Analysis of Grazing Angle and Frequency Dependence on Acoustic Backscattering Pattern

Abstract

This study aims to investigate the underwater acoustic backscattering pattern with respect to grazing angles and frequency. The narrowband impulses signal was generated from 8500 to 9500 Hz with 250 Hz increment, and the grazing angles were 10o, 20o, 30o, 40o, and 50o, respectively. The measurement was carried out in a 9m-width, 5m-wide and 0.5 m-water deep pond with muddy bottom with spatial-aliasing free hydrophone array placed at depth 15 cm and 35 cm. The experiment used ray tracing method for sensor placement. The measurement results showed that bottom backscattering pattern was observed at grazing angle 10o to 30o, and whilst from 40o to 50o the surface backscattering pattern was appeared on the loudness contour. The results also showed the intensity was higher at grazing angle 10o-30o at depth 35 cm, whilst from 40o to 50o was higher at depth 15 cm. It was confirmed from the measurements that the intensity has an exponential decay at grazing angle 10o, 30o, and 50o. Intensity as function of grazing angle was found to fit Lambert's rule at grazing angle 10o-40o at depth 15 cm and grazing angle 10o-30o at depth 35 cm. The results suggest that the acoustic backscattering pattern depends at grazing angle only. The intensity increased as the acoustic waves were backscattered because of reflection as well as interference with walls of the pond, rough bottom, and water surface.