1. 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.
2. 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.
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. Pat. 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. Pat. 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. Pat. No. 4,922,467, discloses an acoustic detection method is based on comparison of the measured "signature" of the object with the 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.
Geohegan, Jr., et al., U.S. Pat. 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.sub.1 and F.sub.2 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.sub.1 and F.sub.2 frequencies as the radiated signals. The frequencies F.sub.1 and F.sub.2 must be within the resonance frequency of the expected target. A processing algorithm subtracts envelopes of received signals with the frequencies F.sub.1 and F.sub.2 looking at the time-variation of the resulting signal due to a resonance "ringing" effect from resonating target.
Pipkin, U.S. Pat. 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. Pat. No. 3,786,405, discloses a communication system which utilizes a well known parametric sonar, first published in 1968 by Westervelt, and is 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. Pat. 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. Pat. 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.