GEOPHYSICAL ASPECTS OF METEORITE IMPACT CRATERS IN ERODED SHIELD ENVIRONMENT, WITH SPECIAL EMPHASIS ON ELECTRIC RESISTIVITY

The traditional geophysical methods of magnetics and gravity, as applied to meteorite impact crater studies are shortly reviewed. Their optimal resolution power with respect to crater size and pre-existing structures is discussed. The morphology of some typical impact craters is illustrated to show the character of the post-impact deformation. A new geophysical method in impact crater studies, the in situ measurement of electrical resistivity, is introduced and illustrated with case studies from simple and complex craters. It is concluded that the resistivity method represents a useful tool for mapping the extent of authigenic brecciation in crater structures and their surroundings. The method constitutes, in combination with detailed elevation data, gravity and magnetic measurements, a powerful tool for constraining the classification of impact craters in crystalline shield environments. The resistivity measurements have revealed a complex brecciation pattern in large central uplift structures consisting of a central, less brecciated, core surrounded by a ring of more intense brecciation. Typical impact brecciated crystalline rocks have resistivities between 2000 and 8000 ohm m. The crater structures treated in this paper are: Bjorko, Dellen, Siljan and Uppland, all located in Sweden.