Fundamental relationships between geoacoustic parameters and predicted bottom loss using a thin layer model

Abstract

Bottom loss curves as a function of grazing angle for the frequency range 50–1500 Hz are obtained using a thin layer geoacoustic model of bottom interaction. A study of the model's sensitivity has revealed simple relationships between certain of the geoacoustic parameters and specific angular regions of the loss curve. Low‐angle losses are caused by losses of the transmitted ray that depend upon the sound speed ratio, gradient, and attenuation in the sediment, whereas high‐angle losses are caused by reflection from the water‐sediment interface and depend primarily on the sediment density. At low frequencies the angular position of the knee of the loss curve is dictated by the sediment thickness. Thinner sediments shift the knee toward shallower grazing angles. These relationships permit bottom losses to be estimated directly from the geoacoustic parameters without the need to exercise the thin layer model. A loss ranking scheme is demonstrated that predicts bottom losses to within ±2 dB of the full model result in over 88% of a random sample of angles and frequencies.