Acoustic Techniques for Imaging the Seafloor

Abstract

The recent development of sophisticated sonar systems for seafloor imaging allows analyses of terrain beyond simple bathymetric mapping. The SEA-BEAM multibeam sonar system produces fullcoverage 12-kHz soundings, and much of the Gorda Ridge has been surveyed by the National Oceanic and Atmospheric Administration and the U.S. Navy. Complete-coverage digital bathymetry provides the basic data for statistical modeling of seafloor roughness through spectral and fractal mathematical techniques. In addition to soundings, the SEABEAM system can provide fundamental information about bottom properties through evaluation of the backscattered acoustical signal. Side-scan sonar systems allow more detailed observations of the variability of acoustic properties. Available sidescan sonar data range from the broad (60-km swath) complete coverage of the GLORIA system collected by the U.S. Geological Survey to 10-km swath Sea-MARC II and 5- to 0.5-km-wide SeaMARC I imaging, which produces pixel spot sizes of as little as 25 cm. The full-coverage, digital nature of these data allows the application of terrain classification techniques developed for satellite imagery. The validity of remote-imaging techniques can only be evaluated when compared to observational ground truth. A recently developed technique for geological mapping from bottom photography facilitates such comparisons.