Note: Descriptions are shown in the official language in which they were submitted.
CA 02296510 2000-O1-14
WO 99/04287 PCT/US98/14443
-1-
TITLE: METHOD AND APPARATUS FOR ACOUSTIC
DETECTION OF MINES AND OTHER BURIED MAN-
MADE OBJECTS
S SPECIFICATION
BACKGROUND OF THE INVENTION
FIELD OF THE INVENTION
The present invention generally relates to a method and apparatus for acoustic
detection of buried man-made objects, and more particularly to a method and
apparatus
which emits an acoustic signal comprising one or more frequencies and measures
vibrations of the ground/sediment surface to detect buried objects such as
mines. The
present invention also relates to a method and apparatus which emits an
electromagnetic RF probing signal and sound or vibration signal {modulating
signal),
detects the reflected electromagnetic signal from the buried object, and
processes the
received signal, identifying the modulation caused by vibration.
RELATED ART
The oldest and probably the most common method of locating land mines
involves prodding the ground with a stick or other implement to locate a mine.
Presently, metal detectors are used to detect mines by measuring the
disturbance of
an emitted electromagnetic field caused by the presence of metallic objects in
the
ground. For ferromagnetic objects, magnetometers are employed. These sensors
measure the disturbance of the earth's natural electromagnetic field. Both
types of
detectors cannot differentiate a mine from metallic debris, leading to 100-
1000 false
alarms for each real mine. In addition, most of the modern antipersonnel mines
are
made of plastic with very few or no metal parts, making them undetectable by
metal
detectors.
SUBSTITUTE SHEET (RULE 26)
CA 02296510 2000-O1-14
WO 99/04287 PCT/US98/14443
-2-
New methods for detecting mines involve ground-penetrating radar, infrared
imaging, X-ray backscatter technique, and thermal neutron activation, Gros and
Bruschini, "Sensor technologies for detection of antipersonnel mines" A survey
of
current research and system developments, International Symposium on
Measurement
and Control in Robotics (ISMCR'96), Brussels, May, 1996. These methods {except
the thermal neutron activation) rely on imaging and cannot differentiate a
mine from
rocks or other debris. The drawbacks of the thermal neutron activation
technique,
apart from system complexity, are the limited depth of penetration and the
potential
danger to the operator due to the neutron source.
There are a number of acoustic methods of detecting buried objects such as
mines. One such method is set forth by Don and Rogers, "Using acoustic
impulses to
identify a buried non metallic object" Journal of Acoustical Society of
America, 95(5),
Part 2, 1994, which describes measuring acoustic reflection from an object and
comparing it to a measurement taken at a microphone positioned over a
homogeneous
matrix. Likewise, the following patents provide the examples of the acoustic
detection
methods:
House. et al., U.S. Patent No. 5,357,063, discloses a method and apparatus for
acoustic energy identification of objects buried in soil. This method
identifies a buried
object by viewing the images of the acoustic energy reflected from the soil
and,
therefore, is unable to differentiate a mine from debris with the similar
acoustic
reflectivity.
Rogers. et al., U.S. Patent No. 5,563,848, compares a reflected signal with a
reference signal reflected from the ground where presumably no buried objects
are
located. The differences between these two signals indicates the presence of
an object.
The drawback of this method is that any variations in the physical properties
of the
ground (density, porosity, moisture content, etc.) as well as the presence of
non-target
objects (rocks, tree and grass roots, debris, etc.) will create a difference
from the
reference signal and, consequently, lead to a high rate of the false alarms.
Caulfield, U.S. Patent No. 4,922,467, discloses an acoustic detection method
is based on comparison of the measured "signature" of the object with the
CA 02296510 2000-O1-14
WO 99/04287 PCT/US98/14443
-3-
predetermined and stored reference "signatures." The signature is derived from
the
properties of the object such as acoustic impedance, absorption, velocity and
the
porosity. This method is intended to identify the substance inside an
enclosure and
may work well for detecting and identifying substances in enclosures with
known
acoustical properties such as a suitcase, mail package, etc. However, when the
enclosure is the earth, this method may not work at all because the acoustical
properties of the earth may vary in wide ranges which cannot be predicted.
Therefore,
these unknown variations in the acoustical properties of the "enclosure"
(earth) will
interfere with the determination of the properties of the buried object.
Geohe~an. Jr.. et al., U.S. Patent No. 4,439,485, discloses a sonar system for
identification of certain resonant body target such as mine. The system
radiates two
acoustic signals of different frequencies F, and Fz which are transmitted
toward the
target and the acoustic returns are separated into the component frequencies,
detected,
and thereafter subtracted from one another. A signal above a threshold value
indicates
a resonant body target. The received signals have the same F, and FZ
frequencies as
the radiated signals. The frequencies F, and FZ must be within the resonance
frequency
of the expected target. A processing algorithm subtracts envelopes of received
signals
with the frequencies F, and FZ looking at the time-variation of the resulting
signal due
to a resonance "ringing" effect from resonating target.
Pipkin, U.S. Patent No. 3,705,381, discloses a resonant target sonar system
for
detection and classification of underwater targets. The system broadcasts two
signals:
one is a high frequency signal, and the other one is a low frequency signal
with the
frequency "substantially similar to the resonant frequency of the target."
This patent
searches resonance targets and requires prior knowledge of their resonance
frequencies. Processing of the signal consists of subtraction (in time domain)
of two
high frequency signals reflected from the target: one is reflected from the
target during
the broadcasting resonant low frequency signal, and another one without
resonant
signal.
Au, et al., U.S. Patent No. 3,786,405, discloses a communication system which
utilizes a well known parametric sonar, first published in 1968 by Westervelt,
and is
CA 02296510 2000-O1-14
WO 99/04287 PCT/US98/14443
-4-
aimed to generate narrow beam low frequency sound signals. It radiates two
high
frequency highly directional signals (primary signals) into a nonlinear medium
such
as water. Nonlinear interaction of the primary signals within the water column
generates narrow beam secondary radiation at a difference frequency. This
phenomenon has nothing to do with a target and takes place in the water
column.
Once the secondary signal is formed, it can be used for various applications
as any
other directly radiated signal.
Bealor. et al., U.S. Patent No. 3,757,287, discloses a sea bottom classifying
sonar with several transducers. It broadcasts and receives acoustical signals
with the
same frequency as ordinary sonar.
Moore, U.S. Patent No. 3,603,919, discloses a radar or sonar system including
a continuous spectrum of electromagnetic or compressional wave energy
transmitted
to define a wide band of frequencies.
None of these previous efforts, taken either alone or in combination teach or
suggest all of the elements, nor the benefits and utility of the present
invention.
CA 02296510 2000-O1-14
WO 99/04287 PCT/US98/14443
-S_
OBJECTS AND SUMMARY OF THE INVENTION
It is a primary object of the present invention to provide a method and
apparatus for the detection of the metal and non-metal man-made objects buried
in the
ground or sea sediments.
It is another object of the invention to provide a method and apparatus to
identify specific buried objects.
It is another object of the present invention to provide a method and
apparatus
to detect unexploded ordinances, (or mines, shells, etc.), in various forms,
buried in
earth or in sediment under water.
It is an additional object of the present invention to provide a method and
apparatus for identifying compliant items buried in earth or in sediment under
water.
It is a further object of the present invention to provide a method and
apparatus
for detecting mines by virtue of vibrating the compliant casings of the mines.
It is still a further abject of the present invention to provide a method and
apparatus for causing a casing of a land mine to vibrate, and then to detect
such
vibration to locate a mine.
It is even a further object of the present invention to provide a low
frequency
signal to penetrate the ground and excite vibrations of a buried object.
It is an additional object of the present invention to provide a method and
apparatus for locating buried mines which employs a sensing signal comprising
two
or more frequencies.
It is yet an additional object of the present invention to provide a method
and
apparatus which measures vibrations caused by compliant articles.
It is an additional object of the present invention to provide a method and
apparatus for detecting buried objects which delivers a seismic probe signal
to the
ground.
It is even an additional object of the present invention to provide a method
and
apparatus for detecting buried objects which includes a sensor placed on or
above the
ground for measuring vibration caused by a compliant article.
These and other objects are achieved by the method and apparatus of the
CA 02296510 2000-O1-14
WO 99/04287 PCTNS98/14443
-6-
present invention which employs low frequency waves containing one or more
frequencies for penetrating into ground, water, or sediments and exciting
vibrations of
a buried object. When these sound waves encounter an acoustically compliant
object
such as a mine, the sound waves vibrate the compliant object, which, in turn,
vibrates
against the boundaries of the surrounding medium, such as the ground or
sediment.
This creates a non-linear distortion of the probing signal, including the
generation of
harmonics and acoustic waves with combination frequencies (nonlinear signals).
These nonlinear vibrating signals are received from the surface by means of a
sensor.
The amplitude of the measured nonlinear signals indicates the presence of an
acoustically compliant object such as a mine. The acoustically compliant
object can
be identified when the probe signal includes more than one frequency.
In another embodiment, the present invention employs the effect of modulation
of a probing RF signal by the vibration of a buried object. The invention
employs a RF
probing signal capable of penetrating underground and an acoustic signal.
These
signals are transmitted toward a target. The acoustic signal excites vibration
of the
buried object. Such vibration is much larger for acoustically compliant
objects such
as mines, unexploded ordinance, pipes and other shell-type object, as compared
with
much less compliant solid objects (rocks, tree roots, etc.).
The RF probing signal reaches the object, reflects back and is then received
by
a receiving antenna. Vibration of the compliant objects causes modulation of
the
reflected RF signal. The presence of this modulation serves as an object
discrimination
characteristic, since less compliant non-target objects (rocks, tree roots,
etc.) reflect the
RF signal without the modulation.
CA 02296510 2000-O1-14
WO 99/04287 PCT/US98/14443
BRIEF DESCRIPTION OF THE DRAWINGS
Other important objects and features of the invention will be apparent from
the
following Detailed Description of the Invention taken in connection with the
accompanying drawings in which:
S FIG. 1 is a schematic diagram of an embodiment of the apparatus of the
present invention.
FIG. 2 is a schematic diagram of another embodiment of the apparatus of the
present invention.
FIG. 3 is a schematic diagram of an experimental apparatus used for
conducting experiments according to the present invention.
FIGS. 4a, 4b, and 4c show schematic diagrams and a corresponding graph of
the spectrum level of the difference frequency signal.
FIGS. Sa, Sb and Sc show schematic diagrams and a corresponding graph of
nonlinear frequency responses.
FIG. 6 is a schematic diagram of another embodiment of the apparatus of the
present invention.
FIG. 7 is a schematic diagram of another embodiment of the apparatus of the
present Invention.
CA 02296510 2000-O1-14
WO 99/04287 PCT/US98/14443
_g_
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to a method and apparatus for the acoustic
detection of buried, man-made objects such as mines. A schematic of one
embodiment
of the apparatus of the present invention is shown in the FIG. 1. The
detecting
apparatus is generally indicated at 10. A probe sound signal is emitted by one
or more
sound sources 12 and 14 suspended above the ground. The probe signal can be
created
with a signal generator 16 and a power amplifier 18. Each of the one or more
sound
sources 12 and 14 emits a signal, preferably a finite duration (burst)
sinusoidal signal,
with a given frequencies such as frequencies f, and f2. The sound sources
could be
electric powered (such as loudspeakers, ete.) or air powered (air horns). In
the latter
case the signal generator and the power amplifier can be substituted with a
compressed
gas source. In the embodiment of the invention shown in the FIG. 2, for
example,
wherein the apparatus is generally indicated at 110, the probe signal is
emitted by an
acoustic (seismic) source (sources) 112 located directly on the ground.
The probe signal penetrates the ground and interacts with a compliant buried
object 8 such as a mine. A compliant object is an object whose compliance in
the
specified frequency range is different from the compliance of the surrounding
media.
Mines have shells which are generally compliant. Acoustic energy is used as a
probe
for a compliant object. As a result of the nonlinear interaction at the object-
medium
interface, a signal with combination frequencies f, ~ fz is generated. This
signal, in
turn, causes vibration of the surface of the ground above the buried object.
This
vibration is received with a sensor 20 or 120 and processed by a processor 22
or 122
to extract the signal with the combination frequencies f~ t f2. This signal
can then be
displayed by display 24 or 124. The receiving sensor ZO or 120 could be an
accelerometer (placed on the ground-contact sensor) or a microphone or
ultrasonic (or
laser) vibrometer suspended above the ground. Additionally, it should be
pointed out
that such sensing can be performed remotely. A signal with the combination
frequencies f, ~ f2 exceeding a predetermined threshold level, which is set
during
calibration of the apparatus, indicates the presence of a compliant object 8.
While the
probe signal is in one frequency range, the received signal, or vibration
signal can be
CA 02296510 2000-O1-14
WO 99/04287 PCT/LJS98/14443
-9-
in a different frequency range.
The method of the present invention can be further enhanced by implementing
the measurement of the nonlinear frequency response of the object. The
nonlinear
frequency response can be obtained by sweeping one or both excitation
frequencies f,
and f2 within the range 0f, or by radiating a mufti-frequency signal in the
same range
D~ Observation of the difference frequency f, - fz, while sweeping, for
example f,,
will produce a nonlinear frequency response of the object in the frequency
range 0f.
It was observed experimentally, that a compliant object produces a resonance-
like
response, while non-compliant objects return practically no response at all.
Therefore,
the observation of the resonance-like nonlinear response can be used, in
addition to the
combination frequency observation, to further increase the detection
probability of the
method of the present invention. It was also experimentally observed that the
nonlinear resonance frequency varies for various objects. This, therefore, can
be
additionally utilized for identification of a particular object. Accordingly,
a reference
nonlinear frequency response can be used for object identification. There is
no need
for a reference signal for object detection.
The experimental setup 210, shown in the FIG. 3, employs two signal
generators 216 and 217 respectively, supplying sinusoidal signals with the
frequencies
f, and f2, respectively, the summing and gating devices 232 and 234 forming a
probe
bi-harmonic burst signal. The duration of the burst is cantrolled with a pulse
generator
236. After amplification by means of power amplif er 218, the probe signal
radiates
from a loudspeaker 212 suspended above the ground where the object 8 is
buried. The
vibration of the ground surface is picked up with an accelerometer 220 and
processed
with a spectrum analyzer 244 after the signal is fed through an amp 242.
FIG. 4 shows examples of the spectral component of the difference frequency,
f~ - f2, received from a compliant plastic container (FIG. 4a), a background
level (no
object is buried) (FIG. 4b), and solid steel disk (FIG. 4c). As can be seen,
the level
of the signal from the compliant plastic container is 16 times greater than
the signal
from the solid non-compliant steel disk, as well as the background signal.
CA 02296510 2000-O1-14
WO 99/04287 PCT/US98/14443
-10-
FIG. 5 depicts examples of the nonlinear frequency responses from two
different compliant objects, namely, a four and one half inch plastic
cylindrical
container (FIG. 5a) a four inch steel disk (FIG. 5b) and a four inch solid
steel
container (FIG. 5c). These spectra show that the response from the compliant
containers have nonlinear resonances while the non-compliant steel object
produces
no such resonances.
FIG. 6 shows another embodiment of the present invention wherein two signal
generators 316 feeds source 312 to create a probe signal to vibrate compliant
object 8.
The source 313 emits a high frequency ultrasonic signal to pick up vibration.
The
vibrations are sensed by sensor 314 and fed to signal acquisition 325 such as
an
Ultrasonic Vibrometer, and then fed to signal processing 323 wherein the
signal can
be processed and displayed.
The present invention is based on the effect of nonlinear interactions between
the compliant housing of the buried object and the surrounding media.
Preferably, a
low frequency (below 5000 Hz) air/water-borne or solid-borne sound waves (the
probe
signal) containing two or more frequencies are utilized. This probe signal
penetrates
into the ground/sediments and excites vibrations of the buried object. For
acoustically
compliant objects such as mines (as opposed to stones, solid metal objects,
bricks, etc.,
which are much less compliant) these vibrations lead to "bouncing" of the
object
boundaries against the surrounding medium. The acoustical manifestation of
this
phenomenon is the nonlinear distortion of the probing signal including the
generation
of harmonics and acoustic waves with the combination frequencies (nonlinear
signals).
These nonlinear vibrating signals are picked up from the surface of the
ground/sediments with a sensor. The amplitude of the measured nonlinear
signals
indicates the presence of an acoustically compliant object. This allows for
the
detection of non-metallic objects (e.g. plastic mines and pipes), with non-
sensitivity
to less-compliant objects such as rocks, solid metal objects, tree roots, etc.
The method of the present invention can be practiced in a portable or semi
stationary mode. Basically, the method includes producing an acoustic signal
such as
a sound or seismic acoustic signal which is directed either through water,
air, or
t
CA 02296510 2000-O1-14
WO 99/04287 PCT/US98/14443
-l I-
sediments to the ground and then into the ground where a mine or other
compliant
object may be buried. The acoustic signal can be emitted by means of
loudspeakers,
air horns, or a seismic source or other means known in the art. The signal may
include
more than one frequency component and may include one or more sources for
emitting
S the signal. The signal travels into the ground where it encounters a
compliant object
and causes the compliant object to vibrate. This vibration impacts the
surrounding
medium and causes same to vibrating, creating a nonlinear distortion and
generating
harmonics and acoustic waves. These vibrations signals received by a sensor on
or
above the surface of the ground or other medium. These signals are fed through
a
processor to analyze same for determination of the existence of a compliant
object.
Referring now to FIG. 7, another embodiment of the present invention is
shown. This embodiment of the invention employs a signal generator 416, a
power
amplifier for amplifying the signal and a source 412 for emitting the acoustic
signal.
The acoustic signal vibrates a compliant object 8. An RF signal generator 417
is
employed to create a ground penetrating RF probing signal. The RF probing
signal
is reflected back to a sensor 414 and then fed into a demodulator 425 and then
to a
signal processor 423 such as a computer. The vibration of the compliant object
caused
by the acoustic signal modulates the reflected RF signal to allow this
embodiment of
the invention to serve as an object discriminator. The RF signal can be a
burst
sinusoidal signal and may be synchronously admitted with a RF transmitter also
suspended above the ground. Both acoustic and the RF signal penetrate into the
ground. The acoustic signal excites vibration of the buried mechanically
compliant
target. This vibration cause a phase or frequency modulation of the RF signal
reflected
from the vibrated target. This modulated signal is then received by the
receiver,
demodulated and analyzed to detect the presence of the modulation frequencies.
The
presence of modulation frequencies indicates the presence of the compliant
target such
as a mine.
There could be various modes of operation utilizing the radio-acoustic
modulation effect. One mode may involve a CW air or solid borne signal causing
the
Doppler shift of a reflected RF probing signal. Another mode may employ a more
CA 02296510 2000-O1-14
WO 99/04287 PCT/US98/14443
-12-
complex acoustic signal, such as a dual frequency {frequencies f, and f2)
signal. This
signal, in addition to vibration excitation of the target with the same
frequencies f, and
f2, causes a nonlinear transformation of target vibration into the combination
frequencies f, + fz and f, - f2 due to the nonlinear interaction of the
vibrating target
interface and the surrounding soil. These frequencies will also produce the
modulation
of the RF signal, further enhancing the discrimination capabilities of the
invented
detection technique.
The processing unit of the device of this invention incorporates the means or
steps of demodulation of the received signal by multiplying the received RF
signal by
a reference signal corresponding to the initially radiated RF signal, low-pass
filtering,
and post-processing such as the spectral analysis to identify the presence of
the
modulating frequencies.
Among the advantages of the proposed technique are: the capability to detect
non-metallic objects (e.g. plastic and wooden mines and pipes); non-
sensitivity to
less-compliant objects such as rocks, solid metal objects, tree roots, etc.;
identification
capabilities, since the measured response depends on structural properties of
the
object; and simplicity and low cost.
The invented technique can be utilized as a stand alone device or combined
with existing target detection devices such as ground penetrating RADAR. In
this case
the GPR's, RF transmitting/receiving equipment can be combined to implement
invented technique. This can be a complimentary mode of operation of GPR,
greatly
enhancing its discrimination capability.
Having thus described the invention in detail, it is to be understood that the
foregoing description is not intended to limit the spirit and scope thereof.
What is
desired to be protected by Letters Patent is set forth in the appended claims.
