A ray path analysis of sediment shear‐wave effects on bottom reflection loss

Abstract

The effect of sediment shear‐wave propagation on bottom reflection loss (RL) from a typical deep sea sediment layer is analyzed by means of a ray path decomposition of the acoustic field. The purpose of this analysis is the development of a simple physical model of the sediment shear‐wave effects reported in a recent computational study of RL from a typical deep sea turbidite layer [P. J. Vidmar, J. Acoust. Soc. Am. Suppl. 1 66, S75 (1979)]. The ray path decomposition uses the interface reflection and transmission coefficients to obtain the amplitude of each shear (S) and compressional (P) wave. These coefficients are analyzed by means of an expansion in the parameter ε which is proportional to the relatively small S wave velocity of marine sediments. Ray paths with amplitudes up to order ε2 are identified and used to develop a qualitative ray path model of RL. P‐wave conversion at the sediment‐substrate interface is the dominant mechanism for sediment S‐wave excitation. To order ε2 RL is the result of the interference of three waves in the water whose amplitudes are proportional to ε0, ε1, and ε2. Comparison with RL obtained from a computational model shows that the ray path model correctly predicts the dependence of RL on frequency, S‐wave speed, S‐wave attenuation, P‐wave attenuation, and sediment thickness. [Work supported by Naval Ocean Research and Development Activity and Naval Electronic Systems Command.]