Detection and target-strength measurements of buried objects using a seismo-acoustic sonar

Abstract

This thesis describes the results of field experiments in which seismo-acoustic interface (Rayleigh) waves were employed to detect and measure the target strength of mine-like test objects buried in the near-surf zone. These experiments were conducted as part of an ongoing NPS research program to develop a seismo-acoustic sonar system for the detection of buried mines in the surf and, near-surf zones. An experimental seismo-acoustic sonar system, using linear force actuators as the wave source and three-axis seismometers as receivers, was deployed at a beach test site. The target strengths of two mine- like test objects, a compressed gas cylinder and a gunpowder can, were measured as a function of target mass and for various emplacement conditions, e.g. very wet sand, not very wet sand, partially buried, completely buried, completely buried and washed over for several days. "Vector polarization filtering" was employed to separate the reflected signal due to Rayleigh waves, for which the particle motion is elliptical, from that of body (P and S) waves, for which the particle motion is linear. The target strength was generally found to increase with increasing target, mass. Typical values observed ranged from approximately -20dB to -10dB for target masses of 70 to 290 kg. Curiously, it was observed that the elliptical particle motion of the reflected wave was of the opposite polarity for those targets which were buried, but slightly exposed, compared to those which were completely buried. It is not known at this time whether this is due to the depth-dependent properties of Rayleigh waves, or whether it is a result of the conditions of source and target emplacement