RESISTIVITY METHODS IN GEOTHERMAL PROSPECTING IN ICELAND

Resistivity techniques have been used successfully to identify and del ineate geothermal resources in Iceland. The most frequently used techn iques include Schlumberger, central loop TEM and head-on profiling. Ge othermal systems in Iceland are located both within and outsite the ac tive volcanic regions. Outsite the active volcanic regions the tempera ture in the upper most kilometer of the geothermal systems is below 15 0-degrees-C whereas the temperature in the geothermal fields within th e active volcanic regions exceeds 200-degrees-C. The resistivity of th e rock in geothermal fields located outside the active volcanic region s ranges from about 10 OMEGAm to some hundreds of OMEGAm, and are char acterized by considerably lower resistivity than of the surrounding ro cks. Most of the geothermal systems within the active volcanic regions , show common resistivity structure with low resistivity of 1-5 OMEGAm surrounding an inner core of higher resistivity. This increasing resi stivity with depth is associated with a change in the conduction mecha nism, from interface conduction to electrolyte conduction due to a cha nge in alteration minerals at about 240-degrees-C. Examples of resisti vity surveys of geothermal fields from both outsite and within the act ive volcanic regions are discussed.