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\ncontrolled source electromagnetic (CSEM) and multichannel reflection\nseismic (MCS) data along one profile in the German North Sea with the\ngoal to reduce ambiguities in interpretation. The shallow water\nenvironment of the North Sea is characterized by a complex geological\ndevelopment which includes rifting, several ice age cycles, a\npropagating shelf margin, mass-transport deposits and salt dome\nformation. Seismic and electromagnetic methods are sensitive to\ndifferent physical properties of the seabed and therefore complement\neach other. We analyse the MCS data with a migration velocity tomography\nand an amplitude variation with offset analysis and discuss seismic\nvelocities and densities. For true amplitude recovery the amplitude\ndistortions are calibrated with in situ logging data. The CSEM data are\nanalysed in 2-D, for which, for the first time, data were included that\nwere acquired while the instrument was towed on the seafloor in addition\nto the stationary sites. The CSEM inversions are constrained by seismic\nhorizons. The joint interpretation focuses on two seismic reflectors:\nOne can be interpreted as an unconformity marking a lithological change\nfrom fresh water-bearing glacial deposits to compacted sediments below,\nand the other one as a layer of fine-grained deposits potentially\ncapping patchy shallow gas occurrences. This exemplary case study shows\nhow the combination of both methods can benefit by interpreting complex\ngeology and eliminating ambiguous explanations.