Characteristics of convergence zone formed by bottom reflection in deep water

Abstract

There appears a convergence effect on the sound filed under the condition of sound channel in the deep sea due to the refraction effect of the sea water. For the deep water environment with an incomplete channel, sea bottom has an important influence on sound propagation. A long-range sound propagation experiment was conducted in the South China Sea in April 2018. Hyperbolic frequency modulated (HFM) signals with a frequency band of 250-350 Hz are transmitted by an acoustic source which is towed at a speed of 4 knots away from a vertical line array (VLA). The VLA consists of 20 hydrophones which are arranged from 85 m to 3400 m with an unequal depth space. Using the data collected in the experiment, the effects of bathymetry variation on sound propagation are studied. The physical causes of the seafloor reflection convergence phenomenon are explained by using the parabolic equation combined with ray theory. The observed phenomenon is different from the convergence phenomenon in the typical deep water environment, the spatial variation of bathymetry contributes to the formation of the seafloor reflection convergence zone in advance, and the sound intensity in part of shadow zone is significantly increased. Due to the reflection from the seabed, two obvious seafloor reflection convergence zones are observed near the range of 20 km and 40 km respectively, in which both gains increase up to 10 dB, and a high sound intensity area is formed in the shadow zone near the range of 11 km, where the gain is less than the gains in the two convergence zones. In addition, the grazing angle of the sound ray reaching the second convergence zone is smaller than that reaching the first convergence zone when the receiving depth is the same as the source depth, and the rays with smaller glancing angle have less reflection loss, which leads to a higher gain in the second convergence zone. As the water depth becomes gradually shallower with range increasing, the convergence zone near the range of 51 km under the SOFAR channel is destroyed, and the sound field energy in the corresponding range is much smaller since the number of arriving refracted sound rays is reduced. In the first convergence zone, the path of arriving rays is gradually increased as the receiver becomes deeper. Therefore, the arrival structure tends to be complicated, and the multi-path effect is more obvious. The study result is meaningful for the performance analysis of sonar in complex deep water environment.