Combined Gravity and Seismic Measurements for Mapping a Buried Tectonic Valley in Western Sweden

Combined seismic and gravity measurements have been used to map an old buried tectonic valley close to the river Klarälven in western Sweden. A gravity low of around 4 mGal is observed along the valley. Interpretation of the gravity data and seismic refraction/wide-angle reflection data indicates that the depth to bedrock exceeds 150 m in the valley. The river Klarälven previously followed the tectonic valley throughout its length, however, at the end of the latest glacial period the river was diverted to a much wider valley. The observation of such large depths to bedrock should have implications for the interpretation of the Quaternary history of the area. The seismic measurements do not indicate that permeable sediments such as coarse-grained sand or gravel occur near the surface in the valley. However, the possibility that such sediments are present at larger depths should justify further investigations for purposes of groundwater extraction and/or for geothermal applications. To conclude, we argue that the combination of gravity and seismic measurements has been a cost-effective method for mapping the buried valley.