TRACKING AND QUANTIFYING FLUID FLOW AT A FRACTURED CARBONATE FIELD SITE WITH 4D GROUND PENETRATING RADAR (GPR)

Characterization of the parameters controlling fluid flow in fractured rock relies largely on 0.01-0.1 m scale sample measurements, upscaling and modeling. We conducted a novel 1-10 m scale field experiment by injecting and monitoring a moving water mass below a fractured limestone quarry floor. Ground Penetrating Radar (GPR) was used to quantify water content changes, delineate flooding/drainage boundaries and determine the influence of faults and deformation bands on fluid flow propagation rates. Datasets were acquired in the Madonna della Mazza quarry located in the inner part of the Majella anticline (Italy). We performed the controlled injection experiment by installing a temporary polyethylene pond (4m diameter) on the quarry floor, in an area where previously acquired 3D GPR datasets revealed the presence of a porous matrix, open faults and deformation bands. The pond wall was sealed to the quarry floor and 3000 liters of water were infiltrated in 30 hours. Overall, we conducted 16 repeated high-resolution 3D GPR surveys with 0.025 x 0.05 m grid spacing and centimeter precise position repeatability; 2 before and 14 after the infiltration, covering a monitoring period of 5 days. The sensitivity of GPR to water content changes allows to characterize the dynamics of wetting, saturation and draining zones within a 20 x 20 x 12 m GPR volume. Event timeshifts and amplitude differences between repeated surveys are related to subsurface water content changes. GPR reflection travel timeshifts from pairs of repeated surveys are extracted with a 3D warp algorithm. Quantitative fields of water content changes and total mass balance are generated by applying the Topp petrophysical transfer function. Results show how the waterbulb is better developed in undisturbed strata compared to zones including deformation bands. The waterbulb inferred from the GPR data show that deformation bands reduce hydraulic conductivity in horizontal direction whereas vertical open fractures represent preferential flow paths. However we also observe how deformation bands facilitate vertical water transport across stratigraphic boundaries. The results obtained from 4D GPR processing are compared with sample plug measurements, small in-situ infiltration/evaporation experiments, and results from Eclipse fluid flow dynamic modeling.