MEASURED REFLECTION STRENGTHS OF UNDERWATER PIPES IRRADIATED BY A PULSED HORIZONTAL DIPOLE IN AIR - COMPARISON WITH CONTINUOUS PLANE-WAVE SCATTERING-THEORY

At Delft Geotechnics the technique of ground-penetrating radar is in use of the detection of buried objects such as pipes. In order to give our 'measurements in the field' a more quantitative interpretation, a series of experiments has been started under well-defined conditions. A cylindrical vessel contains water, simulating wet soil. A pulsed transmitting antenna (TA) is mounted above the water-surface irradiating horizontal underwater pipes. The reflected pulses are detected by a horizontal receiving dipole (AP, i.e. 'air-probe') in the air. The reflecting objects used in the experiments are steel pipes, PVC rods and hollow PVC pipes filled with air or water. The depth of the pipes varies from 0.25 to 2 m. The strength of the reflected pulse depends on the type of pipe, its diameter, its depth, the electromagnetic properties of the water and also on the strength and polarization of the incident E-field. The latter is (mainly) parallel to the axis of the pipe in the present experiments. The experimental results have been compared with calculated results using the theory of a continuous plane wave, incident on an infinitely long cylinder in a homogeneous dielectric medium with the same dielectric constant as water. In a previous paper an experiment was described in which a movable receiving dipole in the water measured the transverse, mutually orthogonal E-phi- and E-theta-components of the pulses emitted by the TA. The amplitudes of E-theta versus depth, measured in that paper, are used in the calculations as amplitudes of the incident field. The attenuation of the scattered field is accounted for by assuming exponential damping. The relative amplitudes of the scattered field for different pipes lying at the same depth are reproduced very well by this simple theory. Also the relative values of the scattered amplitudes for a given pipe lying at different depths are described neatly, provided the refraction of the scattered field at the water-air interface is accounted for.