Fracture imaging within a granitic rock aquifer using multiple-offset single-hole and cross-hole GPR reflection data

Abstract

International audienceThe 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 neighbouring boreholes (70-100 m deep) has 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

Timescales of regional circulation of saline fluids in continental crystalline rock aquifers (Armorican Massif, western France)

“…Hydrodynamic properties of crystalline rock aquifers of the Armorican basement have been intensively investigated at several research sites (Ploemeur and St-Brice-enCogles sites, Fig. 2) (Le Borgne et al, 2004Borgne et al, , 2006Dorn et al, 2012;Leray et al, 2013;Roques et al, 2014;Touchard, 1999) (Fig. 2).…”

Tunnel Assessment with Borehole Radar

“…Other applications, as the detection of tunnels, are established even supplementary data are needed to define proper synthetic models (Di Donato et al, 2012). GPR sounding information was also recently used to evaluate geological fractures, comparing results from single hole reflection data to cross hole data (Dorn et al, 2012).…”

Rock fracture image acquisition using two kinds of lighting and fusion on a wavelet transform

“…In recent years, more and more researchers have started to measure rock fractures by image analysis (Hakami and Wang 2005;Matter and Kelemen 2009;Susan 2010;Kemeny and Randy 2003;Sharifzadeh et al 2008;Dorn et al 2012;Wang et al 2012;Wang 2008Wang , 2011aChae et al 2004;Fekete et al 2010;Mah et al 2013;Takahashi and Takemura 2004;Kevin and Ronald 2004;Seren and Acikgoz 2012;Williams and Carole 2004;Sait and Joerg 2003). A typical example is that of the Swedish government and the European Union conducting one of the world's largest projects (with an investment of at least several millions per year, from 1974 to today) to protect the environment from pollution for generations.…”

“…In recent years, rock fracture image acquisition research has mainly adopted more advanced hardware techniques (Kemeny and Randy 2003;Sharifzadeh et al 2008;Dorn et al 2012;Wang et al 2012;Wang 2008Wang , 2011aChae et al 2004;Fekete et al 2010;Mah et al 2013). When surveying the highway and railway research, many researchers acquire geological data (such as structure, landform, dynamic phenomenon, hydrogeological phenomenon and permafrost zoning) by aerial image analysis.…”