Tem-Resistivity Image of a Geothermal Field in N-Iceland and the Relation of the Resistivity with Lithology and Temperature

Abstract

In Eyjafjordur in N-Iceland several small hot springs were found. Drilling at the various hot springs has shown that the geothermal activity is mainly connected to fractures in the basaltic pile of very low permeability but there are also indications of some near horizontal flow. TEM resistivity soundings have been used to create a resistivity image of the basaltic pile in the area to improve the older resistivity picture obtained by Schlumberger soundings. The TEM soundings were initially inverted to give a layered resistivity model without considering any other geological knowledge. This was done to avoid biasing during interpretation. The result shows several layers of high and low resistivity with southerly dip. This pattern was not resolved in the previous Schlumberger soundings. An additional local low-resistivity zone is observed. Comparison of the TEM result with borehole- and surface data shows that the resistivity layering coincides with the lithological layering. The low resistivity layers coincide with series of 20-30m thick phorphyritic tholeiitic lavas interbedded with several metres thick sedimentary layers and also with series of thick olivine tholeiitic lava. The higher resistivity coincides with series of compound basaltic lavas. The low resistivity anomaly coincides with a temperature anomaly which is interpreted as the main upflow zone of the geothermal activity. After the interpretation in terms of lithology, the borehole data were used to fix the thickness of each lithological unit to give more exact information on the resistivity of that individual layer. Since the layers dip aproximately 6? dip, each resistivity layer descends progressively towards south and the temperature therefore increases. Correlation between the resistivity and the temperature is in agreement with relations between these parameters proposed by Flovenz et al. 1985. Feedzones in boreholes and geohydrological observations show that the series of the thick phorphyritic tholeiites have higher permeability than the surrounding lava pile. In the boreholes the hot water feed zones tend to be within this formations and cold springs are observed where the same formation emerges in the nearby mountain sides. A model, where the local resistivity anomaly denotes the main upflow zone, is proposed. The geothermal fluid migrates laterally away from the upflow zone along thick tholeiitic layers, from where it ascends to the surface in hot springs along some of the numerous dykes and faults in the area and their intersection.