Seafloor acoustic backscattering strength and properties from published data

Abstract

Over the past 50 years, a large number of bottom scattering measurements have been made at a very significant accumulated cost, particularly in the 10 kHz to 1 MHz range. While each measurement is valuable individually, the total collection is more valuable as a whole. The earliest measurements were made in the 1950s using simple sound sources and receivers, such as explosives and omni-directional hydrophones. In the 1960s, a large set of measurements were made with purpose-built sonar equipment by the Applied Research Laboratories of The University of Texas at Austin and others. The results indicated that backscattering strength increased with frequency, but later measurements did not uphold this trend. Another large set of measurements were made in the 1980s by the Applied Physics Laboratory of The University of Washington, which lead to further model development. Since then, many more measurements have been made by numerous institutions and individuals. No one measurement, or program of measurements, has been able to capture the diversity of the seabed. It appears that a database of all the published measurements is needed to fully comprehend the scope of the problem, and the physical processes involved. In parallel with the measurements, a number of models were developed. The earliest ones attempted to link backscattering strength to the sediment type or class, often represented by the mean grain size, in a purely empirical fashion. The earlier data sets appeared to support this approach but the trends are not supported by the database as a whole. Later models were based on physical representations. The seafloor is often modeled as a fluid with volume and roughness scattering mechanisms incorporated. More sophisticated models represented the seafloor as an elastic solid, and more recently as a poro-elastic medium. One objective has been to invert the acoustic backscatter measurements for sediment properties, but the data, so far, shows limited correlation between mean grain size and backscattering strength. The extant database may be used to explore the underlying physics. With respect to the sediment grain size, the scattering regime may be divided into a number of regimes. For grain sizes much smaller than the acoustic wavelength in water, the direct scattering from grains is weak and increases with frequency. This regime is only observed in the laboratory because in the real seafloor, other scattering mechanisms, including seafloor roughness such as sand ripples, and inclusions such as biogenic gas bubbles, and other fauna and flora often dominate the scattering process. In the next regime, where the grain size is of the same order of magnitude as the acoustic wavelength, the backscattering strength is well bounded. In this regime, models and data often agree. Finally, in the regime where the effective grain size is much larger than the acoustic wavelength, there is considerable variability in the data. In this regime, the concept of grain size is no longer useful, because the acoustic scattering is determined by facets rather than grains, and more work is needed to obtain usable relationships.