Accession Number:

ADA210020

Title:

Investigation of High-Frequency Acoustic Backscattering Model Parameters: Environmental Data from the Arafura Sea

Descriptive Note:

Final rept.

Corporate Author:

NAVAL OCEAN RESEARCH AND DEVELOPMENT ACTIVITY STENNIS SPACE CENTER MS

Report Date:

1989-02-01

Pagination or Media Count:

95.0

Abstract:

Bottom backscattering and geoacoustic measurements were made at a 1- sq. km site in the Arafura Sea, north of Australia. Sediment geoacoustic and roughness properties were characterized using box core samples, underwater video, stereo photography, and sidescan sonar imagery. Sidescan sonar images of the bottom were relatively uniform and featureless. Sediments consisted of sand- and gravel-sized mollusk shells, shell fragments, and carbonate rocks embedded in a silt-clay matrix. Sand-gravel fractions averaged 55 and gravel fractions averaged 11 of the total sample by dry weight. Mean grain size averaged 5.3 phi in the four cores analyzed for grain size distribution at 2-cm intervals. Compressional wave velocity ratio 125 kHz averaged 0.989 . Compressional wave attenuation at the same frequency averaged 60 dBm. An average sediment porosity of 69.7 generated a density ratio of 1.49 for cores. The gravel fractions from three box cores were examined for size, weight, and volume distributions for assessing the contribution of shells to sediment volume scattering. X- radiographs were examined for three-dimensional structure and orientation of the shells. Roughness power spectra from bottom height measurements had slopes near -2.2. Model predictions of backscatter strength versus grazing angle using the measured geoacoustic and roughness properties and default values for sediment volume scattering give an underestimate of 4 dB at 20 kHz of scattering strength when compared to actual data. Suggestions are made for characterizing the sediment volume scattering and bottom roughness parameters to improve the accuracy of the composite roughness model.

Subject Categories:

  • Physical and Dynamic Oceanography
  • Acoustic Detection and Detectors

Distribution Statement:

APPROVED FOR PUBLIC RELEASE