Effects of bottom reflectivity on volume (fish) and bottom backscattering in shallow water

Reverberation measurements in shallow water made with fixed source and receiver arrays (D. Weston, 1971) demonstrated that scattering from bladder-bearing fish can dominate backscatter from long ranges in shallow water environments at frequencies of 1 and 2 kHz. In this work, estimates of monostatic scattering levels due to both bottom and volume (fish) were computed using the Generic Sonar Model as a function of three parameters: bottom reflectivity vs grazing angle, bottom scattering strength vs grazing angle and volume backscattering strength. Predictions were made assuming the bottom can be modelled as an infinite absorbing fluid half space. Volume scattering strength estimates were based on previously reported volume scattering experiments conducted in deep water. Bottom scattering strength estimates were made by parameterizing Lambert´s rule coefficient. Comparisons of volume and bottom backscatter are presented as functions of bottom critical angle and bottom absorption coefficient. Results suggest that in shallow water environments where bladder-bearing fish are present, volume reverberation may be expected to dominate where the effective critical angle is small, whereas bottom reverberation may be expected to dominate where the effective critical angle is large. In shallow water environments where the critical angle is small, temporal and spatial variability of fish population densities is expected to make reverberation highly variable and difficult to predict