Underwater ERT Survey for Site Investigation for a New Line for Stockholm Metro

An underwater ERT survey was carried out in central Stockholm as part of pre-investigations for a new line for the Stockholm metro (T-bana). The aim was to identify variations in depth of the bottom sediments, as well as variations in rock quality and the possible presence of weak zones in the rock. The ERT survey was performed with pole-dipole configuration and an electrode cable with 64 electrodes at 7 meter intervals placed on the sea bottom. The water depth was mapped using sonar combined with recording pressure transducers, and water resistivity as function of depth was integrated in the inversion model. The results show that the rather difficult survey conditions could be handled in a satisfactory way thanks to adequate equipment, careful planning and attention to details. The measured data contains information that is relevant for creating coherent models of the variation in depth to rock together with data from drilling. The results also indicate that information in variation in rock quality that can be of critical importance for planning of underground construction can be derived from the data. Further comparisons against reference data are required for a full evaluation of the results.