Side-scan sonar records showing thin-film interference patterns in deep-sea sediments

In 1986 a GLORIA (6.5 kHz) side-scan sonar survey from 4500m in the Bering Sea revealed an unexplained pattern of backscattering that was sub-parallel to track and similar to the pattern generated by the Lloyds mirror effect. In 1987 part of the previous years survey was repeated and the pattern reappeared in an almost identical fashion. It was concluded that the seafloor sediments were responsible. Since then the pattern has been observed on sediments from many of the world´s oceans, and on sonographs from the Institute of Oceanographic Sciences deep towed side-scan (TOBI) which operates at 30 kHz. Several geometries have been considered for the formation of these interference fringes and those in the GLORIA experiments can be attributed to a layer of thickness 3-5 m, the nature of which is a subject of speculation. The implications of these fringes for sonar operators are briefly discussed