Evaluation of low-frequency bottom backscattering strength vs grazing angle by means of multiple beamforming

Abstract

Bottom backscattering strength was measured in deep water as a function of grazing angle in the Balearic Abyssal Plain of the Mediterranean. The measurements were made with a towed, narrowband low-frequency omnidirectional source and a towed, horizontal linear array. The method takes advantage of the multiple beamforming capability of the receiving array and processor to accurately discriminate returns at given grazing angles from interfering returns. Approximations for the evaluation of the backscattering area are described and discussed. The results are presented and compared with previously reported measurements. At the higher grazing angles, specular reflection from normal incidence dominates the returns. However at medium and small grazing angles data have a large plateau region, with small variations. A Lambert's rule relationship, with a coefficient of -33 dB, can approximate results for grazing angles as low as about 20°.