Investigation of Cross-Polarized GPR Signals for Imaging Fracture flow Channeling

摘要

Ground penetrating radar (GPR) can be used to image fractured rock and monitor the flow of fluids in the subsurface.Conventional GPR imaging uses single-polarization,co-polarized signals.This study investigates if cross-polarized GPR signals can image channeling at a discrete horizontal fracture.To assess the response of cross-polarized radar waves to fracture channels 3D FDTD numerical models were used to simulate varying fracture properties (i.e.aperture,channel orientation,fracture water electrical conductivity) and mulitpolarization GPR field data were acquired to monitor saline tracer tests in a subhorizontal fracture.Numerical modeling demonstrated that the cross-polarized data are able to image fracture channels when the axis of the channel is oriented obliquely to the EM wavefield orientation.When the fracture channel is oriented at an oblique angle to the survey line,depolarization of the GPR signal results in scattered energy along the cross-polarized components.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 the survey line all scattered energy is captured by the co-polarized components.Multipolarization,time-lapse 3D GPR field data were acquired at the Altona Flat Rock test site in New York State to investigate GPR imaging of flow channeling in a discrete subhorizontal fracture.The GPR surveys were conducted during background fresh fracture water conditions and during a natural gradient saline tracer test which is used to highlight flow channels at the site.Amplitude analysis of the cross-polarized data reveals flow channeling which is in agreement with the co-polarized GPR data and with independent hydraulic tests.In conclusion,this investigation demonstrates for the first time that cross-polarized components of GPR signals can be used to enhance imaging of channels and monitoring of flow in fractured media.