Apparent aeromagnetic wavelengths of the magnetic signals of ocean swell

Ocean swells have a magnetic signal, caused by the motional induction of seawater moving in the steady main magnetic field of Earth. These signals may be sensed by a low-flying aircraft, carrying out aeromagnetic measurements over the ocean. The apparent spatial wavelength that such signals will have, when observed data are plotted out for geological purposes, can vary greatly. It will depend particularly on the relative speeds and directions of travel of the observing aircraft and the ocean swells. The apparent wavelength of the ocean swell magnetic signal cannot be less than the actual ocean swell wavelength. Generally, it is greater, and it can range up to infinity in value. For observations over continental shelves, the situation is complicated by the dependence of the swell phase-velocity on water depth, by which the swell speed generally slows as land is approached.