Water flow pattern recognition and fracture characterization in near-surface chalks using time-lapse ground-penetrating radar: An experiment

ABSTRACT Predicting subsurface water flow is inherently complex due to the heterogeneity of geologic formations. Water infiltration occurs through various pathways, such as pore throats, fractures, or faults, depending on the sediment type. In an effort to enhance our understanding of water flow within heterogeneous, fractured chalk formations, we integrate ground-penetrating radar (GPR), acoustic measurements, and photogrammetric surveys to examine flow patterns. We first conduct a field experiment of injecting water into chalks and monitor the water flow through a time-lapse GPR reflection survey. Then, we identify the water-affected zone by analyzing the GPR data and correlate the water-affected zone with the identified fractures on the exposed strata. Finally, we verify the water effect through measured acoustic velocities of acquired samples. Our analysis of the GPR data encompasses direct subtraction, traveltime delay, electromagnetic (EM) velocity changes, and water saturation variations. These analyses consistently indicate a predominant southeastward flow direction. This finding is corroborated by the fractures identified through photogrammetry, which closely match the water-affected zone, suggesting that water flow is indeed fracture driven. The measured acoustic velocities are used to estimate water saturation using an isoframe rock-physics model, which is consistent with the saturation estimated through EM velocity, further validating the monitoring results. These findings advance the understanding of fluid flow and its interaction with fracture systems in heterogeneous chalks and provide a reference for future studies by demonstrating the efficacy of integrating photogrammetry-identified fractures, EM velocity, and acoustic velocity to verify monitoring outcomes.