3 Induced seismicity epicentral locations with respect to the injection well during the first hydraulic fracture experiment at Cooper Basin, Australia. Timing of earthquakes with respect to the beginning of injection are shown in grey scale where colours get darker with time following the onset of injection (from Baisch et al., 2006). 

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Context 1

... addition to the explicit imaging of the largest faults, indications of the presence and orientation of small sub-seismic faulting or fracturing can be obtained from the properties of seismic waveforms. The key requirement of these techniques is to have multi-azimuthal data, which can be obtained from high-fold conventional land 3D seismic, from purpose-acquired 2D seismic (e.g. a star configuration around boreholes) and from multi-azimuth Vertical Seismic Profiles (VSP). Multi-azimuthal seismic data provide an 'integrated' measure of rock-mass properties (rather than resolving individual faults), with the fast direction of seismic waves inferred to be parallel to the strike of open fractures. More sophisticated methods employing seismic shear-wave 'splitting' or 'birefringence' could also provide important information on the strike, density and apertures of fractures which are too small to be resolved individually in seismic-reflection lines (e.g. Maultzsch et al., 2003). As with conventional seismic-reflection data time-lapse datasets enable changes in the seismic properties to be tracked, providing insights into changing-rock mass parameters and fluid flow, both within the reservoir, caprock and potentially along faults). In recent years curvature analysis has been applied to processing seismic-reflection volumes for the purpose of imaging fractures that would otherwise be unresolved (e.g. Al-Dossary and Marfurt, 2006;Chopra and Marfurt, 2007;Zhang et al., 2015). Curvature attribute values extracted from seismic volumes enable identification of lineaments on time slices, in some cases in areas where seismic horizons cannot be tracked with confidence, and little information on fault/fractures would otherwise be available. Figure 8.1 compares the results from coherence with the most-positive long-wavelength and short-wavelength curvature analyses. Visual inspection of time slices supports the view that curvature analysis is capable of imaging both the larger faults identified in the coherency time slice and smaller fractures that are below the resolution of conventional seismic reflection images. Confidence in the technique can be further improved by comparing fracture orientations from seismic-curvature analysis with data from image logs or core data (Chopra and Marfurt, ...

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... site-specific earthquake monitoring programmes are implemented induced microseismicity is commonly observed at fluid injection or extraction sites. In map view microearthquakes induced by fluid injection are generally clustered or form a halo around injection wells (e.g. Figure 8.3) and are inferred to reflect rock response to changing pressure and/or temperature conditions. The spatial distribution of induced seismicity may be approximately circular or elongate with the outer margin of the zone often being irregular (Figure 8.3). In a number of cases irregular distributions of microseismicity may arise because the seismicity is locally focused along fault zones (e.g. Ake et al., 2005;Sherburn et al., 2015). The location of the edge of induced microseismicity is influenced by the completeness of the seismicity catalogue. Data from a number of sites suggests that the dimensions of the induced seismicity increase (both temporally at individual sites and between sites) with increasing volumes of injected or extracted fluids (IEAGHG, 2013a;Nicol et al., 2013b). These data suggest that the areal extent of microseismicity is related to the size of the injected fluid plume, however, few data are available to test the model that plume and microseismicity dimensions are approximately equal (i.e. the edge of the plume is marked by the margin of the recorded microseismicity). Microseismic networks are often deployed over geothermal sites where they are inferred to provide information about faulting and fluid flow. In the Coso Geothermal Field (CGF) in California (USA), for example, fluid flow appears to be controlled by a fault and fracture formed at the intersection of the Basin and Range province and the eastern California shear zone ( Wu and Lees, 1999;Kaven et al., 2013;Wamalwa et al., 2013;Kaven et al., 2014). Analysis of the microseismic-event locations reveals seismicity is constrained within two compartments of the geothermal field (referred to as the southwest field and the eastern flank), separated by an aseismic zone which is cooler than the adjacent seismic zones. Within the seismically defined compartments the events are thought to occur on pervasive, small-scale faults and fractures. Along the southern edge of the eastern flank compartment seismicity data define a prominent SW-NE trend which is interpreted to be a large-scale fault or fracture zone. The microseismicity in this zone responds to changing fluid production and injection and is interpreted to be a permeable fault/fracture zone that facilitates fluid flow along the structure while acting as a barrier to cross-structure fluid ...