Combined interpretation of marine controlled source electromagnetic and reflection seismic data in the German North Sea: a case study

Abstract

This paper focuses on the combined analysis and interpretation of controlled source electromagnetic (CSEM) and multichannel reflection seismic (MCS) data along one profile in the German North Sea with the goal to reduce ambiguities in interpretation. The shallow water environment of the North Sea is characterized by a complex geological development which includes rifting, several ice age cycles, a propagating shelf margin, mass-transport deposits and salt dome formation. Seismic and electromagnetic methods are sensitive to different physical properties of the seabed and therefore complement each other. We analyse the MCS data with a migration velocity tomography and an amplitude variation with offset analysis and discuss seismic velocities and densities. For true amplitude recovery the amplitude distortions are calibrated with in situ logging data. The CSEM data are analysed in 2-D, for which, for the first time, data were included that were acquired while the instrument was towed on the seafloor in addition to the stationary sites. The CSEM inversions are constrained by seismic horizons. The joint interpretation focuses on two seismic reflectors: One can be interpreted as an unconformity marking a lithological change from fresh water-bearing glacial deposits to compacted sediments below, and the other one as a layer of fine-grained deposits potentially capping patchy shallow gas occurrences. This exemplary case study shows how the combination of both methods can benefit by interpreting complex geology and eliminating ambiguous explanations.