APPLICATION OF THE RESISTIVITY METHOD IN THE KRÍSUVÍK GEOTHERMAL AREA, REYKJANES PENINSULA, SW-ICELAND

This report describes resistivity measurements made in the Krisuvik geothermal area, Reykjanes Peninsula, SW-Iceland. The report includes the theoretical aspects of the resistivity methods in geothermal exploration and interpretation and comparison of sounding results, based on DC methods (Schlumberger configuration) and EM methods (Central-loop transient electromagnetic - TEM) are made and presented. The resistivity structure of the Krisuvik geothermal area is observed to reveal several resistivity layers, including high resistivity ($200 Sm) near the surface that is related to cold and fresh basaltic rock of Postglacial volcanism, moderately high resistivity (50-200 Sm) related to slightly altered rocks due to groundwater action, moderate resistivity (10-50 Sm) related to moderately hydrothermally altered rocks, and low resistivity (#10 Sm) related to low-temperature (50-200/C) alterations in the smectitezeolite zone. The typical resistivity structure that is observed in most hightemperature geothermal fields of Iceland (e.g. Krafla and Nesjavellir), with high resistivity below a low-resistivity cap rock, is also seen in the Krisuvik geothermal area. This resistivity structure is interpreted as a result of the chlorite alteration zone that is of temperatures higher than 240/C. Comparisons of results of the two methods show that a similar resistivity structure could be obtained using both methods but with slight discrepancies that arise basically from their principles and methodology. Experience in resistivity mapping of geothermal fields of Iceland and this study indicate that the resistivity structure of geothermal prospects is best mapped using the TEM method, especially in areas with dry and highly resistive surface conditions, like volcanic lava, combined with lowresistivity formations at deeper levels, which is common for geothermal fields.