Modeling low-frequency reverberation near the Mid-Atlantic Ridge and comparison with ARSRP data

Abstract

The Acoustic Reverberation Special Research Program (ARSRP) of the Office of Naval Research conducted low-frequency acoustic reverberation experiments just west of the Mid-Atlantic Ridge in 1993. Analyses of these data are supported by a bathymetric survey that produced nearly full-coverage bathymetry gridded to a resolution of 200×200 m. At this scale direct-path reverberation data has been successfully modeled using Lambert’s law applied to the local grazing angle determined from the high-resolution bathymetry. The analysis suggests that there could be a slightly weaker dependence on grazing angle than sine squared. The good agreement of this simple model with data shows that, for this region, two-dimensional seafloor morphology at the proper scale determines the ability to predict reverberation. For a model, such as Lambert’s law, that glosses over the details of seafloor microroughness or texture at the scale the acoustic wavelength (λ=6 m), the seafloor morphology scale that is critical is shown to be an order of magnitude or two greater than an acoustic wavelength. An average Lambert coefficient for the region is between −14 and −17 dB, but it is less than −20 dB for deeply sedimented areas and around −12 dB for rugged exposed rock areas.