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Development of a 2-Degree-of-Freedom Transverse Line Source for a Seismo-Acoustic SONAR

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Master's thesis

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This thesis describes the beach field-testing of a 2-degree-of-freedom 2-DOF transverse line Rayleigh wave source for use in a seismo-acoustic SONAR for buried mine detection. The source is composed of six identical modules, each composed of two radial and two vertical inertial mass shakers, all mounted to a common base plate. The vertical and horizontal excitation of each module is independent. Electrically, the six modules are organized into three independent sets of two. The length of the source is 46 inches 1.17m, the width 6 inches 0.15m and the weight 126 pounds 562N. Field tests were conducted using a 5-cycle, smooth-amplitude-modulated transient excitation signal. Based on past and present experience that the Rayleigh wave speed at the beach test site is approximately 90 ms, a carrier frequency of 100 Hz was employed, producing a Rayleigh wavelength and surface penetration depth of approximately 1 m approximately equal to the source aperture length, which is appropriate for the intended application. The source radiation directional characteristics were investigated for various relative amplitudes and phases of vertical and horizontal shaker excitation. A null result was obtained, that is, no unambiguous evidence of directivity was observed in any of the experiments conducted using this source. These included measurements wherein the source modules were removed from the common base plate and emplaced on the sand. The reason for this result is unknown, but it is suspected that a good possibility is that the beach sand medium just below its surface, whereupon the present source and receivers were deployed, is not homogeneous enough to support coherent spatial beamforming.

Subject Categories:

  • Acoustic Detection and Detectors
  • Seismic Detection and Detectors

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