Research | Concepts | MId-Ocean Ridge EM Exploration

At a fast spreading mid-ocean ridge such as the East Pacific Rise, marine EM can be used to characterize the magmatic and hydrothermal systems. Seafloor electromagnetic receivers are used to measure natural source time variations in Earth's magnetic and electric fields for the magnetotelluric (MT) method, which yields electrical conductivity structure from depths of less than 1 km to a few hundred kilometers. Higher current densities are induced in conductive structure and so the MT method is preferentially sensitive to conductive features such as partial melt and hydrothermal fluids.

For the controlled-source or active-source EM method a horizontal electric dipole antenna is towed close to the seafloor and transmits a few hundred amps of current into the seafloor at frequencies of 0.1 to 10 Hz. The transmitted electric field travels through both the seawater and the seafloor. Because seawater is usually more conductive than the seafloor, the electric field in seawater attenuates away more rapidly. Thus the measured field on the seafloor EM receivers is that of the field that travels through the subsurface. The spatial character of the attenuation of the measured EM fields yields information on the subsurface conductivity structure. Because the electric field propagates to larger distances for more resistive structures, the controlled-source EM method is preferentially sensitive to resistive features. The penetration depths are less than for the MT method, so controlled-source EM is more suited to mapping structures at crustal depths.