Effects of density gradients on bottom reflection loss for a class of marine sediments

The sensitivity of bottom reflection loss to a density gradient is examined within the context of a physically meaningful model consisting of a fluid sediment layer having depth-dependent density and sound speed overlying a substrate having solid properties. The bottom grazing-angle range is divided into two regions, the low-angle and the high-angle regions. In the low-angle region sediment sound-speed gradients refract incident energy upward before it encounters the substrate. In the high-grazing-angle region, energy impinges on the substrate. Through the use of a numerical plane-wave reflection coefficient model it is shown that the effect of a density gradient on low-angle bottom loss is small. An expression indicating this effect is derived and is shown to agree with numerical bottom-loss calculations. The high-angle bottom loss is more influenced by a density gradient because of the impedance changes at the sediment–substrate interface which accompany a density gradient. It is shown that introduction of density gradient of 0.002 g/cm3/m can cause the high-angle bottom loss to change on the average by 12.5%, 7%, and 3% in 100-m clay, silt, and sand layers, respectively.