Accession Number:

ADA255712

Title:

Application of Ray-Born Scattering and Boundary Perturbation Methods to Acoustic Reverberation

Descriptive Note:

Final technical rept. 15 Jun 1989-30 Sep 1991

Corporate Author:

MASSACHUSETTS INST OF TECH CAMBRIDGE DEPT OF EARTH ATMOSPHERIC AND PLANETARY SCIENCES

Personal Author(s):

Report Date:

1992-08-01

Pagination or Media Count:

34.0

Abstract:

This study utilizes ultrasonic water tank modeling to examine three- dimensional scattering trends from a random set of parallel grooves, and compares this with theoretical results obtained from two-dimensional finite- difference calculations. Ultrasonic laboratory modeling is carried out using computer-controlled source and receivers with an aluminum bock submerged in a water tank. The blocks upper interface is plane for the reference model and grooved for the test model. The grooves measure about one-third the center source wavelength and have a Gaussian distribution with a mean of 1 wavelength and a standard deviation of 13 wavelength. This experiment places both the source and receiver at the waters surface with the receiver array in the horizontal plane. The receiver line is then positioned at various angles to grooves. A staggered-grid finite-difference scheme is used for theoretical computations and comparisons with laboratory data. These theoretical results matched experimental data well for both the plane interface and the grooved model. Specifically, this study shows that scattering mechanisms are different for propagation normal to grooves than those parallel to the grooves. In the first case scattering takes place in the form of point diffractors. This causes reduction of the specular reflections. Amplitudes decrease by more than 60, relative to a plane interface, when the incidence angle exceeds 45 degrees. Snapshots of finite-difference synthetics helped to clarify details of scattering. In the second case, where the wave front is parallel to the grooves, scattering takes e form of guided head waves and continuous diffractions giving rise to constructive and destructive interference. This gives the illusion of broken reflectors at depth.

Subject Categories:

  • Seismology
  • Acoustics

Distribution Statement:

APPROVED FOR PUBLIC RELEASE