Porosity estimates from marine controlled source electromagnetic dipole-dipole data at the Scanner Pockmark in the North Sea

As part of the EU Horizon2020 ‘STEMM-CCS’ project, controlled source electromagnetic (CSEM) and seismic data were acquired in 2017 at the Scanner Pockmark in the UK sector 15/25 of the North Sea, which is actively venting methane gas, to contribute to the evaluation of risk from potential fluid pathways to the sequestration of carbon dioxide in geological formations. We will present some preliminary results and relate electrical resistivities to sediment properties such as porosity and gas saturation.

The CSEM data presented were acquired with a University of Southampton deep-towed electric dipole source and two towed three axis dipole receivers (Vulcan, Scripps) along 12 profiles across an active pockmark. The data were processed in the frequency domain and the electrical resistivity structure was inferred with a 2D regularized inversion algorithm (MARE2DEM, K. Key).

To estimate porosities and their uncertainties to about 200 m below the seafloor, we use the empirical Archie’s law and calibrate Archie’s coefficient using physical properties measured with the multi-sensor core logger on gravity cores and sediment cores from the British Geological Survey Rock Drill 2 rig. Geological horizons identified on reflection seismic data are used as constraints in the resistivity model. The resulting porosity profile decreases with depth due to compaction and can be related to marine and glacial deposition environments.