The bi-azimuthal scattering distribution of an abyssal hill

Abstract

High-resolution bistatic images of a typical abyssal hill on the western flank of the Mid-Atlantic Ridge are made with a low-frequency towed-array system operating remotely at 1/2 convergence zone (∼33.3 km) stand-off. Comparison with modeled images, generated from high-resolution supporting bathymetry sampled at 5-m intervals, roughly the wavelength scale, reveals that steep scarps return the strongest echoes because they project the largest area along the acoustic path from the source to receiver. Prominent returns deterministically image scarp morphology when the cross-range axis of the system’s resolution footprint runs along the scarp axis. Statistical fluctuations inherent in the scattered field prevent the system from distinguishing smaller-scale anomalies on the scarps, such as canyons and gullies (∼100–200 m scale), that would otherwise be resolvable in range, in certain bistatic geometries. The mean bi-azimuthal scattering distributions of the two major scarps on the abyssal hill are identical and have strengths equal to the constant −17 dB±8 dB. This suggests that long-range reverberation from prominent geomorphological features of the world’s mid-ocean ridges can be adequately modeled as Lambertian with albedo π/101.7, given supporting bathymetry sampled with sufficient frequency to resolve the projected area of these features.