ASA 125th Meeting Ottawa 1993 May

3aUW1. Application of sidescan sonar to geological sea floor mapping.

David C. Twichell

Thomas F. O'Brien

US Geological Survey, Woods Hole, MA 02543

Digital sidescan-sonar images collected during a systematic mapping of the deep water part of the US continental margins, show a more complex near-surface geology than anticipated from previous mapping programs. For example, reconnaissance mapping with the GLORIA system (6.5-kHz operating frequency) in the Gulf of Mexico revealed channels extending hundreds of kilometers across the basin floor and discontinuous high-backscatter deposits in areas previously inferred to be laterally uniform deposits. Subsequent studies over part of the same area using the 27- to 30-kHz SeaMARC system permitted a detailed mapping of smaller channels and the high-backscatter deposits around their ends while cores permitted a link between seafloor geology and the imagery. Sandy and silty beds ranging from 10--118 cm thick occur in the areas of high backscatter and muds from areas of low backscatter. The imagery and cores in some cases permit mapping the extent and geometry of individual deposits. The deposits are a series of laterally discontinuous sand and silt lenses with abrupt edges rather than a continuous sheet of sand as previously was inferred for this area. This complex geology needs to be incorporated in efforts to understand the interaction of acoustic signals with the seabed.