The estimation of buried pipe diameters using ground penetrating radar

The generalised 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 probe is scanned along an axis, y, perpendicular to the pipe, shows an arc whose shape depends on four unknown variables: y0, the position of the centre of the pipe along the scan, z0, the depth of the pipe centre, R, its radius and V the velocity in the medium. Analytic expressions for the solution of these variables have been obtained. They use sets of between 1 and 4 times ti at corresponding positions yi, along the arc, depending on the number of variables to be determined. In the generalised 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 600 MHz frequency (around 0.16 m wavelength in soil) the estimated radius was 0.174 ± 0.059 m.