GPR-based imaging and inference of heterogeneous aperture fields: Part I—experimental validation and correlation analysis

SUMMARY Ground penetrating radar (GPR) is a widely used geophysical technique for detecting and characterizing subsurface structures such as fractures. While it is effective for determining fracture geometry, accurately characterizing heterogeneous aperture fields remains a challenge. We investigate the latter through a combination of numerical simulations and controlled laboratory experiments. We created a fracture aperture field by modelling its rough-wall surfaces with spatially correlated random fields. We used this digital model in numerical simulations and to fabricate a physical replica via 3-D printing and gypsum casting. We then simulated and measured GPR reflection data on a 2-D measurement grid above the fracture, and after amplitude normalization and averaging, computed both relative and absolute reflection coefficients. This data set allowed us to assess how aperture heterogeneity is expressed in the GPR response and to extract spatial statistics. In particular, our results show that the correlation length of the aperture field can be reliably inferred from reflection data through variogram analysis, even after normalization. Our results are significant for subsurface fracture characterization, where alternative methods to remotely estimate apertures do not exist.