Influence of Bottom Trawling on the Normal- Incidence Reflection Coefficient

A North Sea area characterized by muddy, sandy, and gravelly sediments is surveyed with a single-beam echo sounder operating at 12, 38, and 200 kHz. Closely spaced survey legs and a high repetition rate of five sounder pings per second allow the production of echo energy maps with a high spatial resolution. These energy maps reveal a series of string-like features, which are believed to be due to beam trawling activities. The trawler plough marks feature a dramatic increase in the echo energy, up to 12 dB above that of their surroundings. This effect, the strength of which is frequency dependent, may be used to monitor fishing activities and demersal habitats. The present paper illustrates the marked acoustic signature of the furrows and discusses several candidate responsible physical mechanisms. Disturbance of the mass density gradient in the benthic layer is considered the most likely candidate.