P. Falorni

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.

Combined holographic subsurface radar and infrared thermography for diagnosis of the conditions of historical structures and artworks

This paper describes a new application of holographic radar for non-destructive testing applied to cultural heritage items. A holographic radar, called RASCAN, operates in continuous wave multi-frequency mode at 4 GHz range and produces images with high in-plane resolution (about 2 cm). Marble items and other stones have been investigated to validate the method to search for subtle cracks, moisture or to unveil details of structures at shallow depth (up to 2 wavelengths). Other important materials are aged wood items that are investigated for tunnels made by insects. Then the holographic radar imaging has been experimentally compared with infrared thermography to understand the advantages and disadvantages of these methods and to derive indications for solving the problem common in all geophysics of inherent non-uniqueness.

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.

Holographic Subsurface Radar as a Device for NDT of Construction Materials and Structures

A relatively rare type of subsurface radar – holographic radar – is described in this paper as a tool for non-destructive testing (NDT) of construction materials and structures. Its principle of operation, advantages and disadvantages are considered. Holographic subsurface radar, operating several discrete frequencies, is used to illuminate a sufficiently extensive area of a surface of opaque dialectical medium to be inspected to register interference between reflected from objects and reference waves. In a lossy media with low level of microwaves attenuation, reconstruction algorithms could be applied for obtaining the subsurface image in such a manner as in optical holography. An attempt is made to highlight significant application areas and problem cases where this type of radar could potentially be applied as a device for NDT of construction materials and structures. The paper describes results of different building surveying including objects of historical heritage. Space shuttle thermal protection system tiles were investigated in some other experiments. Each application area is illustrated by relevant data acquired in laboratory experiments or field tests.

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.

Optical method for the positioning of measurement points

When data analysis is based on imaging techniques, a good correlation between measures and their spatial position is needed. To reach this goal, scanning is commonly performed along a rectangular grid (raster scan) and measures are associated with a local Cartesian (planar) reference. Unfortunately this method reveals inadequate in many situations. For example when the scanned object is not planar, or a uniform sampling grid does not reflect the underlying density of details. The proposed method addresses these and other issues thanks to three features: free scan is enabled in 3D space, each measurement is automatically associated with its position in space and all scanning points can be shown on a picture of the scanned object along with a representation of the measured property. The presented method is particularly useful for high frequency hand held GPR and in general for multisensors measurement heads.