On the exponent in the power law for the attenuation at low frequencies in sandy sediments

Abstract

Shallow water transmission loss measurements yield intrinsic attenuation estimates for acoustic waves in the underlying sediment, with results that are consistent with attenuation being proportional to frequency raised to a power n, with n between 1.6 and 1.87. Plausible theory suggests that n should be identically 2. The discrepancy can be explained because the inverse analysis inferences were made with the neglect of an additional attenuation mechanism where generated lower velocity shear waves carry energy downwards out of the waveguide. The shear wave effect has a weaker dependence on frequency than the intrinsic attenuation, so the apparent exponent is shifted downward.

Investigation of Complex Range-Dependent Shallow Water Sound Transmission

“…A recent paper discusses calculations used to explain the less than quadratic frequency dependence of compressional wave attenuation [7,8]. To quantify the attenuation properties of shallow-water waveguides, various physical mechanisms were considered as possible influences on propagation.…”

“…For shear wave speeds of approximately 300 m/s, these effects are in general agreement with less than quadratic expected values. [8] To better quantify the effects that shear has on shallow-water sound propgation, an experiment, Nantucket Sound II, was conducted during August of 2007. The experiment involved a bottom mounted source and a near-bottom array system towed behind a small autonomous vehicle.…”