On the exponential power law for low frequency attenuation in shallow water

Abstract

Reviews of measured shallow-water sound transmission over sandy-silty bottoms show the intrinsic attenuation for low frequencies (< fo=1 kHz) to vary with as (f/fo) to the 1.8 power. Since plausible theory suggests this dependence should be (f/fo)to the 2.0 power, then why are the observed values different? Calculations incorporating scattering, layering,and gradients were found not to explain this observation. The discrepancy can be explained when an additional attenuation mechanism where generated lower velocity shear waves carry energy downwards out of the waveguide is considered. Since the intrinsic attenuation and the apparent attenuation due to shear wave conversion are small, the observed attenuation can have a different frequency dependence. If the removal of energy from the field due to shear wave conversion is comparable to the intrinsic attenuation at the lower frequencies but less than the intrinsic attenuation at the higher frequencies; the inferred frequency dependence can be less than quadratic. Inversions with fluid sediments account for this effect by the site-specific factors that accounts for the loss of energy from the compressible field due shear wave conversion.