Analysis of GPR Polarization Phenomena

Abstract

Polarization is an important consideration in designing GPR data measurement strategy, implementing GPR data processing and performing GPR data interpretation for the following reasons: (1) most ground penetrating radar (GPR) antennas are dipoles that radiate linearly polarized waves, (2) dipole receiving antennas are sensitive to the polarization of waves scattered by buried targets, and (3) certain buried targets scatter waves preferentially depending on the polarization of the incident wave mean that polarization is an important consideration in designing GPR data measurement strategy, implementing GPR data processing and performing GPR data interpretation. The subsurface far field pattern of a dipole GPR antenna is predominately polarized parallel to the long axis of the antenna, although the radiated cross-polarized components become significant away from the broadside direction and GPR polarization measurements are commonly made with at least one receive antenna oriented perpendicular to the transmit antenna. A receive antenna oriented perpendicular to the transmit antenna is sensitive to the cross-polarized components parallel to its long axis in the portion of the scattered field it receives. The cross-polarized vector components may be generated by depolarization at the target surface or simply be comprised of scattered cross-polarized components that were present in the field incident on the target. Knowledge of the depolarization properties of specific targets or sources of clutter can be used to design the optimal GPR antenna configuration for a specific study. Interpretation of GPR polarization data requires that the polarization of the fields incident on the target and the target's location relative to the GPR antennas are known or can be estimated from the data. The polarization scattering properties of two important GPR targets, the flat plane and the long circular cylinder are compared and contrasted. The back scattered field from a flat plane is not depolarized but the back scattered field from a circular cylinder can be significantly depolarized depending on the orientation of the cylinder relative to the incident polarization. A major contributing factor giving rise to depolarization of fields scattered from a flat plane or long circular cylinder is the contrast in amplitude between the parallel polarized and perpendicular polarized scattering coefficients (e.g. Fresnel's coefficients for the case of the flat plane). The greater the amplitude contrast in the scattering coefficients, the greater the potential for depolarization. The polarization sensitivity of a particular GPR antenna configuration depends on the relative positions of the transmitting and receiving antennas, the field patterns of the antennas, and on the depolarization properties of the target. The scattering polarization loss factor (SPIF), defined as the square of the dot product of the incident and scattered unit polarization vectors, can be calculated to analyze the various incidence angles and incident polarizations which produce depolarized scattered waves for a specific target geometry. The scattered-to-incident polarization ratio (SIPR) is introduced as a parameter that is calculated to predict the polarization sensitivity of a particular GPR antenna setup to a scatterer positioned at a known location in the subsurface.