Experimental and Numerical Demonstration of Anomalous Enhanced Backscatter by Subsurface Spherical Dielectric Anomalies

Abstract:

Anomalous signal enhancement appears in ground-penetrating radar (GPR) profiles recorded over electrically resistive ground containing dielectric anomalies. Examples of the ground matrix include sands, glacial till, sub-lacustrine diamicton, glaciers and even icy moons. The likely anomalies are cylindrical or spherical water-filled conduits, boulders or saturation bulbs. In such cases refractive focusing within the anomaly may be the cause, whereby waves propagating along the curved surface continually refract along short cuts through the anomaly, which results in enhanced backscatter that emanates from the near the anomaly base. Although Mie scattering and optical ray path propagation of this phenomenon are well understood, the associated backscatter attributes of amplitude, dispersion and phase are not conceptually obvious from the mathematics. Here we try to provide some understanding of these attributes with theory, laboratory experiments with GPR pulses and higher permittivity spheres buried in lower permittivity sand, and numerical models that simulate the experiments. Our experiments clearly show amplitude enhancement of waves whose time delay is predicted by the surface wave explanation, yet with an unexpected reversal of waveform phase polarity. Theoretically we provide a geometrical model that predicts the position of the focal point as a function of the geometric and dielectric parameters, which we evaluate to find those that cause the focus to occur inside the sphere, and the unexpected phase change.