Hydrogeologic characterization of fractured carbonate aquifers employing ground-penetrating radar.

摘要

Ground-penetrating radar (GPR) surveying is proposed as a remote sensing method for high-resolution characterization of fractured carbonate aquifer hydrogeologic properties in three dimensions. Understanding a geologic formation's fluid flow properties is important to hydrogeologic and environmental studies and the petroleum industry. Fractured anisotropic carbonate aquifers are major sources of water supply. To predict flow in these aquifers, fracture network and rock matrix hydraulic properties must be characterized accurately in three dimensions. GPR is employed to investigate rock matrix and fracture hydraulic properties by direct imaging (reflection surveying), signal attribute analysis (velocity, amplitude and phase) and waveform analysis.;Overlapping GPR surveys were collected over the fractured Byron Dolomite aquiferin Door County, Wisconsin. A high-resolution, single-offset 3-D volume and 2-D lines successfully imaged varying carbonate lithofacies, lithologic discontinuities, dissolution zones, bedforms and horizontal fractures (also referred to as bedding plane discontinuities). Prominent flow conduits were clearly identifiable in the GPR data volumes and delineated in three dimensions by distinct continuous reflections. Velocity analysis of common mid-point surveys identified cyclic velocity variations that correlate to cyclic alternating middle and inner shelf carbonate facies. Within each facies, velocity trends controlled by the volumetric water content of the rock matrix were resolved to 0.5 m vertically. Radar reflector amplitude variation up to one order of magnitude along known flow conduits suggested centimeter-scale conduit aperture variation, with considerable effects on flow properties.;New data acquisition techniques exploiting the polarization properties of electromagnetic waves demonstrated that the location and orientation of areas of anisotropy (i.e. vertical fractures) can be determined by acquiring multi-azimuth data of varying polarization. Phase analysis of these data sets yielded consistent observations relating the polarization of the electric field to the orientation of the fracture plane.;GPR data collected during a pumping test identified fracture drainage patterns and showed that GPR surveying can be used for real-time observation of aquifer response to hydraulic tests.;This study demonstrates that in addition to the traditional GPR reflection surveying, radar signal attributes can be used to provide information about the hydrogeologic properties of the subsurface.