Ground penetrating Radar
From Wiki Against Mines
The text on this site is published with permission of RAND and taken from "Alternatives of Landmine Detection" Jacqueline MacDonald et.al, RAND report, ISBN 0-8330-3301-8, Document Number: MR-1608-OSTP, Year: 2003
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Ground Penetrating Radar
Description:
GPR detects buried objects by emitting radio waves into the ground and then analyzing the return signals generated by reflections of the waves at the boundaries of materials with different indexes of refraction caused by differences in electrical properties. Generally, reflections occur at discontinuities in the dielectric constant, such as at the boundary between soil and a landmine or between soil and a large rock. A GPR system consists of an antenna or series of antennas that emit the waves and then pick up the return signal. A small computerized signal-processing system interprets the return signal to determine the object’s shape and position. The result is a visual image of the object or an audio signal indicating that its shape resembles a landmine, based on comparison with a mine reference library. The major design control in a GPR system is the frequency of the radio wave. The scale at which GPR can detect objects is proportional to the wavelength of the input signal, so the quality of the image improves as the wavelength decreases and the frequency increases. However, at high frequencies, penetration of the incident wave into the soil can be poor. As a result, the designer must make a tradeoff between quality of the image and required penetration depth. The optimal design for maximizing image quality while ensuring sufficient penetration depth changes with environmental conditions, soil type, mine size, and mine position. Various alternative GPR designs are being explored to optimize the tradeoff between penetration depth and image quality under a wide range of conditions. Also critical in the design of a GPR system are signal-processing algorithms, which filter out clutter signals and select objects to be declared as mines. GPR is a mature technology, but it has not yet been widely deployed for mine detection. GPR was first used in 1929 to measure the depth of an Austrian glacier. The Army tested rudimentary GPR techniques for mine detection in the 1940s. The first commercial GPR systems were developed in 1972. Since then, use of GPR for locating buried objects ranging from utility pipelines to archaeological artifacts has proliferated. Although GPR is well established for these other uses, understanding how different environmental factors and mine characteristics affect its performance is far from complete.
Strengths:
GPR has a number of advantages. First, it is complementary to conventional metal detectors. Rather than cueing exclusively off the presence of metal, it senses changes in the dielectric constant and therefore can find mines with a wide variety of types of casing (not just those with metal). Generating an image of the mine or another buried object based on dielectric constant variations is often possible because the required radar wavelength is generally smaller than most mines at frequencies that still have reasonable penetration depth. Second, GPR is a mature technology, with a long performance history from other applications. Finally, GPR can be made lightweight and easy to operate, and it scans at a rate comparable to that of an EMI system.
Limitations:
Natural subsurface inhomogeneities (such as roots, rocks, and water pockets) can cause the GPR to register return signals that resemble those of landmines and thus are a source of false alarms. In addition, GPR performance can be highly sensitive to complex interactions among mine metal content, interrogation frequency, soil moisture profiles, and the smoothness of the ground surface boundary. However, theoretical investigations indicate that increased soil moisture and interrogation frequency may actually strengthen the return signal for non-metallic mines, but nonuniform soil moisture profiles (e.g., a wet surface and dry subsurface) and rough ground surfaces present difficulties. For the same mine, a given GPR can be very effective or ineffective, depending on soil moisture and mine location; such complex interplays make performance highly variable and difficult to predict. An additional limitation is that unless the GPR system is tuned to a sufficiently high frequency, it will miss very small plastic mines buried at shallow depths because the signal “bounce” at the ground surface (caused by the electrical property differences between air and soil) will mask the return signal from the mine. Finally, the GPR system designer must make a tradeoff between resolution of the return signal and depth, because high-frequency signals yield the best resolution but do not penetrate to depth.
Summary Evaluation:
Current-generation GPR technology has the potential for high performance. In addition, alternative approaches to GPR design have the potential to yield significant advancements over the available systems. However, the ability to model the radar response from different kinds of landmines and natural clutter is essential for yielding the expected performance gains. So far, such modeling is in its infancy. Ideally, GPR systems would be able to provide high-resolution images to a signal-processing system that could decide whether a buried object is a root, rock, clutter object, or landmine. Development of a “library” of clutter signatures is suggest to aid in this task.
Bibliography
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GROUND-PENETRATING RADAR (PAPER II); James Ralston, Anne Andrews, Frank Rotondo, and Michael Tuley, Institute for Defense Analyses, Originally published by the Institute for Defense Analyses, Alexandria, Va.
