Remote optical seismic surveying and detection and imaging of underground objects

#The disclosed technique relates to optical seismic surveying, in general, and to methods and systems for remotely surveying an area of interest using seismic waves and optics and for detecting and imaging underground objects, in particular.

#Underground threats in the context of war and military activities refer to objects and tactics employed in attacking an enemy force which make use of the ground as a camouflage for concealing these threats. For example, underground threats can include landmines, buried explosives, booby traps placed within the ground, underground tunnels, covered holes and pits, roadside charges against convoys and the like. Underground threats can also include changes in ground composition which may pose a threat to heavy military equipment and military personnel, such as muddy terrain, swamps, quicksand and the like. With the blurring of the boundary between straight out war and guerilla or terrorist activity, underground threats have increased in number over the years as they are in general easily acquired and built (such as in the case of landmines or explosives) yet difficult to detect. In addition, underground threats are very difficult to neutralize in a military context as underground threats need to be detected in real-time without the knowledge of an expert.

#One known method for detecting underground objects is underground imaging using techniques taken from the fields of mineral and oil exploration. In such techniques, a geological survey is taken of an area or region of interest. Based on the geological survey, an image of the ground and what lies beneath can be reconstructed and objects or threats in the ground can be determined. In general, geological surveys are major operations which take a significant amount of time to set up and complete. Also, field experts are usually required to read the geological surveys and interpret the data to determine what objects lie in the ground beneath.

#The disclosed technique will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which:

#FIG. 1 is a schematic illustration of a system for detecting and imaging underground objects, constructed and operative in accordance with an embodiment of the disclosed technique;

#FIG. 2A is a schematic side view illustration of the system of FIG. 1 being used to detect and image underground objects, constructed and operative in accordance with another embodiment of the disclosed technique;

#The disclosed technique overcomes the disadvantages of the prior art by providing a novel system and method for seismic surveying. The novel method and system enables a seismic map to be remotely generated for an area of interest without having to substantially be in physical contact with the ground of the area of interest. In one embodiment of the disclosed technique, a seismic source for providing a seismic wave is in physical contact with the ground of the area of interest. Using the generated seismic map, an image of the area of interest can be generated and underground objects of varying sizes can be detected. According to the disclosed technique, the area of interest is illuminated by a laser beam, or an array of laser beams, such that a matrix of laser spots is formed on the ground of the area of interest. Alternatively, the laser beam may be scanned across the ground. At least one seismic wave is then provided to the area of interest which modifies the matrix of laser spots. A detector then receives and detects reflections of the laser spots from the ground of the area of interest as a modified speckle pattern. From changes between the laser spots transmitted to the ground and reflected from the ground, variations in the phase of the laser spots can be determined. These variations are due to the at least one seismic wave traversing the ground. The variations in phase of the laser spots are then used to determine the amplitude and phase of the at least one seismic wave. Due to the presence of a matrix of laser spots, a complete seismic map of the area of interest can be determined in real-time according to the disclosed technique. The seismic map may be a single point, a 1D map or a 2D map of the area of interest. In addition, according to the disclosed technique, the seismic map of the area of interest can be used to generate an image of the area of interest and can thus be used to detect and determine the presence of underground objects in the area of interest. The disclosed technique can thus be referred to as a technique for optical seismic surveying. As mentioned above and described below, a seismic wave is propagated through an area of interest using known procedures, yet the detection of the seismic response of the area of interest is determined by optically imaging the surface of the area of interest using a laser source.

#According to one embodiment of the disclosed technique, the detector is a high speed camera. In this embodiment, a detector array in the high speed camera is divided into a plurality of segments. Each segment substantially determines changes in phase of a different single laser spot on the ground of the area of interest. Processing the changes in phase of each laser spot thereby enables properties of the seismic wave to be determined or reconstructed. According to another embodiment of the disclosed technique, the detector is a single detector with a high speed moving grating positioned before the opening of the detector. The received speckle pattern of the laser spots is substantially modulated due to the presence of the high speed moving grating before the single detector. According to this embodiment, the modulated speckle pattern can be demodulated and properties of the seismic wave can be determined or reconstructed. According to a further embodiment of the disclosed technique, a substantially large area of interest can be imaged by a matrix of laser spots in real-time which is then used to determine properties of a propagated seismic wave in the area of interest. According to the disclosed technique, the seismic wave can be generated by a controlled or active seismic source, such as a large hammer repeatedly striking the ground. In addition, the seismic wave can be generated by an uncontrolled or passive seismic source, such as background seismic activity of the ground of the area of interest. According to the disclosed technique, the detector can determine differences in the speckle pattern of the reflected matrix of laser spots even when the speckle pattern is modified by only background seismic activity. This embodiment thereby obviated the need for an active seismic source to generate a seismic map for an area of interest. According to another embodiment of the disclosed technique, underground objects can be detected by determining changes in the properties of the propagated seismic wave over time. Different types of objects change the properties of the propagated seismic wave in different ways. According to this embodiment, specific frequencies of the propagated seismic wave as a function of time can be used to determine the size of underground objects. According to the disclosed technique, the size of underground objects is inversely proportional to the frequency of the propagated seismic wave in time such that lower frequencies indicate substantially large objects and higher frequencies indicate substantially small objects.

#Throughout the description, the expression “area of interest” is used to denote an area or region of interest in which properties of a seismic wave propagating through the area of interest are to be determined. An area of interest herein substantially represents a volume of interest that includes the ground surface of the area of interest as well as what is beneath the ground surface. Therefore an area of interest may include underground objects. In addition, the expression “underground objects” as used in the description of the disclosed technique can refer to physical objects in the ground, such as rocks, ore, landmines, buried charges, powder kegs and the like. Underground objects can also refer to changes in ground composition, such as from rock to sand, or to open spaces, like covered holes and pits as well as to tunnels, cavities, caves, bunkers, manholes and air pockets in the ground. Underground objects can also be referred to as underground anomalies or underground abnormalities. In general, according to the disclosed technique, underground objects of varying sizes can be determined, in particular objects as small as 5 centimeters in length (such as rocks or very small landmines) or as large as 50 meters in length (such as underground tunnels). In addition, underground objects may refer to any object underground having a seismic impedance that is detectably different than the seismic impedance of the ground surrounding the underground object.