Urban underwater ERT for site investigation in lake Mälaren, Sweden

An underwater ERT survey was made along a planned road tunnel for The Stockholm Bypass Project (Forbifart Stockholm). The aim was to assess the ability to identify variations in the depth of the bottom sediments, as well as variations in rock quality and the possible presence of weak zones in the rock. Reference data from a seismic survey and geotechnical drilling was used for evaluation the result. An underwater layout of 121 underwater and 37 land based electrodes at a separation of 5 m was used. Repeated measurements were made to assess the temporal variation in urban noise, which lead to doing the actual survey during the quiet night hours. The resistivity data are of good quality and the inversion results in low residuals. The invested section shows a low resistivity layer corresponding to sediments on top of bedrock with varying resistivity, where the interpreted depth to bedrock is in good accordance with the reference data. A major low resistive zone corresponds well with poor quality rock according to drilling and a low velocity zone.