The Björkö geothermal energy project
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Impact crater formation results in changes in the physical properties of rocks that can be geophysically determined and therefore are of importance for the mapping structures at depth. Especially electrical properties have direct bearing on the remaining porosity found in impact structures in crystalline target rocks. Several studies made in the Dellen, Siljan and Lockne craters show typical changes in rock physica properties. Although a significantly increased porosity in the crater bbasement seems to occur, the hydraulic conductivity appears to be rather small. The total volume of brecciated rocks in impact structures may be very large and that volume could, after reactivitation of the fractures, be used as a heat exchange structure for geothermal energy retrieval, exploiting the normal geothermal gradient at greater dephts.\rThe Bj\u00F6rk\u00F6 structures is a c. 10 km-diameter impact crater located just west of Stockholm in lake M\u00E4laren. Studies made so far on islands show the characteristic features of intense brecciation and increased porosity known from other craters. The Bj\u00F6rk\u00F6 Energy Project is designed to assess the potential for geothermal energy retrieval by mapping the structure at depth with geophysical methods and by drilling. The project is financed by the Swedish National Energy Administration and runs for 2 years, starting in October 2000.