The effect of sediment rigidity on bottom reflection loss

Abstract

The effect of sediment rigidity on bottom reflection loss, RL = −20 log10 (|R|) where R is the plane wave reflection coefficient, is studied using a computational model [P. J. Vidmar and T. L. Foreman, J. Acoust. Soc. Am. 65, S43 (1979)]. A single inhomogeneous turbidite layer overlying a homogeneous substrate is treated with an emphasis on low frequencies (10–200 Hz) and low grazing angles (0°–45°). We find that sediment rigidity can be neglected for thick (∼500 m) layers while it can produce significant increases in RL for thin layers (∼36 m). The frequency dependence of RL for thin layers has a low frequency regime characterized by large peaks (∼25 dB) recurring at short (∼2 Hz) intervals. At high frequencies there remains a persistent increase in RL (∼4 dB). A ray picture, based on an expansion of the reflection and transmission coefficient in the small sediment S wave speed, provides a basis for understanding these results. The most important mechanism for sediment S wave excitation is found to be compressional wave conversion at the sediment‐substrate interface. [Work supported by the Naval Ocean Research and Development Activity and the Naval Electronic Systems Command.]