Buried object detection with seismic sonar in the surf zone
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Authors
Muir, Thomas
Baker, Steven
Gaghan, Frederick
Fitzpatrick, Michael
Sheetz, Kraig
Subjects
seismic
surf zone
mines
detection
sonar
surf zone
mines
detection
sonar
Advisors
Date of Issue
2016
Date
2016
Publisher
IEEE
Language
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.
Type
Article
Description
Published in: OCEANS 2016 MTS/IEEE Monterey
The article of record as published may be located at http://dx.doi.org/10.1109/OCEANS.2016.7761013
The article of record as published may be located at http://dx.doi.org/10.1109/OCEANS.2016.7761013
Series/Report No
Department
Physics
Organization
Naval Postgraduate School (U.S.)
Identifiers
NPS Report Number
Sponsors
Funder
Format
6 p.
Citation
Distribution Statement
Rights
This publication is a work of the U.S. Government as defined in Title 17, United States Code, Section 101. Copyright protection is not available for this work in the United States.