Shear wave splitting applications for fracture analysis and improved imaging: some onshore examples

Bale, Richard; Gratacos, B.; Mattocks, B.; Roche, Steven L.; Poplavskii, Kostya; Li, X.

doi:10.3997/1365-2397.27.1304.32448

Cited by 37 publications

(3 citation statements)

References 3 publications

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“…In an anisotropic media, shear-wave splitting occurs when an incident shear wave exhibits birefringence, resulting in two shear waves that are polarized parallel and perpendicular to the fracture strike, respectively (Crampin and Peacock, 2008). The shear wave S 1 propagates with faster speed than S 2 (Bale et al, 2009). Therefore, this anisotropy related phenomenon may be described by the difference in arrival times of shear waves originated from a single source (Simm and Bacon, 2014).…”

Section: Introductionmentioning

confidence: 99%

Investigation of fractured carbonate reservoirs by applying shear-wave splitting concept

Diaz-Acosta

Bouchaala

Kishida

et al. 2022

Adv. Geo-Energy Res.

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In this study, fracture orientations in carbonate reservoirs were determined using a multicomponent velocity analysis based on shear wave splitting. The analysis is based on the estimated velocities of large seismic events with different polarizations. In a fractured zone with a dominant orientation, weak amplitude split shear events, including shear noise, result in shear waves that are polarized toward the symmetry and anisotropy axes and propagate with a common fast and slow velocity, respectively. Thus, a velocity stack should show high coherency anomalies in directions parallel and orthogonal to the fracture strike. Furthermore, because the analysis is applied locally at a specific depth range, it is less susceptible to the effects of overburden anisotropy and noise. The dominant fracture orientations from carbonate reservoirs of four oilfields were compared to those interpreted from fullbore microimager and core data. Fractures in two offshore reservoirs strike NNE-SSW and NW-SE, which are related to Zagros stress. Fractures in two onshore reservoir strikes NE-SW, while in deeper onshore reservoir fractures are aligned with N-S direction. The findings of this study are promising, particularly for the fractured reservoirs especially those located in Abu Dhabi, which are characterized by high heterogeneity and complex fracture network related to complex tectonic history. In order to obtain geometrical parameters of fractures at seismic scale, it is recommended to implement the analysis adapted in this study after acquiring three component zero-offset vertical seismic profiling.

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Section: Introductionmentioning

confidence: 99%

Investigation of fractured carbonate reservoirs by applying shear-wave splitting concept

Diaz-Acosta

Bouchaala

Kishida

et al. 2022

Adv. Geo-Energy Res.

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“…The split S-waves propagate through the anisotropic medium, and are subsequently polarized along (fast, S1) and across (slow, S2) the vertical features, resulting in a difference between the detected arrival times and signal amplitudes (Lynn and Thomsen, 1990;Thomsen, 1999;Tsvankin et al, 2010). The time delay between S1 and S2 observed on the radial component (Figure 1B), and the amplitude nulls observed on the transverse component (Figure 1C), can be used respectively to constrain the percentage of anisotropy and therefore the spatial density, and the orientation of aligned vertical fractures exhibiting HTI anisotropy (e.g., Crampin, 1985;Bale et al, 2009). SWS analysis of active source data recorded on ocean bottom seismometers (OBSs) has been successfully applied for marine slope stability assessment on the west Svalbard continental slope (Haacke and Westbrook, 2006) and for the identification of vertical fluid migration pathways within hydratebearing sediments in the Storrega slide offshore Norway (Exley et al, 2010).…”

Section: Introduction Overviewmentioning

confidence: 99%

Seismic Anisotropy Within an Active Fluid Flow Structure: Scanner Pockmark, North Sea

et al. 2021

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Understanding sub-seabed fluid flow mechanisms is important for determining their significance for ocean chemistry and to define fluid pathways above sub-seafloor CO2 storage reservoirs. Many active seabed fluid flow structures are associated with seismic chimneys or pipes, but the processes linking structures at depth with the seabed are poorly understood. We use seismic anisotropy techniques applied to ocean bottom seismometer (OBS) data, together with seismic reflection profiles and core data, to determine the nature of fluid pathways in the top tens of meters of marine sediments beneath the Scanner pockmark in the North Sea. The Scanner pockmark is 22 m deep, 900 m × 450 m wide and is actively venting methane. It lies above a chimney imaged on seismic reflection data down to ∼1 km depth. We investigate azimuthal anisotropy within the Scanner pockmark and at a nearby reference site in relatively undisturbed sediments, using the PS converted (C-) waves from a GI gun source, recorded by the OBS network. Shear-wave splitting is observed on an OBS located within the pockmark, and on another OBS nearby, whereas no such splitting is observed on 23 other instruments, positioned both around the pockmark, and at an undisturbed reference site. The OBSs that show anisotropy have radial and transverse components imaging a shallow phase (55–65 ms TWT after the seabed) consistent with PS conversion at 4–5 m depth. Azimuth stacks of the transverse component show amplitude nulls at 70° and 160°N, marking the symmetry axes of anisotropy and indicating potential fracture orientations. Hydraulic connection with underlying, over pressured gas charged sediment has caused gas conduits to open, either perpendicular to the regional minimum horizontal stress at 150–160 N or aligned with a local stress gradient at 50–60 N. This study reports the first observation of very shallow anisotropy associated with active methane venting.

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“…It has been used as a measure for characterising crack density and orientation or crystal alignment (Crampin et al, 2015;Savage et al, 2016). It has been studied throughout the Earth, ranging in depth from hydrocarbon reservoirs in the shallow crust (Crampin and Peacock , 2008;Verdon et al, 2009;Bale et al, 2009) to the core and deep mantle (Wookey and Helffrich, 2008). Seismic anisotropy is often considered to be an indicator of stress orientation in the crust, because closure of cracks due to differential stress leads to (transverse) waves polarized parallel to the cracks traveling faster than the orthogonal direction (Crampin, 1984;Savage, 1999).…”

Section: Seismic Anisotropymentioning

confidence: 99%

Measuring Crustal Seismic Anisotropy through Shear Wave Splitting

Graham¹

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<p>This thesis involves the study of crustal seismic anisotropy through shear wave splitting. For the past three decades, shear wave splitting (SWS) measurements from crustal earthquakes have been utilized as a technique to characterize seismic anisotropic structures and to infer in situ crustal properties such as the state of the stress and fracture geometry and density. However, the potential of this technique is yet to be realized in part because measurements on local earthquakes are often controlled and/or affected by physical mechanisms and processes which lead to variations in measurements and make interpretation difficult. Many studies have suggested a variety of physical mechanisms that control and/or affect SWS measurements, but few studies have quantitatively tested these suggestions. This thesis seeks to fill this gap by investigating what controls crustal shear-wave splitting (SWS) measurements using empirical and numerical simulation approaches, with the ultimate aim of improving SWS interpretation. For our empirical approach, we used two case studies to investigate what physical processes control seismic anisotropy in the crust at different scales and tectonic settings. In the numerical simulation test, we simulate the propagation of seismic waves in a variety of scenarios. We begin by measuring crustal anisotropy via SWS analysis around central New Zealand, where clusters of closely-spaced earthquakes have occurred. We used over 40,000 crustal earthquakes across 36 stations spanning close to 5.5 years between 2013 and 2018 to generate the largest catalog of high-quality SWS measurements (~102,000) around the Marlborough and Wellington region. The size of our SWS catalog allowed us to perform a detailed systematic analysis to investigate the processes that control crustal anisotropy and we also investigated the spatial and temporal variation of the anisotropic structure around the region. We observed a significant spatial variation of SWS measurements in Central New Zealand. We found that the crustal anisotropy around Central New Zealand is confined to the upper few kilometers of the crust, and is controlled by either one mechanism or a combination of more than one (such as structural, tectonic stresses, and gravitational stresses). The high correspondence between the orientation of the maximum horizontal compressive stress calculated from gravitational potential energy from topography and average fast polarization orientation around the Kaikōura region suggests that gravitationally induced stresses control the crustal anisotropy in the Kaikōura region. We suggest that examining the effect of gravitational stresses on crustal seismic anisotropy should not be neglected in future studies. We also observed no significant temporal changes in the state of anisotropy over the 5.5 year period despite the occurrence of significant seismicity. For the second empirical study, we characterized the anisotropic structure of a fault approaching failure (the Alpine Fault of New Zealand). We performed detailed SWS analysis on local earthquakes that were recorded on a dense array of 159 three-component seismometers with inter-station spacing about 30 m around the Whataroa Valley, New Zealand. The SWS analysis of data from this dense deployment enabled us to map the spatial characteristics of the anisotropic structure and also to investigate the mechanisms that control anisotropy in the Whataroa valley in the vicinity of the Alpine Fault. We observed that the orientation of the fast direction is parallel to the strike of the Alpine Fault trace and the orientations of the regional and borehole foliation planes. We also observed that there was no significant spatial variation of the anisotropic structure as we move across the Alpine Fault trace from the hanging wall to the footwall. We inferred that the geological structures, such as the Alpine Fault fabric and foliations within the valley, are the main mechanisms that control the anisotropic structure in the Whataroa valley. For our numerical simulation approach, we simulate waveforms propagating through an anisotropic media (using both 1-D and 3-D techniques). We simulate a variety of scenarios, to investigate how some of the suggested physical mechanisms affect SWS measurements. We considered (1) the effect on seismic waves caused by scatterers along the waves' propagation path, (2) the effect of the earthquake source mechanism, (3) the effect of incidence angle of the incoming shear wave. We observed that some of these mechanisms (such as the incidence angle of the incoming shear wave and scatterers) significantly affect SWS measurement while others such as earthquake source mechanisms have less effect on SWS measurements. We also observed that the effect of most of these physical mechanisms depends on the wavelength of the propagating shear wave relative to the size of the features. There is a significant effect on SWS measurements if the size of the physical mechanism (such as scatterers) is comparable to the wavelength of the incoming shear wave. With a larger wavelength, the wave treats the feature as a homogeneous medium.</p>

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Shear wave splitting applications for fracture analysis and improved imaging: some onshore examples

Cited by 37 publications

References 3 publications

Investigation of fractured carbonate reservoirs by applying shear-wave splitting concept

Investigation of fractured carbonate reservoirs by applying shear-wave splitting concept

Seismic Anisotropy Within an Active Fluid Flow Structure: Scanner Pockmark, North Sea

Measuring Crustal Seismic Anisotropy through Shear Wave Splitting

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