High‐frequency bottom backscattering in the Arafura Sea

Abstract

Detailed backscatter and geoacoustic measurements were made at a 1‐km2 site in the Arafura Sea, north of Australia. Data were collected in collaboration with the Royal Australian Navy Research Laboratory in May 1984. Sediment geoacoustic and roughness properties were characterized using box core samples, underwater television, stereophotography, and sidescan sonar. Sediments consisted of a silty‐clay matrix, mixed with up to 30% whole and broken sand and gravel‐sized molluscan shells. X‐radiograpbs of the sediment showed fine scale volume inhomogeneities due to burrowing animals and shell fragments. Compressional wave attenuation (65 dB/m at 125 kHz; CV = 47.5%) was much more variable than the sediment/water sound velocity ratio (0.989 at 125 kHz; CV = 0.66%). Backscatter measurements were made over the frequency range of 15–45 kHz and over a wide range of grazing angles. Scattering strength at a 20°‐grazing angle ( −27 dB) was independent of frequency. Comparison of measured results with those predicted by physical scattering models suggests that backscattering was primarily due to sediment inhomogeneity. Measurements obtained along a 144‐km track showed that this scattering process was remarkably constant over a wide geographic area. [Work supported by NAVSEA 63‐R and administrated by NORAD.]