GPR-based imaging and inference of heterogeneous aperture fields: Part II – Inferring spatial heterogeneity from frequency-dependent reflection coefficients

Aperture is a key property of fractures, as it controls fluid flow and mechanical behavior in fractured rock, yet it remains difficult to observe directly. Ground Penetrating Radar (GPR) offers non-invasive means of imaging fractures and is commonly used to determine their geometry. The amplitude of the reflected GPR signal, which contains valuable information about fracture aperture and filling material, is often underutilized in quantitative interpretation. In this study, we present a workflow for estimating heterogeneous fracture aperture fields by inverting GPR reflection data in the frequency domain. The approach relies on the thin-bed approximation and uses the effective-dipole forward model. We first validate the workflow using laboratory data with a known aperture field, demonstrating accurate recovery of aperture distributions in high-sensitivity regions. We then apply the method to field data from the Bedretto Laboratory, where it successfully reconstructs a 1D relative aperture profile along a target fault. From this, we extract the correlation length, providing additional insight into fracture structure. These results demonstrate that the approach can deliver robust, spatially resolved estimates of fracture aperture and its variability, even under realistic field conditions.