Relative contribution of surface roughness and bottom attenuation to propagation loss in shallow water

Abstract

A simulation study has been performed to investigate the relative importance of the contributions of surface roughness and bottom attenuation to propagation loss under differing environmental conditionso The NRL normal mode model, which was used in the calculations, is restricted to range independent environments but can accommodate a sound velocity profile in the water layer which varies arbitrarily with depth. The bottom sediment is treated as a fluid of constant density and sound velocity with a small frequency dependent attenuation coefficient. The surface of the water layer is allowed to be rough and by using an ocean surface wave spectrum model, the roughness .is characrerized by a wind speed and wind direction. Modal attenuation coefficients have been calculated as a function of frequency and parameterized according to bottom sediment type, velocity profile and wind direction. Propagation loss at a given range has been calculated as a function of wind velocity, where frequency, sediment type and velocity profile were varied. The results show that for isovelocity and downward refracting profiles the contribution of surface roughness to the total propagation loss is important when the wind speed exceeds a threshold value, the latter depending upon the environmental conditions. Surface roughness can be the major attenuation mechanism for upward refracting profiles above a certain windspeed.