C. Windsor

The Estimation of Buried Pipe Diameters by Generalized Hough Transform of Radar Data

The generalized Hough transform method is applied to the measurement of the diameters of buried cylindrical pipes by Ground Penetration Radar (GPR). 600 MHz radar scans across long pipes, buried in one metre or so of soil, show complex reflection patterns consisting of a series of inverted hyperbolic arcs. The time of flight t(y) as the radar probe is scanned along an axis, y, perpendicular to the pipe, shows an arc whose shape and position depends on 4 unknown variables: y0, the position of the center of the pipe along the scan, z0, the depth of the pipe center, R0, its radius and V0 the velocity in the medium. Analytic expressions for the solution of these variables have been obtained. They use sets of times ti at corresponding positions yi, along the arc, depending on the number of variables to be determined. In the generalized Hough method many such sets of times are chosen randomly from points on the arcs. The results are presented for example as peaks in an accumulator space for each variable. The method is demonstrated for a 0.18 m radius concrete pipe buried at a nominal 1 m depth in a road. Using data acquired at 600MHz frequency (around 0.16m wavelength in soil) the estimated radius was 0.174 ± 0.059 m.

ESTIMATION OF RELATIVE PERMITTIVITY OF SHALLOW SOILS BY USING THE GROUND PENETRATING RADAR RESPONSE FROM DIFFERENT BURIED TARGETS

Combined ground penetrating radar and metal detector equipment are now avail- able (e.g., MINEHOUND, ERA Technology-Vallon GmbH) for landmine detection. The perfor- mance of the radar detector is in∞uenced by the electromagnetic characteristics of the soil. In this paper we present an experimental procedure that uses the same equipment for the detec- tion and calibration by means of signal processing procedures for the estimation of the relative permittivity of the soil. The experimental uncertainties of this method are also reported.

A Data Pair-Labeled Generalized Hough Transform for Radar Location of Buried Objects

A method is presented for isolating the overlapping hyperbolic arcs found when a radar scan is made over several adjacent buried objects. The reflected signal is first converted into a series of data pairs (yj, tj) giving, for a radar antennae position yj along the scan, the times-of-flight tj of the maxima or minima in the reflected radar amplitude. The generalized Hough transform method is extended to record in an associative store the sets of data pairs contributing to each bin in the Hough accumulator space. A cluster of high bins, defining a peak in this space, may then be broken down to reveal its contributing data pairs. This gives the important advantage that a conventional least-squares algorithm can be used to reveal the object position, depth, and radius or velocity. The method is demonstrated on real radar data from buried pipes. The radius of a 0.18-m radius concrete pipe at 1-m depth is estimated at 0.14 m.

RASCAN holographic radar for detecting and characterizing dinosaur tracks

The development of high lateral resolution holographic radar imaging has stimulated new research on mapping of exposed dinosaurs and detection of tracks hidden in the uppermost layers of potential track-bearing rocks, as well as for the characterization of rock features around tracks. This project involves experiments on tracksites, museum specimens, and laboratory models from around the world. Preliminary experiments indicate that holographic radar will be able to provide comprehensive tracksite mapping with objectivity and total non-invasiveness.

Holographic radar: A strategy for uneven surfaces

Holographic radar gives a useful image of sub-surface objects under favorable conditions. However when the surface has undulations of many millimeters, or is broken by stones, or elevated areas of harder material, then images are obtained which may well be a partial representation to the surface height, but may obscure the buried objects under investigation. If these are, for example, anti-personnel mines or Improvised Explosive Devices, the consequences could be serious. A simple possible solution is explored here: that is to cover the rough surface with a smoothed layer of appropriate sand. A simulated mine was buried at 40 mm depth in rough sand containing many hard sandstones, several breaking the surface by a centimeter or so. The image was full of detail correlating with the surface structure and largely obscuring the mine. A layer of sand from the same source was then covered over the rough surface and approximately smoothed by a straight edge. The simulated mine became clear.

Analysis of Time Domain Ultra-Wide-Band Radar Signals Re∞ected by Buried Objects

The aim of this work is the analysis of the signal composition observed in a single radar sweep during an underground investigation with an ultra-wide-band (UWB) radar. The electromagnetic (EM) response of a buried object, the radar pulse spectrum and the antenna set-up, all strongly in∞uence the accuracy of the time of ∞ight estimate. The analysis of the time domain signal will discuss the efiects of the antenna coupling with the ground (flrst arrival pulse from air-soil interface) and the interference of overlapping pulses due to multiple interfaces and multiple re∞ections. The results of this analysis are based on simulations with parameters characteristic of an investigation of layered medium and signal processing schemes to extract information about soil and buried objects composition will be addressed. DOI: 10.2529/PIERS061004063107

Theoretical and experimental analysis of an equivalent circuit model for the investigation of shallow landmines with acoustic methods

Acoustic methods have been recently investigated for the detection of shallow landmines. Some plastic landmines have a flexible case which can made to vibrate by an airborne excitation like a loudspeaker. The soil-mine system shows a resonant behavior which is used as a signature to discriminate from other rigid objects. The mechanical resonance can be detected at the soil surface by a remote sensing systems like a laser interferometer. An equivalent physical model of the mine-soil system has been investigated having the known physical characteristics of mine simulants. The authors designed and built a test-object with known mechanical characteristics (mass, elasticity, damping factor). The model has been characterized in laboratory and the results compared with the classic mass-spring loss oscillator described by Voigt. The vibrations at the soil surface have been measured in various positions with a micro machined accelerometer. The results of the simulations for the acceleration of the soil-mine system agree well with the experiment. The calibrated mine model is useful to investigate the variation of the resonance frequency for various buried depths and to compare the results for different soils in different environmental conditions.

Quantitative interpretation of RASCAN holographic radar response from inclined plane reflectors by a theoretical model

Holographic radar of RASCAN type provides an output signal that is amplitude-modulated by the phase variation between transmitted and received signals. In this work RASCAN radar response is compared with a model by several experiments performed in air with a 4GHz probe scanning a planar metallic reflector inclined at different angles. The radar response shows a series of dark and light stripes with spacing dependent on signal frequency and velocity, and reflector inclination angle. The model is used to interpret the amplitude variations as a function of probe position.

A Single Display for RASCAN 5-frequency 2-polarisation Holographic Radar Scans

The RASCAN holographic radar system has been developed by the Remote Sensing Laboratory of Bauman Moscow Technical University. The present design uses flve frequencies and two polarisations to give 10 distinct images of scan from buried objects. Because of the sinusoidal phase variation of the interference signals, all displays show a complex picture of dark and light phases which vary in a complicated way between difierent frequencies and polarizations. This is a preliminary investigation into the optimal presentation of the 10 images as a single composite image. The objective is to display as much as possible of the information present in the original image. The solution presented here is to sum the absolute values of the background-corrected amplitude over both the flve frequencies and the two polarizations. The method is justifled using an experiment in which nine US pennies, and 9 metal washers, were buried in sand at increasing depths in the range 0 to 56mm. The method is illustrated by example images from the flelds of civil engineering and mine detection.

Comparison of ROC curves for landmine detection by holographic radar with ROC data from other methods

In de-mining or UXO work, classification of subsurface targets into bins such as “mine” vs. “clutter” is critical. So is statistical evaluation of classification accuracy. For radar systems with automated target recognition involving some threshold parameter, one can plot an ROC curve showing the detection rate versus the false alarm rate across a range of threshold values. However, for visual interpretation of images, there is no parameter. Instead, the operator makes a “judgment call” based on training and experience. We propose that for visual interpretation of radargrams, differences in judgment between operators are a proxy for a variable threshold parameter. To test this, we recorded holographic radar images of a test bed containing plastic mine casings and clutter items. Each image contained between 0 and 3 mines, and 3-7 clutter items. University students with no prior training in mine or UXO recognition where given minimal training, and then asked to interpret the images. The detection and false alarm rate for each test subject across all of the images yielded a single point on an ROC curve. In addition, the false alarm rate for each clutter item was determined individually. Based on this, it appeared that rounded rocks are the most frequent false alarm. ROC curves for the “worst operators” were compared to published ROC data from other landmine detection methods, and fall within the range of performance for these other methods - even for testing by trained operators. We propose that in appropriate conditions, the holographic method will provide competitive detection metrics, even by minimally-trained lay-people such as de-miners recruited from within mined communities, and that the described method for developing ROC data can be used to quantify their performance with statistical significance.