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Normal Incidence Reflection Loss from a Sandy Sediment

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Technical rept.

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The detection of mines buried in the seabed is of continuing concern. The optimization of sonars for such a purpose depends critically on the understanding of the interactions of acoustics with the seabed. A well-advertised limitation to search geometries is the existence of a critical angle for the incidence of an acoustic wave on the seabed. Beyond this angle most of the acoustic energy is reflected back into the water. In practice, this limitation is ill-defined because of small-scale sediment variability and water-sediment interface roughness. There have been a number of experimental and theoretical attempts to further understand the nature of the critical angle limitation. There are models of varying complexity that may be used to describe the acoustic properties of the sediment. This report provides, through the analysis of a simple normal incidence experiment, evidence to support the appropriateness of one model over another. Acoustic reflection loss at normal incidence of a sandy sediment in the Biodola Gulf on the north side of the Island of Elba, Italy, was measured in the band 8-17 kHz using a self-calibrating method. The water depth was approximately 11 meters and the sand was pure with a mean grain diameter of 0.2 millimeters. The measured reflection loss 11 dB, - 2 dB is consistent with measurements in the published literature. The computed reflection loss for an interface between water and a uniform visco-elastic media with the same properties was 8 dB, - 1dB. The theoretical and experimental values do not significantly overlap, which leads to the conclusion that the visco-elastic model is inappropriate. The Biot model is suggested as a better alternative, but more work is needed to ascertain the appropriate parameter values. 15 figures, 16 refs.

Subject Categories:

  • Undersea and Antisubmarine Warfare
  • Acoustic Detection and Detectors
  • Acoustics

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