High resistivity is an indicator of hydrocarbon presence.
High-power source transmits low frequency EM signals
The source, a powerful horizontal electric dipole, is towed about 30 meters above the seafloor. The dipole source transmits a low frequency electromagnetic signal (0.05 to 10 Hz) into the surrounding water column and the underlying sediments. EM energy is rapidly attenuated, or weakened, in the conductive seafloor sediments, but in higher resistive layers, as for example in hydrocarbon filled reservoirs, EM energy is attenuated less and propagation is faster along the layer.
Seabed receivers measure the energy
Lines or grids of seabed receivers measure the energy that has propagated through the sea and the subsurface. Crucially, some of the energy is guided with low attenuation by resistive bodies, such as hydrocarbon reservoirs. The detection of this guided energy is the basis of marine EM.
Maps, cross sections and 3D volumes
Processing, post-modelling and inversion of EM data are performed after the survey, resulting in maps, cross sections and 3D volumes that show the location and the depth of resistive bodies.