Comparison of seafloor character as measured by calibrated vertical-incidence profiler and sidescan sonar

Abstract

Sidescan sonar backscatter from a variety of continental shelf bottom types reveals considerable divergence from vertical-incidence reflection coefficients. Acoustic data are from a 300-kHz sidescan sonar and a calibrated, constant beam width, 10–50 kHz vertical incidence profiler, and are supported by extensive physical, geoacoustic, textural, and morphologic measurements. Bottom types include a wide range of quartzose and carbonate sands and silts, seagrass, coral heads, coral reef, and hardbottoms. The two systems show notable divergences over seagrass and carbonate hardbottoms, which produced low normal-incidence reflection coefficients but high backscatter from sidescan sonar, and smooth, packed sands, which gave high normal-incidence reflection coefficients but low sidescan backscatter. The divergences appear to be caused by bottom roughness. Narrow-beam, vertical incidence systems are subject to signal loss from out-of-beam scattering over rough bottoms, whereas roughness increases backscatter at the low grazing angles over which the sidescan system operates. Although both systems will accurately map bottom character, there is no direct correspondence betwen the two vis-à-vis bottom type. Therefore, to extend seafloor classification from vertical incidence profiling into an areal approach with sidescan sonar requires more than simple lateral extrapolation. [Work supported by ONR.]