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Backscattering from Bioturbated Sediments at Very High Frequency

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Technical rept.

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The environment significantly affects mine hunting system performance. This is especially true in mine hunting conditions to detect low-contrast mines in cluttered environments. Inexact performance predictions result in poor evaluations of mine clearance which, in turn, lead to inaccurate assessments of remaining risk to follow-on forces in the channel or area. This report provides a quantification and understanding of environmental phenomena critical to acoustic backscattering at mine hunting frequency. The study demonstrates the unambiguous importance of seabed interface roughness, internal bioturbation, and upper sediment structure for object detection. In some cases, mine hunting sonar detection performances are believed to be strongly reduced. To remedy these environmental limitations, mine hunting operations need to quantitatively account for the environmental conditions and adapt to them in near real time. Recent backscattering measurements made in the Gulf of La Spezia Italy using a sonar operating at 140 kHz combined with thorough seabed interface and volume ground truth illustrate the dominance of seabed volume scattering. Three-dimensional fluctuation statistics of density variability and vertical density gradients, both of which relate directly to the level of bioturbation e.g., sea shell fragments, burrows, water pockets were quantified using X-ray computed tomography. Two-dimensional interface roughness spectra also were determined using a digital stereo photogrammetry system. The combined ground truth has allowed a backscattering model to be fully constrained. Measured backscattering strength versus angle is compared to a model that includes the effects of varying density and sound speed. Data-model comparisons show that scattering from the volume of strongly inhomogeneous sediments can often be a primary contributor to seafloor scattering away from normal incidence. 28 figures, 27 refs.

Subject Categories:

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

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