Very Low Frequency (VLF)
VLF techniques measure the perturbations in a plane-wave radio signal (3-30 kHz) emanating from one of several world-wide radio transmitters used for submarine communications. The primary field (the transmitted radio signal) causes eddy currents to be induced in conductive geologic units or structures. The radio wave will produce an electric field and hence an electric current in a conductive media. Those eddy currents create a secondary magnetic field which is measured by the VLF receiver.