3D Inversion of Transient EM Data – A Case Study From the Alvheim Field, North Sea

We present a case study leading to the 3D inversion of transient electromagnetic (EM) data for delineating reservoir extent at the Alvheim field in the Norwegian sector of the North Sea. The survey was conducted in July and August 2008 using one method of marine EM surveying, namely a two ship operation and ocean bottom cables. One ship laid a receiver cable with 30 receivers on the sea floor, and the second ship placed a source cable on the sea floor which was used to generate a coded transient signal. The configuration of the source and receiver spread was analogous to 2D seismic acquisition, as the system was rolled along to obtain multi-fold coverage of the subsurface. The survey spanned 20 km, resulting in measurements of 1270 source-receiver locations. The measured electric field for each source-receiver pair was deconvolved for the measured source current to determine the impulse response function. Preliminary inversions were made for each source-receiver pair using a 1D model, and the results were stitched to a 2D image. Having defined a background model, all data were then simultaneously inverted in 3D with focusing regularization. This revealed high resistivity volumes corresponding to the known hydrocarbon-bearing reservoirs of the Alvheim field.