THE RELATIONSHIP BETWEEN GROUND PENETRATING RADAR PHASE RESPONSE, FLUID SALINITY, AND FRACTURE APERTURE OF A FRACTURED BEDROCK ANALOG THOUGH COMPARISON OF THEORETICAL AND EXPERIMENTAL DATA

Abstract

Accurate characterization of subsurface fractures and associated groundwater channelization is indispensible for contaminant transport and fresh water resource modeling because discharge is cubically related to the fracture aperture; thus, minor errors in aperture estimates may yield major errors in a modeled hydrologic response. Surface ground penetrating radar (GPR) has been successfully used to noninvasively estimate fracture aperture for sub-horizontal fractures at the outcrop scale, but limits on vertical resolution are a concern. Theoretical formulations and field tests from our previous work demonstrate increased GPR amplitude response with the addition of a saline tracer in a sub-millimeter fracture, but robust verification of existing theoretical equations without an accurate measure of aperture variation across a fracture surface is difficult. We therefore investigated both the amplitude and phase response of a 1 GHz PulseEKKO Pro transducer to a sub-millimeter, fluid-filled bedrock fracture using a physical model composed of two plastic (UHMW-PE) blocks, where fracture aperture ranged from 0-40 ± 0.01 mm and fluid conductivity ranged from 0-5700 ± 5 mS/m. Comparison of the measured GPR response to analytical and numerical modeling suggests that numerical modeling best predicts response variations for both changes in fracture aperture and conductivity; however, amplitude response is relatively conductivity independent at low conductivities (<1000 mS/m). Statistical evaluation of the data shows that variations in aperture and conductivity as well as the interaction between the two are significant, although the effect of conductivity is minor over a wide range. For more accurate characterization of variations in conductivity, we examined GPR phase response. We will present GPR data where the instantaneous phase at the fracture reflection is calculated and unwrapped. Measured instantaneous phase is compared to results from theoretical equations and numerical modeling, and provides the first controlled test of those predictions.