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 ͑ϳ33.3 km͒ 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 ͑ϳ100-200 m scale͒, 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 Ϫ17 dBϮ8 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 /10 1.7 , given supporting bathymetry sampled with sufficient frequency to resolve the projected area of these features.
The bistatic scattering characteristics of two geologically distinct abyssal hills located on the western flank of the Mid-Atlantic Ridge, known as B' and C', are experimentally compared using data acquired with low-frequency towed-array systems at 1/2 convergence zone (approximately 33 km) stand-off. The comparison is significant because the abyssal hills span the two classes of elevated seafloor crust that cover the Mid-Atlantic Ridge. The highly lineated B' feature is representative of abyssal hills composed of outside corner crust, the most commonly occurring category, whereas the domed C' promontory is representative of the rougher, low-aspect-ratio abyssal hills composed of inside corner crust. The latter are less common and usually restricted to segment valley margins. The mean biazimuthal scattering distributions of the two abyssal hills each exhibit Lambertian behavior with comparable albedos, suggesting that the distinction between abyssal hills composed of differing crust is not significant in modeling long-range reverberation. The adverse effect of using bathymetry that undersamples seafloor projected area in scattering strength analysis is also quantified with data from the B' ridge. Specifically, the use of undersampled bathymetry can lead to significant overestimates in the strength of seafloor scattering.
A comparison is made of bistatic scattering from two distinct deep-ocean ridges. The ridges are located along a segment valley on the western flank of the Mid-Atlantic Ridge (MAR). The acoustic data sets were acquired during the Main Acoustics Experiment of the Acoustic Reverberation Special Research Program. Detailed analysis of bistatic scattering from one of these ridges, named B′, has been previously presented by some of the authors. The purpose of this paper is to present a similar analysis for the second ridge, C′, for comparative purposes. This comparison is important because the ridges span the two geologically distinct classes found in the MAR. There is a possibility, therefore, that their scattering characteristics may be extrapolated to ridges throughout the MAR. For example, B′ is an ‘‘outside corner’’ and therefore is composed of highly lineated scarps and terraces, while C′ is an ‘‘inside corner’’ that is characteristically amorphous with irregular large-throw normal faults. While major scarps on these ridges can be easily resolved by the towed-array system, smaller-scale anomalies such as gullies and trellises are often underresolved by nearly an order of magnitude. These anomalies are found to introduce large variances in the bistatic scattering characteristics of a given scarp.
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