The effect of shear wave dispersion on bottom loss from unconsolidated sediments with rough interfaces.

Abstract

Shear waves are often neglected for unconsolidated ocean bottom sediments like sands; however, the shear mode in these sediments can influence propagation and bottom loss especially for relatively high shear wave speeds and rough sediment/water interfaces. Although measured shear wave speeds are low, these measurements were made at frequencies much lower than the operating frequencies of most imaging or bathymetric sonars. New models for unconsolidated sediments suggest that shear wave speed may have a steep dispersion implying significant shear wave speeds at operating sonar frequencies. [Chotiros and Isakson, J. Acoust. Soc. Am., 116, 2011–2022 (2004).] Additionally, measurements for bottom loss suggest a dependence of shear speed on frequency. [Isakson, Yarbrough, Chotiros, and Piper, J. Acoust. Soc. Am., In review.] Lastly, although there is often low coupling between waves of vastly different sound speeds for shallow grazing angles on flat interfaces such as in long range propagation, interface roughness can steepen the local grazing angle leading to increased coupling. In this study, the effects of predicted shear wave dispersion and coupling through rough interface scattering on bottom loss and propagation for shallow water waveguides with unconsolidated sediment bottoms will be explored using finite element modeling. [Work sponsored by Office of Naval Research, Ocean Acoustics.]