Stress orientation inferred from shear wave splitting in basement rock at Cajon Pass
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Abstract:
S‐waves generated by mode conversion of P‐waves from surface compressional sources and recorded by oriented geophones at depths between 1300 and 1820 meters in the Cajon Pass scientific drillhole arrive at higher apparent velocities when polarized along the direction N70°E than when polarized in the orthogonal direction. The observed polarization dependent velocities are consistent with seismic anisotropy due to in situ fractures or cracks that are preferen tially aligned N70°E (subnormal to the San Andreas fault). This result is consistent with the maximum principal horizontal stress direction at Cajon Pass also being subnormal to the San Andreas fault.Keywords:
Geophone
Shear wave splitting
The polarizations of three‐component shear wavetrains carry unique information about the internal structure of the rock through which they pass: specifically, commonly observed shear‐wave splitting may contain information about the orientation of crack distributions. This information cannot usually be recovered from shear waves recorded at the free surface because of interference with the interaction of the shear wave with the surface, even for nearly vertical incidence. However, shear‐wave splitting in synthetic three‐component vertical seismic profiles, in some cases, may be interpreted directly in terms of the direction of strike of vertical cracks. Because the human eye is not skilled at identifying the phase relationships between three‐component seismograms played out conventionally as parallel time‐series, the polarizations are displayed in orthogonal sections of the particle displacements to facilitate recognition and evaluation of the shear‐wave splitting. Estimating the orientations of cracks, and hence of preferred directions of flow, by seismic investigations could be of crucial importance to production and reservoir engineering.
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We investigated the use of seismic sensors as small seismic sources. A voltage signal is applied to a geophone that forces the mass within the geophone to move. The movement of the mass generates a seismic wavefield that was recorded with an array of geophones operating in the conventional sense. We observed higher-frequency (25 Hz and above) surface and body waves propagating from the geophone source at offsets of 10 s of meters. We further found that the surface waves emitted from geophone sources can be used to generate a surface-wave group velocity map. We discuss potential developments and future applications.
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Vertical seismic profile
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Passive seismic
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Shear waves propagating through an anisotropic medium split into two approximately orthogonal phases with different velocities. For a single set of fractures, the faster shear wave is polarized along the fracture planes, while the slower shear wave is polarized in a direction orthogonal to the fracture planes. For multiple sets of fractures, the faster shear is not polarized along one specific fracture plane. In this paper, shear-wave splitting analysis for walkaround VSP data was applied for the characterization of multiple sets fractures. Dispersion of the downgoing S- and reflected P-waves was also investigated as a way to discriminate between open and closed fractures.
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Shear wave splitting analysis is one of the most commonly used techniques in crust and mantle deformation. In this paper, we show the derivation process of theoretical equations of the split wave, introduce several shear wave splitting analysis methods, and then summarize the progress of research on crustal anisotropy using shear wave splitting analysis. Although the shear wave splitting analysis method is widely used in the study on crustal anisotropy, it also has some inherent limitations. A reliable method and procedure of data processing to accurately obtain shear wave splitting parameters was expected in future.
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Seismic anisotropy
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Shear wave splitting
Seismogram
Shear waves
Stress field
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A major difficulty in classifying seismic events in the near field is the existence of multiple wave types and their lack of time to separate from one another. During an impulsive seismic event, as the seismic wave components travel through a medium, the difference in their velocities results in a superimposed signal that will look drastically different at varying distances. It would be most beneficial to detect, classify and localize targets creating impulsive events if seismic sensor data could be reduced to a single wave type that has an expected shape and consistent features that do not change as a function of distance. Research was conducted to determine if measuring seismic data from within enclosures of specific architectural design could be used to attenuate specific wave types while maintaining energy of other wave types. The resulting waves produced by these geophone enclosures were then subject to testing using various algorithms to determine their ability to detect, classify, and localize seismic targets.
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This is the final paper in a series on the 3D multicomponent seismic experiment in Oman. In this experiment a 3D data set was acquired using three‐component geophones and with three source orientations. The data set will subsequently be referred to as the Natih 9C3D data set. We present, for the first time, evidence demonstrating that shear waves are sensitive to fluid type in fractured media. Two observations are examined from the Natih 9C3D data where regions of gas are characterized by slow shear‐wave velocities. One is that the shear‐wave splitting map of the Natih reservoir exhibits much larger splitting values over the gas cap on the reservoir. This increase in splitting results from a decrease in the slow shear‐wave velocity which senses both the fractures and the fracture‐filling fluid. Using a new effective‐medium model, it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect. Secondly, an anomaly was encountered on the shear‐wave data directly above the reservoir. The thick Fiqa shale overburden exhibits a low shear‐wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content. No such effects are evident in the conventional P‐wave data. This feature is interpreted as a gas chimney above the reservoir, a conclusion supported by both effective‐medium modelling and the geology. With this new effective‐medium model, we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves (S2), polarized perpendicular to the fracture orientation, whilst leaving the vertical compressional‐wave velocity largely unaffected. This conclusion has direct implications for seismic methods in exploration, appraisal and development of fractured reservoirs and suggests that here we should be utilizing S‐wave data, as well as the conventional P‐wave data, as a direct hydrocarbon indicator.
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Gemology
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Shear-wave splitting is a phenomenon that has received a lot of attention primarily because of its connection with vertically aligned cracks or fractures within reservoirs. However, in most cases the largest amount of shear-wave splitting is observed to occur in the near-surface layers where the rocks are the least consolidated, and so are least likely to be stiff enough to support cracks. Ironically, we have observed during the processing of many multicomponent data sets in western Canada that the largest amounts of shear-wave splitting occur in an area where we least expect to see it— in the highly unconsolidated sediments that comprise the heavy oil plays in the northwestern part of Alberta. At first we thought that the rocks in this area were surely too soft to support cracks and therefore that shear-wave splitting would be smaller than observed elsewhere. The data have taught us that the opposite is true.
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