Buried object detection with seismic sonar in the surf zone

Abstract

The surf zone presents a difficult environment for remote detection of naval mines and military ordnance, which become buried in the sediment as a result of tidal action, currents and other causes. We have investigated this problem in the course of several thesis projects done at the Naval Postgraduate School in Monterey CA. Experiments were conducted on the Del Monte Beach on Monterey Bay, utilizing seismic interface waves of the Rayleigh type. Experimental and analytical research on Rayleigh wave sources, interface wave propagation, reflection from buried targets, and echo and signal processing was addressed. The Del Monte Beach site is composed of sand sediments with a mean grain size of 0.2 mm, a horizontal slope of 1:40, with mean tides of 2 meters. Several types of seismic sources were utilized, of both impact and electromagnetic shaker types, which were made to emit toneburst signals to realize pulseecho sonar operation. Receivers consisted of moving coil geophones as well as tri-axial seismometers, and seismometer arrays, all buried in the sediments. Rayleigh wave propagation was measured out to ranges of 130 ft. (40m) and hodograms, Hankel wave plots and range stacked seismograms computed from the transmitted signals’ vertical and horizontal components confirmed the vector wave nature of propagation in elliptical orbits, associated with this type of wave. Both single sources and source arrays were utilized, as were both single geophones and arrays of geophone receivers. It was shown that both propagating and target-reflected Rayleigh waves possessed sufficient spatial coherence to enable formation of seismic sonar beams, thereby reducing reverberation and backscattering from inhomogeneties. Buried targets used in the experiments included a 1,000 lb. Mk 63 naval mine shape, a Mk. 19 anti-tank mine shape, as well as a powder keg and a steel cylinder, both of which could be loaded with lead bricks to study mass loading effects. Signal processing methods included coherent receiver array beamforming, in some cases - coherent subtraction (echo data with target minus that without target), and vector polarization filtering, whereby the complex power of the echo is computed from the vector product of the Hilbert transform of the radial and vertical signal components. The measured complex-power target strengths of the various reflector targets are reported, as are the measured spreading and attenuation loss factors, as well as the performance of the seismic sonar beamforming and signal processing methods. A discussion is given on experience gained with studies of shaker source design, coupling of the sources and receivers to the sediment, the expected ranges for practical parameters of both the environment, the expected targets, false targets, backscattering, as well as the difficulties and the successes of beamforming and signal processing, particularly vector polarization filtering.