3-D mapping of permeable structures affecting a deep granite basement using isotropic 3C VSP data

Abstract

International audienceThis paper illustrates the efficiency of vertical seismic profiling (VSP) for the investigation of dipping and hydraulically conductive structures affecting a granitic basement covered by sediments. A three-component (3C) VSP data set has been acquired in the GPK1 and EPS1 wells of the Soultz-sous-Foreˆts enhanced geothermal system (EGS) located within the Upper Rhine Graben (URG). Our study focuses on the isotropic processing of profiles acquired with vertical vibrator P and their subsequent interpretation. Mainly P-S converted reflections are identified from the analysis of the 3C records. These P-S conversions occur on steep permeable faults that are positioned in space by traveltime modelling. These faults cut the granite basement in several hectometric-scale blocks, and represent the main fluid paths between the boreholes. These faults are thought to be inherited from late Variscan and Alpine deformation periods, reactivated by the current stress field. When properly processed and interpreted, VSP allow the scale gap between surface and borehole data to be bridged

Evidence for tectonic, lithologic, and thermal controls on fracture system geometries in an andesitic high‐temperature geothermal field

“…The vein location distribution at Waihi is also inferred to have been controlled both by the initial jointing of the lava and by subsequent faulting events, especially close to faults where the lithological controls are overridden by tectonic controls [Brathwaite et al, 2001;Spörli and Cargill, 2011]. Further, borehole (e.g., resistivity or microresistivity image logs [Davatzes and Hickman, 2010]) or near-borehole (e.g., vertical seismic profiling [Place et al, 2011]) measurements would inform on the nature and thickness of these mechanical layers in the Rotokawa andesitic reservoir.…”

Deep Borehole Field Test Research Activities at LBNL

“…Seismic acquisition methods that utilize borehole receivers are advantageous because of their proximity to the fractures and the ability to record higher frequency waves. These two factors have opened the possibility for imaging discrete fractures using VSP, cross-well, and single-well acquisition geometries (Meadows & Winterstein, 1994;Majer et al 1997;Coates et al 1998;Cosma et al 2001;Place et al 2011). …”

Initial seismic observations from a deep borehole drilled into the Canadian Shield in northeast Alberta

“…By placing the geophones directly into the borehole versus surfaceplanted geophones in a seismic survey, and also its higher frequency content, VSP imaging overcomes hurdles of seismic attenuation and reveals reflectors that could not always be resolved by surface seismic data. These reflectors have proven to associate with dipping geological structures and flow anomalies that affect the development of a geothermal reservoir (e.g., Nakagome et al 1998;Melosh et al 2010;Place et al 2011). This form of borehole geophysics provides in situ measurements of rock properties including velocity, acoustic impedance, attenuation, and anisotropy (Stewart 2001).…”

“…However, the complex structures of fractured reservoirs in the crystalline basement cannot always be properly resolved at the reservoir scale (e.g., Schutter 2003;Luthi 2005;Al-Ali et al 2009;Place et al 2011). Consequently, borehole geophysical surveys such as the vertical seismic profiling are utilized to minimize the ambiguity in the interpretation of seismic reflection surveys.…”

“…Detailed borehole seismic surveys have been carried out at Kola (Pavlenkova 1992;Carr et al 1996;Ganchin et al 1998), KTB (Li and Richwalski 1996;Lüschen et al 1996;Harjes et al 1997;Okaya 2004;Rabbel et al 2004), and Outokumpu (Duo 2011;Heinonen et al 2011;Kukkonen et al 2012;Schijns et al 2012). Surveys in similar crystalline lithologies but in more tectonically active zones include studies at Soultzsous-Forêts, France (Place et al 2011), and Cajon Pass, California (Rector 1988), for mineral exploration (e.g., Miao et al 1994;Eaton et al 1996), and for repository characterization (e.g., Cosma et al 2001).…”