Investigating the Capabilities of Ground Penetrating Radar for Imaging Shallow Experimental Fractures

We have used ground penetrating radar (GPR) to image fractures formed in shallow sediments as a result of high-pressure injection. Understanding fracture formation and behavior is important for a variety of reasons, ranging from validating fracture formation theories to characterizing fracture networks induced for enhancing recovery schemes in low permeability rocks. GPR is a high resolution geophysical method that is sensitive to electromagnetic property changes in the subsurface. The resolution of GPR is, however, typically on the order of ¼ of the wavelength, which for the 900MHz GPR data is on the order of 2-5cm. Thus it was not clear prior to the experiment whether it would be possible for GPR to image the fractures formed during the injection. We found that the GPR was indeed able to image the fractures very well as they evolved through time. Over the course of the experiment, we were able to collect pseudo-3D data that allowed us to monitor the growth of the fracture over time. The experiment was also repeated for different injection materials to examine how the fill in the fractures impacts the GPR signal. From the GPR data we are able to reconstruct the approximate three-dimensional shape of the facture over time. At the end of the experiment, the experimental cells were trenched so that the actual fracture distribution could be mapped. Overall, the GPR interpretation showed reasonable agreement with what we could observed in the trenches. The experimental results suggest that GPR characterization of fractures is feasible.