oa Magnetic signals generated by ocean swells

Ocean waves and swells generate magnetic signals which may be spurious for aircraft carrying out magnetic surveys over ocean areas, and particularly over continental shelves. To check the character of such signals at the sea surface, a magnetometer has been set free from a ship to float unrestricted on the ocean surface for periods of several days. The path of the magnetometer was tracked by satellite; this procedure enabled also the eventual recovery of the magnetometer by the ship.

Superimposed upon a background of slow change of magnetic field, as the magnetometer drifted across different patterns of crustal magnetisation, are high-frequency signals generated by the strong ocean swell present at the time. These wave-generated signals are typically 5 nT trough-to-peak, consistent with theory for their generation by ocean swells several metre trough-to-peak in size.

The magnetic signals reflect the oceanographic effects of wave dispersion, and changing sea-state. In particular, the power spectra for the observed magnetic field exhibit a strong (-7) power fall-off with increasing frequency above the peak of 13 s. This strong fall-off is consistent with oceanographic observations of the spectra of surface swell, and suggests higher-frequency disturbances in such situations will generally be negligibly weak in aeromagnetic data.