KR100552931B1 - Method and apparatus for acoustic detection of mines and other buried artifacts - Google Patents

Abstract

An apparatus 10 is provided that uses an acoustic signal having one or more frequencies to penetrate the ground, water or deposits and to vibrate the compliant buried object 8. When these acoustic signals encounter an acoustically compliant object 8, such as a mine, they generate vibrations around the boundary of the surrounding medium, such as ground deposits, of the compliant object 8, creating a nonlinear distortion of the probing signal. Nonlinear distortion produces harmonics and acoustic waves having a combined frequency (nonlinear signal). This nonlinear vibration signal is received from the ground surface by the sensor 20. The amplitude of the measured nonlinear signal indicates the presence of an acoustically compliant object such as a land mine. The present invention also relates to a method and apparatus for radiating electromagnetic RF signals and acoustic or vibration signals (modulation signals), detecting electromagnetic signals reflected from buried objects, and processing received signals to identify modulations caused by vibrations. Related.

Description

Method and Apparatus for Acoustic Detection of Landmines and Other Buried Man-made Objects {Method and Apparatus for Acoustic Detection of Mines and Other Buried Man-made Objects}

FIELD OF THE INVENTION The present invention generally relates to acoustic detection methods and apparatus for artifacts, in particular to detecting buried objects such as mines by emitting acoustic signals comprising one or more frequencies and measuring vibrations of the ground / sediment surface. The invention also emits electromagnetic RF probing signals and acoustic or vibration signals (modulation signals), detects electromagnetic signals reflected from buried objects, and processes the reflected signals to generate the vibrations. Identifies the modulation.

The oldest and most common way of locating landmines is to pin the ground with a stick or other tool to locate them. Currently, metal detectors are used to detect land mines by measuring disturbances in the radiated electromagnetic fields caused by the presence of metallic objects underground. Magnetometers are used for ferromagnetic objects. These sensors measure the disturbances of the Earth's natural electromagnetic field. Both types of detectors do not distinguish between metallic debris and landmines, which actually triggers 100-1000 false alarms per mine. In addition, most modern antipersonal mines are made of plastic with little or no metal parts, making them undetectable by metal detectors.

New methods for detecting landmines include ground-penetrating radar, infrared imaging, X-ray backscatter technique, and Gros and Bruschini's "Sensor technologies for detection." of antipersonal 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 thermal neutron activation) rely on imaging techniques and do not distinguish mines from rocks or other debris. A disadvantage of thermal neutron activation techniques is that, apart from the complexity of the system, the penetration depth is limited and there is a potential risk for the operator due to the neutron source.

There are many acoustic methods for detecting buried objects such as land mines. One such method is the "Using Acoustic impulses to identify a buried non metallic object" published by Don and Rogers, Journal of Acoustical Society of America, 95 (5), Part 2, 1994, which measures acoustic reflections from objects. It is disclosed to compare it with the measurement measured with a microphone located on a homogeneous matrix. Likewise, the following patents provide examples of acoustic detection methods.

US Pat. No. 5,357,063 to House et al. Discloses a method and apparatus for identifying acoustic energy of an object embedded in the ground. This method identifies buried objects by looking at the image of acoustic energy reflected from the ground, and thus cannot distinguish mines from objects with similar acoustic reflectivity.

US Pat. No. 5,563,848 to Rogers et al. Compares the reflected signal with a reference signal reflected from the ground that is supposed to be buried. The difference between these two signals indicates the presence of an object. The disadvantage of this method is that not only objects that are not targets (rocks, trees and grass roots, debris) but also any changes in the physical properties of the ground (density, porosity, moisture content, etc.) produce differences from the reference signal, resulting in false alarms. The rate of false alarm is high.

Caufield's US Pat. No. 4,922,467 discloses an acoustic detection method based on a comparison of a measured "signature" of an object with a predetermined and stored reference "signature." The signature is obtained from properties such as acoustic impedance, absorption, velocity and porosity. This method identifies materials inside the enclosure and is effective in detecting and identifying materials inside the enclosure with known acoustic properties such as suitcases and mail boxes. However, if the enclosure is soil, this method may be ineffective at all because the acoustic properties of the soil can vary over an unpredictable range. Therefore, unknown changes in the "enclosure" (soil) acoustic properties interfere with the determination of the properties of the buried object.

U.S. Patent 4,439,485 to Geohegan, Jr. discloses a sonar system for identifying a specific resonant body target, such as a land mine. The sonar system emits two acoustic signals having different frequencies F ₁ and F ₂ , which are transmitted towards the target, and acoustic returns are separated by component frequency and detected. Then they are deducted from each other. A signal above the threshold indicates a target with a resonant body. The received signal has the same F ₁ and F ₂ frequencies as the emitted signal. The frequencies F ₁ and F ₂ shall be within the resonant frequencies of the expected target. The processing algorithm subtracts the envelope of the received signal with frequencies F ₁ and F ₂ , noting the time variation of the resulting signal due to the resonance “ringing” effect from the resonance target. .

Pipkin's US Pat. No. 3,705,381 discloses a resonant sonar target system for detection and identification of underwater targets. The sonar system emits two signals, one of which is a high frequency signal and the other of which is a low frequency signal having a frequency substantially similar to the resonance frequency of the target. This patent searches for resonance targets and requires prior knowledge of their resonance frequencies. The processing of the signal includes the subtraction (in time domain) of two high frequency signals reflected from the target, one reflected from the target while radiating the resonance low frequency signal, and the other reflected without the resonance signal. .

U.S. Patent No. 3,786,405 to Au et al. Utilizes a well-known parametric sonar, first published by Westervelt in 1986, to produce a narrow beam low frequency sound signal. A target communication system is disclosed. It radiates two highly directional high frequency signals (first signal) to a nonlinear medium such as water. Nonlinear interaction of the first signal in the water column produces a narrow beam second radiation of a different frequency. This phenomenon is not related to the target and occurs in the order. Once the second signal is formed, it can be used for a variety of applications, such as other signals emitted directly.

US Patent No. 3,757,287 to Bealor et al. Discloses a sea bottom classifying sonar having a plurality of transducers. It emits and receives acoustic signals of the same frequency as a conventional sonar.

Moore's US Patent No. 3,603,919 discloses a radar or sonar system that includes a continuous spectrum of transmitted electromagnetic or compressed wave energy to define a wide range of frequencies.

None of the above attempts explain or suggest all the elements, advantages, and usefulness of the present invention, even when each of them is considered alone or in combination.

It is a primary object of the present invention to provide a method and apparatus for detecting metal and nonmetallic artifacts embedded in underground or underwater deposits.

Another object of the present invention is to provide a method and apparatus for identifying a particular buried object.

It is another object of the present invention to provide a method and apparatus for detecting various types of pyrotechnics (or mines, shells, etc.) embedded in underground or underwater deposits.

It is another object of the present invention to provide a method and apparatus for identifying compliant items embedded in underground or underwater deposits.

It is an object of the present invention to provide a method and apparatus for detecting landmines by vibrating the compliant casing of the landmines.

It is an object of the present invention to provide a method and apparatus for vibrating a casing of landmines and detecting such vibrations.

It is an object of the present invention to provide a low frequency signal for exciting the vibration of an embedded object through the ground.

It is an object of the present invention to provide a method and apparatus for discovering buried mines using sense signals comprising two or more frequencies.

It is an object of the present invention to provide a method and apparatus for measuring vibrations generated by a compliant article.

It is an object of the present invention to provide a method and apparatus for detecting a buried object by transmitting a vibration probe signal to the ground.

It is an object of the present invention to include a method and apparatus for detecting a buried object, including a sensor in contact with or located on the ground to measure vibrations generated by a compliant object.

These and other objects are achieved by the method and apparatus of the present invention using low frequencies comprising one or more frequencies for exciting the vibrations of buried objects through ground, water or deposits. When such sound waves encounter an acoustic compliant object such as a mine, the sound wave vibrates the compliant object and then vibrates the boundary with the surrounding medium such as the ground or the deposit. This results in nonlinear distortion of the probe signal, which produces harmonics and acoustic waves having a combinational frequency (nonlinear signal). These nonlinear vibration signals are received from the ground through the sensor. The amplitude of the measured nonlinear signal indicates the presence of an acoustically compliant object such as a land mine. An acoustically compliant object can be identified if the probe signal contains one or more frequencies.

In another embodiment, the present invention takes advantage of the modulation effects of the probing RF signal by the vibration of the buried object. The present invention uses a probing signal that can penetrate underground and sound wave signals. These signals are sent towards the target. The sonic signal excites the vibration of the buried object. These vibrations are much higher for acoustically compliant objects such as mines, pyrotechnics, pipes and other shell-type objects compared to solids (rocks, tree roots, etc.) with very low compliant properties.

The RF probing signal reaches the object, is reflected and received by the receive antenna. Vibration of the compliant object causes modulation of the reflected RF signal. Non-compliant targets (rock, tree roots, etc.) reflect RF signals without modulation, so the presence of this modulation serves as an object identification feature.

Other important objects and features of the present invention will become apparent from the following detailed description of the invention taken in conjunction with the accompanying drawings.

1 is a schematic diagram of an embodiment of the apparatus of the present invention.

2 is a schematic diagram of another embodiment of the apparatus of the present invention.

3 is a schematic diagram of an experimental apparatus used to perform an experiment according to the present invention.

4A, 4B and 4C are schematic diagrams and corresponding graphs of spectral levels of signals having different frequencies.

5A, 5B and 5C are schematic diagrams and corresponding graphs of nonlinear frequency responses.

6 is a schematic diagram of another embodiment of the apparatus of the present invention.

7 is a schematic diagram of another embodiment of the apparatus of the present invention.

The present invention relates to a method and apparatus for acoustic detection of buried artifacts such as land mines. 1 shows a schematic of an embodiment of the device of the invention. The detection device is generally indicated by reference numeral 10. The probe sound wave signal is emitted by one or more sound sources 12 and 14 suspended on the ground. The probe signal may be generated by the signal generator 16 and the power amplifier 18. Each of the one or more sound sources 12, 14 emits a signal of a predetermined frequency, such as f ₁ and f ₂ , preferably a sine wave of finite duration. The sound source can be powered by electricity (loudspeakers, etc.) or air (air horn). In the latter case, the signal generator and power amplifier can be replaced with a compressed gas source. For example, in the embodiment of the invention in which the apparatus of the present invention is shown in FIG. 2, denoted by reference numeral 110, the probe signal is emitted by a sound source 112 located directly on the ground.

The probe signal penetrates the ground and interacts with a compliant buried object 8 such as a land mine. Compliant objects are objects whose response to a particular frequency range is different from the response of the surrounding medium. Landmines generally have a compliant shell. Acoustic energy is used as a probe for compliant objects. As a result of the non-linear interaction at the object-medium interface, signals with combined frequencies f ₁ and f ₂ are produced. This signal then causes vibrations of the earth's surface on the buried object. This vibration is received by the sensor 20 (FIG. 1) or the sensor 120 (FIG. 2) and processed by the processor 22 (FIG. 1) or the processor 122: FIG. 2 to combine frequencies f ₁ ± Extract the signal of f ₂ . This signal can then be displayed on the display device 24 (FIG. 1) or on the display device 124 (FIG. 2). The receiving sensor 20 (FIG. 1) or the receiving sensor 120 (FIG. 2) may be an accelerometer (located above the ground-contact sensor) or a microphone or ultrasonic (or laser) oscillometer suspended on the ground. May be). It should also be noted that the sensing can be performed remotely. A signal of combined frequency f ₁ ± f ₂ above a predetermined threshold level, where the threshold level is set during the calibration of the device, indicates the presence of the compliant object 8. While the probe signal is in one frequency range, the received signal or vibrating signal may be in another frequency range. The method of the present invention is improved by performing a measurement of the nonlinear frequency response of the object. The nonlinear frequency response can be obtained by sweeping either or both of the excitation frequencies f ₁ and f ₂ within the Δf range or by radiating a multi-frequency signal within the same Δf range. For example, the difference frequency while sweeping the f ₁ (difference frequency) f ₁ -f ₂ when observed, a non-linear frequency response of the frequency range △ f of the object is generated. It has been observed experimentally that compliant objects produce a resonance-like response while non-compliant objects do not actually return any response. Therefore, observation of the similar resonance response can be used to further increase the detection probability of the method of the present invention, in combination with observation of the combined frequency. It has also been experimentally observed that the nonlinear resonant frequency varies with various objects. Therefore it can be used for identification of a particular object, and thus a reference nonlinear frequency response can be used for object identification. No reference signal is required for object detection.

The apparatus 210 shown in FIG. 3 includes two signal generators 216 and 217 for supplying sinusoids of frequency f ₁ and f ₂ , respectively, and a summation to form a probe bi-harmonic bust signal. Device 232 and gating device 234 are used. The duration of the burst is controlled by pulse generator 236. After amplification by the power amplifier 218, a probe signal is emitted from the loudspeaker 212 suspended on the ground on which the object 208 is embedded. The vibration of the ground is picked up by the accelerometer 220 so that the signal passes through the amplifier 242 and is processed by the spectrum analyzer 244.

4A-4C are examples of spectral components of the differential frequencies f ₁ -f ₂ received from a plastic container (FIG. 4A), a background level (no object buried) (FIG. 4B), and a solid steel disc (FIG. 4C). It is shown. As shown, the level of the signal from the compliant plastic container is 16 times greater than the signal from the solid non-compliant steel disc as well as the background signal.

5A-5C are examples of nonlinear frequency responses from three different compliant objects, namely a 4.5 inch plastic cylindrical container (FIG. 5A), a 4 inch steel disc (FIG. 5B) and a 4 inch solid steel container (FIG. 5C). It is shown. These spectra show that non-compliant steel objects have no non-linear resonance, while the response from the compliant container has non-linear resonance.

FIG. 6 illustrates another embodiment of the invention in which one of the two signal generators 316, 317 connects to a source 312 and generates a probe signal to vibrate the compliant object 8. It is shown. Source 313 picks up vibrations by radiating high frequency ultrasonic signals. The vibration is sensed by the sensor 314 and supplied to a phase demodulator 325 such as an ultrasonic vibrometer, and then to a signal processing device 323 where 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 medium. Preferably, a low frequency (less than 5000 Hz) air / water-borne sound or solid-borne sound wave containing two or more frequencies is used. This probe signal penetrates the ground / sediment and excites the vibration of the buried object. In acoustically compliant objects such as landmines (as opposed to rocks, solid metals, bricks, etc., which have much less compliant properties), these vibrations cause bouncing of the object's boundaries with respect to the surrounding medium. When this phenomenon is acoustically represented, this phenomenon results in nonlinear distortion of the probing signal, including the generation of acoustic waves (nonlinear signals) having harmonics and combined frequencies. This nonlinear vibration signal is picked up by the sensor from the surface of the ground / sediment. The amplitude of the measured nonlinear signal indicates the presence of an acoustically compliant object. This prevents the detection of less compliant objects such as rocks, solid metals, tree roots, etc., and the detection of nonmetallic objects (eg, plastic mines and pipes).

The method of the present invention can be implemented in a portable or semi-stationary mode. Basically, the method involves generating an acoustic signal, such as a sonic or vibrational acoustic signal, where the acoustic signal penetrates water, air, or ground sediment to the ground where landmines or other compliant objects may be buried. do. The acoustic signal may be emitted by loudspeakers, air horns or seismic sources or other known means. The signal may include one or more frequency components and may include one or more sources that emit a signal. The signal is transmitted to the ground where it encounters a compliant object to vibrate the compliant object. These vibrations collide with the surrounding medium and cause the same vibrations, producing nonlinear distortions and generating harmonics and acoustic waves. This signal is received by sensors located on or in contact with the ground or other medium. These signals are supplied to the processor and analyzed to determine the presence of a compliant object.

In Fig. 7, another embodiment of the present invention is shown. This embodiment of the present invention uses a signal generator 416, a power amplifier 418 that amplifies the signal, and a sound source 412 that radiates the acoustic signal. The acoustic signal vibrates the compliant object 8. RF signal generator 417 is used to generate a ground-transmitting RF probing signal. The RF probing signal is reflected and returned to the sensor 414 to be supplied to the demodulator 425 and then to a signal processing device 423 such as a computer. Vibration of the compliant object generated by the acoustic signal modulates the reflected RF signal, making this embodiment of the present invention available as an object discriminator. The RF signal may be a burst sinusoidal signal, and may be allowed in a synchronous manner by using a transmitter suspended on the ground. Both acoustic and RF signals penetrate the ground. The acoustic signal excites the vibration of the embedded mechanically compliant target. This vibration generates phase or frequency modulation of the RF signal reflected from the vibrating target. This modulated signal is then received by the receiver, demodulated and analyzed to sense the presence of a modulation frequency. The presence of a modulation frequency indicates the presence of a compliant target such as a land mine.

There may be various modes of implementation using radio-acoustic modulation effects. One mode may involve CW air or a solid borne signal that generates a Doppler shift of the reflected RF probing signal. Other modes may use more complex acoustic signals, such as dual frequency (frequency f ₁ and f ₂ ) signals. This signal not only vibrates the target at the same frequencies f ₁ and f ₂ , but also nonlinear conversion of the target vibration to the combined frequencies f ₁ + f ₂ and f ₁ -f ₂ by the nonlinear interaction of the vibrating target interface with the surrounding medium. Let's do it. These frequencies produce modulation of the RF signal, further improving the identification of the detection technique of the present invention.

To identify the presence of the modulation frequency, the processing unit of the apparatus of the present invention multiplies the received RF signal with a reference signal corresponding to the initially radiated RF signal, low-band-pass filtering, and then spectral analysis Means for demodulating the received signal via post-processing, such as the like.

Advantages of the techniques presented herein include: the ability to detect non-metallic objects (eg plastics, wood mines and pipes); No response to objects with less compliant properties such as rocks, solid metal objects, tree roots, and the like; Identification of the measured response depending on the structural properties of the object; And simplicity and low cost.

The technique of the present invention can be used as a stand alone device or in combination with existing target detection devices such as ground penetrating radars. In this case, GPR and RF transmission / reception equipment may be combined to implement the techniques of the present invention. This may be a complementary mode of operation of the GPR and may greatly improve the identification function.

Although the present invention has been disclosed in detail, the above disclosure is not intended to limit the scope of the present invention. Matters to be protected by the patent are set forth in the claims below.

Claims (32)

A device for remotely detecting land mines in a compliant housing embedded underground, Signal generating means for generating probe signals having first and second frequencies; Source means for outputting the probe signal; Receiving means for receiving a non-linear frequency response vibration signal having a third frequency generated by the probe signal oscillating the boundary of the compliant object with respect to the medium surrounding it; And Processing means for processing the vibration signal Remote land mine detection device comprising a. The method of claim 1, And a display means for displaying the vibration signal. The method of claim 2, And the processing means compares the vibration signal with a predetermined signal level and issues an alarm when the vibration signal exceeds the predetermined signal level. The method of claim 1, Remote mine detection device wherein said source means comprises one or more loudspeakers. The method of claim 1, Remote mine detection device wherein the source means comprises one or more air horns. The method of claim 1, And a seismic source for the source means to vibrate the ground directly. The method of claim 1, And a remote mine detection device wherein said source means comprises an underwater sound source. The method of claim 1, And a remote mine detection device in which the receiving means is placed in contact with the ground. The method of claim 1, And the probe signal comprises a single frequency for detecting a compliant object. The method of claim 1, And the probe signal comprises more than one frequency for identifying a compliant object. An apparatus for remotely detecting buried mines in a compliant container, Signal generating means for generating a signal having first and second frequencies; Source means for outputting said generated signal in contact with or above ground; Means for receiving a vibration signal in contact with or on the ground, the vibration signal having a third frequency equal to the difference between the first and second frequencies, wherein the vibration signal causes the compliant buried object to adhere to the surrounding medium. Receiving means having a third frequency signal caused by the generated signal to vibrate against; And Processing means for processing the vibration signal Remote land mine detection device comprising a. The method of claim 11, And a power amplifying means for amplifying the generated signal. The method of claim 11, And a display means for displaying the vibration signal. The method of claim 11, And the processing means compares the third frequency vibration signal with a predetermined signal level and issues an alarm when the vibration signal exceeds the predetermined signal level. The method of claim 11, Remote mine detection device wherein said source means comprises one or more loudspeakers. The method of claim 11, Remote mine detection device wherein the source means comprises one or more air horns. The method of claim 11, Remote mine detection device, said source means comprising an earthquake source for directly vibrating the ground. The method of claim 11, Remote mine detection device wherein the source means comprises an underwater sound source. The method of claim 11, A remote land mine detection device wherein said receiving means is positioned in contact with the ground. In the method of remotely confirming the position of the compliant landmine in the compliant container, Generating a probe signal having at least first and second frequencies; Radiating the probe signal from a source; Generating a vibration signal by vibrating a compliant object relative to a medium around the probe signal by the probe signal to generate a nonlinear vibration signal having a third frequency; Receiving a non-linear vibration signal generated by bouncing a probe signal on the boundary of a compliant object to a medium of interest; And Processing the nonlinear vibration signal Landmine positioning method comprising a. The method of claim 20, And comparing the amplitude of the vibration signal with a predetermined signal level. The method of claim 20, Generating the probe signal comprises generating two separate signals having different frequencies. The method of claim 22, And wherein the nonlinear vibration signal is in a different frequency range than the probe signal. An apparatus for detecting a compliant buried object, Signal generating means for generating probe signals having first and second frequencies to vibrate the compliant buried object; RF signal generating means for generating an RF signal; Receiving means for receiving an RF signal after the RF signal is modulated by a vibration embedding object; And Processing means for processing a signal received by said receiving means Compliant embedded object detection device comprising a. The method of claim 24, And the signal generating means includes power amplifying means for amplifying the probe signal. The method of claim 25, And the signal generating means generates a dual frequency signal. The method of claim 26, And an additional demodulation means for demodulating the received signal. The method of claim 27, And the RF signal generating means generates a ground transmission radar (RADAR) signal. In the method of confirming the position of the buried object, Generating a probe signal; Oscillating a target by directing the probe signal to a target; Generating a second signal; Directing the second signal to the target such that the vibrating target modulates the second signal; And Receiving the modulated second signal by a receiving means Buried object positioning method comprising a. The method of claim 29, And generating the second signal comprises generating an RF signal using an RF signal generator. The method of claim 30, And generating the RF signal comprises generating a ground transmission radar (RADAR) signal. The method of claim 31, wherein And demodulating the modulated second signal.

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