Marine Controlled Source EM (mCSEM) comes from the adaptation of both MTEM and onshore MT measurement to an offshore environment by the Scripps Institution of Oceanography to study the oceanic lithosphere. Then, Oil & Gas market extensively adapted the technology with innovative ideas about acquisition design, controlled source and with a substantial work on modeling and inversion issues.

mCSEM involves transmitting a low-frequency EM signal from a subsea source towed behind a ship. The signal returned from sediments below the seafloor is then recorded by a number of receivers dropped on the sea bottom at differing distances from the source.

More recently (2014), PGS ( has developed a towed EM system that can be used together with seismic acquisition. HED source and receivers are towed near by the sea surface. The system is designed to acquire 3D EM as a stand-alone service over existing or planned 3D seismic or to acquire 2D GeoStreamer seismic and EM data simultaneously using a single vessel.

The CSEM signals involve both vertical and horizontal current flow, which could be interrupted by oil or gas reservoirs to provide sensitivity to these geologic structures even when they are quite thin.

The current section is composed of 6 lessons: