Evaluating GPR polarization effects for imaging fracture channeling and estimating fracture properties

Abstract

This study investigates the polarization properties of GPR signals for imaging flow channeling in a discrete fracture. In particular this study examines if cross-polarized components could be used to image channels in a horizontal fracture. To understand how the polarization of radar waves affects imaging of channelized flow in a horizontal fracture, i) a series of numerical forward models was created with varying fracture aperture, channel orientation, and varying fracture water electrical conductivity, and ii) mulitpolarization field data were used to monitor dipole flow saline tracer tests in a subhorizontal fracture. Numerical modeling demonstrated that the cross-polarized data held useful information about channels but only when the channel is oriented oblique to the E-W wavefield orientation. When the channel is oriented oblique to survey line, summation of the cross-polarized and co-polarized components results in an accurate representation of the total scattered energy from the channel. When the channel is oriented parallel or orthogonal to survey line summation the co-polarized components represent the total scattered energy. In addition to numerical modeling multipolarization, time lapse GPR field data was acquired at the Altona Flat Rock test site in New York State. These surveys were conducted under varying artificial hydraulic gradients, to investigate channeled transport of different concentrations of saline tracer through the fracture and to highlight flow channels between wells. Amplitude analysis of the cross-polarized components reveals flow channeling in an E-W orientation which suggests good well connectivity in that direction. N-S amplitude trends suggest poor hydraulic connectivity. In conclusion, this investigation reveals that cross-polarized components of GPR signals contain useful information for imaging channeled flow in fractured media.