River-crossing Resistivity Survey for Bridge Foundation Design - A Case-study from Skarnes, Norway

An ERT survey was performed in 2015 across the Oppstadåa river in Norway to map the bedrock topography for the design of a bridge foundation. Two pre-existing boreholes, 150 m apart, had shown that the bedrock is 20 m lower on one side of the river than on the other. The aim of the ERT investigation was to reveal the bedrock topography between these two boreholes and across the river. Five ERT profiles were acquired, four across and one along the river. The river is quite narrow and some of electrodes could be deployed on the ground surface while others were deployed along the water surface. Floating electrodes are usually preferred over sea-bottom electrodes in the case of shallow water depth because of better efficiency and lower cost of fieldwork. Although the investigation was performed in a complex geological area and challenging site, the data quality is good and inversion results reliable. The interpretation agrees with previous geotechnical soundings, it provides a seamless depth to bedrock surface and it also gives a rough characterization of the soils.