Advances in marine electromagnetic (EM) efficiency with a towed acquisition system

Abstract

A towed marine EM system has been developing since 2004 where both source and receivers are towed behind the same vessel in an arrangement similar to 2D streamer seismic. This is an ideal technology for reducing risk in hydrocarbon targets in general and low saturation gas in particular, as well as the monitoring of CO2 sequestration. The dipole source is 400 or 800 m long and towed at 10 m below the sea surface. The receiver cable is towed at 100 m depth and has receiver offsets between 500 and 8,000 m. A transient source signal is used, allowing deterministic deconvolution of the source signature, which can be of any shape; for example, square wave, PRBS, or optimised repeated sequence (ORS). There are multiple benefits of the towed EM system: Similar in operation to a marine streamer seismic. Improved survey efficiency with source and receivers towed by the same vessel. Real-time monitoring of source and receivers, and quality control of incoming data. Onboard pre-processing. Dense sub-surface sampling. Receivers towed above the seafloor—the influence of strong local anomalies at the seabed is thus minimised. Facilitates simultaneous acquisition of EM and 2D seismic. Successful field tests were conducted in mid-2010 over the Peon gas field and the Troll oil and gas field in the Norwegian sector of the North Sea. A total of 615 line km were acquired during 138 hours, and the data has been successfully processed and inverted to delineate all targets.