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Seismic anisotropy for understanding the dynamics of crust and upper mantle
Published in Rajib Biswas, Recent Developments in Using Seismic Waves as a Probe for Subsurface Investigations, 2023
Seismic anisotropy is a unique property of underlying rocks/foliated formations which affect the passage of seismic waves leading to directional dependence of seismic wave velocity (Babuška and Cara, 1991; Savage, 1999). There are several ways through which the anisotropic characteristic of the propagating medium gets manifested in seismic wave propagation. These include the birefringence or splitting of shear waves, the directional dependence of P-wave travel times, the characteristic conversion of P-to-S waves, the splitting of normal modes, and the difference between the propagation of Love and Rayleigh surface waves, and so on (Wei et al., 2016; Alvizuri and Tanimoto, 2011). However, the most popular tool to study the physical phenomenon of anisotropy is shear-wave splitting which has emerged as a standard seismological tool over the past several decades (Mainprice and Silver, 1993; Savage, 1999; Park and Levin. 2002; Long and Becker, 2010; Hazarika et al., 2013; Long, 2013; Wu et al., 2015; Zhao et al., 2016; Gao et al, 2020; Liu et al., 2020; Paul et al., 2017, 2021; Sivaram et al., 2022). The principle of the shear wave splitting technique is that, when a shear wave propagates through an anisotropic medium, it splits into two components (quasi-S waves) that have different polarizations and travel at different speeds (Figure 12.1). This phenomenon is referred to as shearwave splitting (Babuška and Cara, 1991; Savage, 1999). The shear-wave splitting mechanism is the elastic analog of the birefringence phenomenon observed in the polarized microscopy technique (Savage, 1999; Park and Levin 2002). The polarization direction of the faster wave is termed as ‘fast polarization direction’ (FPD or Φ) and the time difference between the fast and slow split waves is called delay time (δt). Both these parameters’ orientation of anisotropy and delay time provide an estimate of the strength of anisotropy. Seismic anisotropy has been observed in different depth ranges of the Earth's interior, including the crust, the upper mantle, the transition zone, the D″ layer, and the inner core. Most of the scientific interest in delineating and interpreting seismic anisotropy is driven by the link between deformational processes and anisotropic structure (Crampin and Lovell, 1991; Mc Namara and Owens, 1993; Silver, 1996). Deformation in the Earth often leads to seismic anisotropy, either through the crystallographic or lattice-preferred orientation (CPO, LPO) of anisotropic constituent minerals or through the shape-preferred orientation (SPO) of materials. Because of this link between deformation and anisotropy, the characterization of anisotropic structure can yield some of the most direct constraints on dynamic processes in the Earth's interior.
Fault and fracture study by incorporating borehole image logs and supervised neural network applied to the 3D seismic attributes: a case study of pre-salt carbonate reservoir, Santos Basin, Brazil
Published in Petroleum Science and Technology, 2022
Amir Abbas Babasafari, Guilherme Furlan Chinelatto, Alexandre Campane Vidal
Apart from the seismic data and quality of image logs, the available seismic data was limited to post-stack narrow azimuth marine data. Therefore, the fracture azimuth study was not properly feasible. The strike azimuth of some wells is in compliance with the calculated strike azimuth of the supervised discontinuity attribute map, while a few wells seem to contradict the seismic strike azimuth trend (Figure 12a). The change in the elastic properties of seismic waves as a function of propagation direction is called seismic anisotropy. The azimuthal anisotropy can be caused by a system of fractures. Generally, for seismic fracture orientation study, the wide azimuth marine seismic data acquired by ocean-bottom seismometers (OBS) is required. In such a dataset, a particular seismic data processing is conducted to classify data into the common offset – common azimuth volumes for amplitude variation with angle and azimuth study (AVAZ) after anisotropic velocity analysis. Furthermore, shear wave splitting analysis using multi-component seismic data is another approach for fracture azimuth study. It is worth noting that both above-mentioned techniques are not applicable in this study because the particular data is not available. Hence, the seismic anisotropy analysis was not performed.