HR: 1340h
AN: H23A-1104    [Abstracts]
TI: What are the Physical Causes of GPR Reflections in an Oolitic Carbonate Environment? Excavation, TDR, and Modeling Results
AU: * Vega, S
EM: svega@rsmas.miami.edu
AF: Comparative Sedimentology Laboratory, RSMAS. University of Miami. 4600 Rickenbacker Causeway, Miami, Fl 33149 United States
AU: Grasmueck, M
EM: mgrasmueck@rsmas.miami.edu
AF: Comparative Sedimentology Laboratory, RSMAS. University of Miami. 4600 Rickenbacker Causeway, Miami, Fl 33149 United States
AU: Truss, S
EM: struss@rsmas.miami.edu
AF: Comparative Sedimentology Laboratory, RSMAS. University of Miami. 4600 Rickenbacker Causeway, Miami, Fl 33149 United States
AU: Or, D
EM: dani@engr.uconn.edu
AF: Departament of Civil and Environmental Engineering, University of Connecticut. 261 Glenbrook road, Storrs, Ct 06269 United States
AU: Chen, Y
EM: yoc03001@engr.uconn.edu
AF: Departament of Civil and Environmental Engineering, University of Connecticut. 261 Glenbrook road, Storrs, Ct 06269 United States
AU: Drasdis, J
EM: drasdis@engr.uconn.edu
AF: Departament of Civil and Environmental Engineering, University of Connecticut. 261 Glenbrook road, Storrs, Ct 06269 United States
AB: Ground Penetrating Radar (GPR) can produce sub-meter resolution 3D images of internal anatomy and moving water in the Miami Oolitic Limestones. However, the exact physical causes of GPR responses are still unclear. In order to reach our long-term goal of extracting hydrologically relevant and quantitative parameter volumes from 3D and 4D GPR data we have initiated an integrated study involving full-resolution 3D GPR imaging, excavation, fresh outcrop inspection, sample analysis, TDR measurements and synthetic GPR modeling. Just before the excavation for a multistory building began, we acquired a full-resolution 3D GPR survey covering an area of 16 m x 14 m with a 0.05 m x 0.10 m grid using 250 MHz antennae, reaching the watertable at a depth of 4.5 m. The entire rock volume of 1000 cubic meters was subsequently excavated, allowing the exceptional opportunity to directly compare 3D radar data with a 3D rock cube. Qualitative comparisons between the 3D GPR results and the excavated rock faces show that the technique is valid; the GPR shows the main stratigraphic boundaries seen in the real rock. However, some reflections and attenuated zones in the radar data do not fit with any visible structure. To help explain these discrepancies we took large (0.05 cubic meter) samples from within the rock volume that corresponded with either an anomaly location or a known stratigraphic reflector, and analyze their petrophysical characteristics under laboratory conditions. The samples were cut into cubes and tested at a range of saturations from dry (0%) to fully saturated (100%) using 10cm long Time Domain Reflectometry (TDR) probes, which provide accurate values of dielectric constant and allow us to investigate its relationship with water saturation. We find that the relationship between dielectric constant and water saturation depends on sample stratigraphy, porosity, mineralogy, and permeability. This result suggests that each different oolite sample produces a characteristic GPR response. To upscale these results from the TDR scale (0.002 cubic meter) to 3D GPR volume (1000 cubic meter), we use two synthetic models that are compared with field data. One of the models shows how GPR can detect and quantify water saturation and fluid flow. This finding indicates that we can estimate water saturation and fluid flow from 4D GPR data. The second model indicates that sink holes, a common feature in this geological environment, can be detected in GPR surveys as attenuated data surrounded with time-shift anomalies. We compare these models with the original 3D GPR results to help us to better interpret these data. The results from this study will be used to design future 4D GPR experiments, which will enable us to more fully understand the detailed structure and quantify the hydogeological behavior of oolitic carbonates.
DE: 5100 PHYSICAL PROPERTIES OF ROCKS
DE: 1800 HYDROLOGY
DE: 1829 Groundwater hydrology
DE: 1857 Reservoirs (surface)
DE: 1875 Unsaturated zone
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting