Classification of Buried Objects from Series of Aligned Hyperbolic Arcs or "pendants" in Radar Scans: Estimating a Buried Pipe Diameter.

In our experiments, 600 MHz radar scans across long parallel objects, such as pipes, buried in one metre or so of soil, show complex reflection patterns consisting of a series inverted hyperbolic arcs characteristic of the object. A classification of the objects has been achieved by an analysis of the arcs, which gives rise to a series of “apex” points defining the lateral position (y) and depth (z) and amplitude of each arc. For objects of size larger or comparable with the wavelength (20cm), several points with alternating positive and negative phases are obtained. Code has been written to associate series of apexes which may all arise from the same object. For example these should all lie within a specified vertical area, and which have appropriately spaced depths, with each ripple having the correct alternating phase. The relative intensities of these apexes provide the necessary features for classification by, for example, a neural net. The method is simulated using two-dimensional examples provided by pipes buried under a road. Different pipes can be identified and readily separated from small objects giving background-scattered signals. Section 1. Radar reflections from buried pipes large compared to the wavelength Radar has now become a standard method for detection of buried pipes. (1) A sharp radar pulse is emitted close to the ground. The radar waves penetrate readily into a metre or so of ground. Any surfaces from buried pipes or other objects, whose dielectric constant differs appreciably from the ground, will reflect some of the incident radiation and can be detected by a receiver. The range of the object can be calculated from the time difference between the reflected signal and the incident pulse, and the propagation velocity of the microwaves in the ground. For a small pipe at a position (y0 , z 0) whose diameter is small compared with the typical wavelength of 20cm, the range d follows the hyperbolic form d=[(y-y0) 2 + zo 2 ] 1/2 shown by the lower dashed line in figure 1. If the object size is larger, for example a buried pipe of diameter D , there are two effects. Firstly, as illustrated in figure 2, the initially narrow signal pulse suffers multiple reflections from the surfaces of the pipe. The result obtained by summing of this series of reflections is a broadened wavelet whose shape is characteristic of the object. Secondly the range of the reflection from the upper surface of the pipe will be reduced to d=[(y-y0) 2 + zo 2 ] 1/ 2 - D/2 and have the non-hyperbolic form shown by the upper dark curve in figure 1. The more flattened apex of the curve is clearly seen in the middle light curve of figure 1 which shows the signal from a pipe at depth zo + D/2, where the apex range is the same as for the point-like object.