A sideways-looking acoustic-survey system which uses the reflection of high-frequency (30–110 kHz) sound waves from a surface to map the texture of that surface. It is most commonly used to map sea-floor features (sand ripples, rock outcrops, pipes, wrecks, etc.) but it can also be suspended vertically from a vessel to investigate vertical surfaces, e.g. the submerged parts of icebergs and major ice fronts.