Jacques R. Chamuel
Sonoquest Advanced Ultrason. Res., P.O. Box 81153, Wellesley Hills, MA 02181-0001
A two-dimensional horizontal hydrophone array when used to receive acoustic waves backscattered from an entire ocean basin and its margin can identify individual features in azimuth and in range. Localized anomalous strong low-frequency acoustic backscatter from the Canada Basin near 73.2 (degrees)N, 139 (degrees)W was observed by Dyer et al. [J. Geophys. Res. 87, C12, 9477--9488 (1982)] using a long-range 2-D hydrophone array, however, recent bathymetric charts do not reveal the existence of large bathymetric highs nearby [A. Grantz et al., J. Geophys. Res. 89, C2, 2105--2108 (1984) and I. Dyer, J. Geophys. Res. 89, C2, 2109 (1984)]. The author hypothesizes that the anomalous observed backscatter is likely caused by the limitation of a 2-D acoustic array in presence of focused multiple scattering. When direct scattering is assumed, range is calculated based on total travel time, however, when multiple scattering is ignored, range tends to be overestimated. Scaled 3-D laboratory ultrasonic modeling results are presented demonstrating that long near-elliptical segments of the Beaufort slope focus the acoustic waves onto raised bottom features (water depth about 3500 m) near (a) 73.5N, 140.4W (b) 72.9N, 147.4W, and (c) 73.1N, 144.3W oriented to reflect the waves back toward the source. The range of these features can be overestimated by 45%--65% when focused multiple scattering is ignored. Future bathymetric measurements need to be conducted at the corresponding range predicted when multiple scattering is included. [Work supported by ONR.]