The promise of elastic anisotropy (original) (raw)
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Seismic anisotropy of the crystalline crust: what does it tell us?
Terra Nova, 1996
The study of the directional dependence of seismic velocities (seismic anisotropy) promises more refined insight into mineral composition and physical properties of the crystalline crust than conventional deep seismic refraction or reflection profiles providing average values of P-and S-wave velocities. The alignment of specific minerals by ductile rock deformation, for instance, causes specific types of seismic anisotropy which can be identified by appropriate field measurements. Vice versa, the determination of anisotropy can help to discriminate between different rock candidates in the deep crust. Seismic field measurements at the Continental Deep Drilling Site (KTB, S Germany) are shown as an example that anisotropy has to be considered in crustal studies. At the KTB, the dependence of seismic velocity on the direction of wave propagation in situ was found to be compatible with the texture, composition and fracture density of drilled crustal rocks.
Seismic anisotropy in exploration and reservoir characterization: An overview
GEOPHYSICS, 2010
Recent advances in parameter estimation and seismic processing have allowed incorporation of anisotropic models into a wide range of seismic methods. In particular, vertical and tilted transverse isotropy are currently treated as an integral part of velocity fields employed in prestack depth migration algorithms, especially those based on the wave equation. We briefly review the state of the art in modeling, processing, and inversion of seismic data for anisotropic media. Topics include optimal parameterization, body-wave modeling methods, P-wave velocity analysis and imaging, processing in the [Formula: see text] domain, anisotropy estimation from vertical-seismic-profiling (VSP) surveys, moveout inversion of wide-azimuth data, amplitude-variation-with-offset (AVO) analysis, processing and applications of shear and mode-converted waves, and fracture characterization. When outlining future trends in anisotropy studies, we emphasize that continued progress in data-acquisition technol...
Temporal changes in seismic anisotropy can be interpreted as variations in the orientation of cracks in seismogenic zones, and thus as variations in the stress field. Such temporal changes have been observed in seismogenic zones before and after earthquakes, although they are still not well understood. In this study, we investigate the azimuthal polarization of surface waves in anisotropic media with respect to the orientation of anisotropy, from a numerical point of view. This technique is based on the observation of the signature of anisotropy on the nine-component cross-correlation tensor (CCT) computed from seismic ambient noise recorded on pairs of three-component sensors. If noise sources are spatially distributed in a homogeneous medium, the CCT allows the reconstruction of the surface wave Green's tensor between the station pairs. In homogeneous, isotropic medium, four off-diagonal terms of the surface wave Green's tensor are null, but not in anisotropic medium. This technique is applied to three-component synthetic seismograms computed in a transversely isotropic medium with a horizontal symmetry axis, using a spectral element code. The CCT is computed between each pair of stations and then rotated, to approximate the surface wave Green's tensor by minimizing the off-diagonal components. This procedure allows the calculation of the azimuthal variation of quasi-Rayleigh and quasi-Love waves. In an anisotropic medium, in some cases, the azimuth of seismic anisotropy can induce a large variation in the horizontal polarization of surface waves. This variation depends on the relative angle between a pair of stations and the direction of anisotropy, the amplitude of the anisotropy, the frequency band of the signal and the depth of the anisotropic layer.
Comparison of Signal Processing Techniques for Estimating the Effects of ANISOTROPY1
Geophysical Prospecting, 1991
Three-component recordings of shear-waves in exploration surveys provide an opportunity to measure crustal anisotropy, which may be important in estimating the geometrical and physical parameters of reservoir rocks. VSPs are particularly important for this purpose as they are less subject to the complex interactions of the shear wavefield with the free surface. The first stage in characterizing the subsurface anisotropy requires that the distinctive phenomenon of shear-wave splitting must be examined for every arrival at each geophone. This effect may be defined by two parameters: the polarization of the leading shear-wave and the time-delay between corresponding split shear-waves. A variety of techniques have been designed to estimate these parameters of shear-wave splitting. Here, we classify the published techniques into four main categories and review their properties. Representative procedures from each group are applied to a common synthetic data set contaminated with signal-generated noise. The results allow some general statements to be made about the utility of these methods for processing shear-waves in VSP data.
Laboratory determination of velocity anisotropy
2004
Over the past decade technological advances have brought us the capacity to acquire and process high-resolution seismic data in seismic exploration. The physical properties of crust rocks are anisotropic to some extent as demonstrated by data from seismic studies, borehole logs and laboratory. In fact, anisotropy is a fundamental property of rocks. But, our understanding of the anisotropic properties of many rocks is still limited. Consideration of anisotropy in seismic analysis and processing will improve their resolution. So, it is important to understand the intrinsic properties of the rocks through which the seismic waves pass.
How seismic anisotropy changes with scale
By: D. Baden, P. Henry G. Saracco, L. Marie, A. Tonetto, Y. Guglielmi, S. Nakagawa, Gerard Massonat, JP Rolando, in:Proceedings, SEG 2017, 87th Annual Meeting, Houston, Texas, USA, pp. 305-309., 2017
Physical properties of carbonate rocks cannot be fully captured from laboratory-sized samples. Indeed, heterogeneous facies distribution and/or diagenetic alterations may lead to significant variations in petrophysical properties within few meters. In carbonates, diagenetic transformations are tightly related to nature of fluids flowing through the formations, e.g. via fractures network. Consequently, reservoir properties may have patchy distribution, and may not be correlatable (e.g. using facies distribution or wells-logs correlations) within few meters. Our works aim at characterizing carbonates anisotropy at different scales, and are subject of two presentations at SEG's 87 th Annual Meeting. This abstract deals with the second part of our approach, that's to say characterizing impact of diagenetic alteration on reservoir properties and seismic anisotropy, from centimeter to multi-meter scale. This part of the works integrate data from centimeter-scale (mini-cores), decimeter-scale (5" cores), multi-meter (ultrasonic crosshole), and hectometer-scale (seismic), which have been measured at suitable frequency ranges (1MHz, 250kHz, 50kHz, and 1-100Hz, respectively). Although anisotropy is measureable at every scales, its origins vary according to scale. In this study, it is shown that matrix of porous samples are weakly anisotropic as a result of minerals orientation, and inter-crystalline pores. At centimeter-scale, anisotropy can also be related to: (1) patchy distribution of some physical properties, (2) local cracks distribution, and (3) thick single fractures. The lack of correlation between stiffness components from seismic-scale measurements, and laboratory to multi-meter scale ones emphasizes the fact that, when fracturing dominates, measured anisotropy is dominated by fracture/fault related anisotropy and matrix-related anisotropy may be lost. So that, scale effect must be handled carefully in anisotropy analyses, especially for carbonate formations.
1] We used 6 h of continuous vertical records from 2320 sensors of the Valhall Life of Fields Seismic network to compute 2,690,040 cross-correlation functions between the full set of sensor pair combinations. We applied the "Helmholtz tomography" approach combined with the ambient noise correlation method to track the wave front across the network with every station considered as a virtual source. The gradient of the interpolated phase travel time gives us an estimate of the local phase speed and of the direction of wave propagation. By combining the individual measurements for every station, we estimated the distribution of Scholte's wave phase speeds with respect to azimuth. The observed cosine pattern indicates the presence of azimuthal anisotropy. The elliptic shape of the fast anisotropy direction is consistent with results of previous shear wave splitting studies and reflects the strong seafloor subsidence due to the hydrocarbon reservoir depletion at depth and is in good agreement with geomechanical modeling. Citation: Mordret, A., N. M.
A review of methods, techniques and approaches on investigation of rock anisotropy
An extensive review on the anisotropy of rock samples has been carried out to characterize the velocity and strength behaviors under a variety of geometrical and mechanical conditions. Primarily, the causes and impacts of anisotropy is discussed to further understand the importance of the effect of such material from an engineering point of view. The strength anisotropy is investigated in laboratory using the standard strength testing practices (UCS, Triaxial, direct shear and etc..) to perceive the directional dependence of strength for anisotropic rocks and the velocity anisotropy using the ultrasonic scanning of the samples under destructive tests to evaluate the cracks propagation, density and orientation. Then, thorough literature review is done to highlight the significant observations that have been previously elicited. Furthermore, the mathematical determination methods of the degree of anisotropy are explored. Finally, this paper summarize that the strength and velocity anisotropy might be influenced by almost the same factors; however, the behavior of each anisotropy may not be the same considering the rock matrix and failure criteria.