Introduction

The use of sidescan sonar technology has greatly expanded in recent years. One impediment to interpreting sidescan sonar images, which are a representation of the amount of sound backscattered from the seafloor, is the incomplete understanding of the physical meaning of acoustic backscatter intensity variations. Ground-truth studies can help us to understand the causes of variations in backscatter. We need to measure physical and geometric properties of seafloor sediment and correlate them with variations in sidescan sonar acoustic backscatter. We present in this paper comparative ground-truth studies of two deep-sea fan depositional lobes. We show that sediment lithology influences sidescan sonar images, but that the relation between backscatter intensity and sediment grain size is not uniquely defined.

Some of the seafloor characteristics that are potential causes of variations in acoustic backscatter intensity are surface roughness, variations in sediment composition, grazing angle of insonification, and seafloor slope, including topographic variability (Urick 1983). The influence of each of these and the subbottom depth range over which sediment compositional variations are important will vary with the characteristics of the sidescan system, including frequency, pulse length, bandwidth, time-varying gains, and footprint size. The number of variables needs to be kept to a minimum in order to simplify a ground-truth study. The distal parts of deep-sea fans are good locations for such studies because they tend to have nearly horizontal seafloor surfaces. Thus, the topographic variability – bottom slope effect can be ignored.