that, as there is no certain sign of a high-resistivity core. Chemical evidence.....