Title: Fracture imaging within a granitic rock aquifer using multiple-offset single-hole and cross-hole GPR reflection data,
Abstract: The sparsely spaced highly permeable fractures of the granitic rock aquifer
at Stang-er-Brune (Brittany, France) form a well-connected fracture network of
high permeability but unknown geometry. Previous work based on optical and
acoustic logging together with single-hole and cross-hole flowmeter data
acquired in 3 neighboring boreholes (70-100 m deep) have identified the most
important permeable fractures crossing the boreholes and their hydraulic
connections. To constrain possible flow paths by estimating the geometries of
known and previously unknown fractures, we have acquired, processed and
interpreted multifold, single- and cross-hole GPR data using 100 and 250 MHz
antennas. The GPR data processing scheme consisting of time-zero corrections,
scaling, bandpass filtering and F-X deconvolution, eigenvector filtering,
muting, pre-stack Kirchhoff depth migration and stacking was used to
differentiate fluid-filled fracture reflections from source-generated noise.
The final stacked and pre-stack depth-migrated GPR sections provide
high-resolution images of individual fractures (dipping 30-90°) in the
surroundings (2-20 m for the 100 MHz antennas; 2-12 m for the 250 MHz antennas)
of each borehole in a 2D plane projection that are of superior quality to those
obtained from single-offset sections. Most fractures previously identified from
hydraulic testing can be correlated to reflections in the single-hole data.
Several previously unknown major near vertical fractures have also been
identified away from the boreholes.