Marine Electromagnetics

Marine electromagnetics is a geophysical exploration methods to derive the electrical properties of seafloor. The electrical conductivity (or its reciprocal the electrical resistivity) is an important parameter to characterize seafloor sediments as well as oceanic crust and mantle. The conductivity of marine sediments mainly depends on the porosity and the conductivity of the pore fluids. Seawater has high salinities and is therefore conductive. In contrast, naturally occurring hydrocarbons like oil, gas and gas hydrates are electrically resistive. The formation resistivity is therefore elevated where they accumulate in the pore space replacing conductive pore fluid. In the oceanic crust and mantle the rock conductivity rather depends on rock composition, fluids, hydrothermal activity and partial melts.

Model study of electrical field propagation of a horizontal electrical dipole (TX) located at the seafloor above a homogeneous seafloor (left), and in presence of a 3D resistive target (right). The fields recorded at the receivers (RX) is clearly biased by the 3D body

Electromagnetic (EM) methods can be separated into active or controlled source electromagnetic (CSEM) and passive methods (magnetotellurics). Marine CSEM is used for shallow seafloor explorations down to a sediment depth of about 1-2 km. The main targets are oil, gas and gas hydrate deposits, offshore groundwater discharge and seafloor massive sulfide deposits in context wit hydrothermal activity. Magnetotelluric measurements target at the exploration of deeper sediment basins, sub-salt, sub-basalt structures, as well as deeper oceanic crust and mantle studies. The experimental setup differs depending on target depth and type.

Marine CSEM Principle Source: BGR

EM methods are often used complementary to seismic methods. While reflection seismic data provide high resolution structural images, EM yields volume information for characterization and quantification of pore fluids
BGR develops marine CSEM methods and instrumentations for shallow seafloor exploration targets. HYDRA is a seafloor-towed system which has been used for gas hydrate exploration offshore New Zealand and in the Black Sea. GOLDEN EYE is a deep-sea EM profiler for seafloor massive sulfide exploration on the German license areas in the SW Indian Ocean.