# Seismic Exploration Methods for Structural Studies and for Active Fault Characterization: A Review
## 3 Developments in Seismic Imaging
### Seismic Anisotropy  
In geology, the term anisotropy refers to the directional dependence of rock physical properties. Anisotropy can be caused by many factors, such as mineral alignment, fracture orientation and fluid-filled cracks. Seismic anisotropy [64; 65; 66] studies the variation of seismic wave velocity along different directions of propagation caused by anisotropy. Seismic anisotropy has important applications in structural geology. For example, in regions of tectonic deformation, seismic anisotropy can reveal the orientation and intensity of faulting and folding [67]. In sedimentary basins, seismic anisotropy can be used to image the orientation and distribution of minerals in sedimentary layers, and it has the potential to be used as an indicator of rock lithology [68].  
Many studies of seismic anisotropy are based on the phenomenon of shear-wave splitting: when a shear wave travels through an anisotropic medium, it splits into two orthogonal polarization modes, which are known as the fast shear wave and slow shear wave [69; 70; 71; 72; 73; 74]. An example of application can be found in [PERSON] et al. [75], who modeled the splitting of the shear-wave observed in crystalline rock bordering an active, normal fault-zone at Oroville, California, to quantitatively interpret P-wave polarization anomalies observed in the three-component seismograms recorded in the Oroville fault zone.  
Azimuthal velocity measures seismic velocity variation with respect to the direction of propagation. [PERSON] [76] introduced an efficient and effective way to separate the fast and slow shear modes in the sedimentary crust, which is now called Alford rotation. By using the rotation method, it was possible to detect the existence of azimuthal anisotropy in the upper crust, which is caused by deformation in the Earth's crust and upper mantle [77; 78; 79].  
Figure 4: Comparison between results of RTM using a standard and a FWI improved velocity model (from [PERSON] et al. [29]). The figure shows a sub-line segment that includes the input and output velocity models for FWI in (**a**) and (**b**), respectively, along with their corresponding RTM images shown in (**c**) and (**d**).  
The dependence of anisotropy on many factors has limited its applications in seismic exploration. However, over the past two decades, technological advances in parameter estimation, PSDM, 3D survey coverage, and the acquisition of high-quality multicomponent data have significantly increased the role of anisotropy in seismic exploration, and many seismic processing and inversion methods now operate with anisotropic models [80]. Since the mid-1990s, high-quality multicomponent offshore surveys have shown that PP- and PS-wave sections cannot be tied in depth without considering anisotropy in the velocity model [80]. Although anisotropy-induced distortions in P-wave imaging are less dramatic, the progress in P-wave processing has largely been due to breakthroughs in the parameterization of transversely isotropic models [81; 82]. Recently, PSDM has exposed the inadequacy of isotropic velocity models, with transversely isotropic models with a vertical and tilted axis of symmetry becoming standard in PSDM imaging projects worldwide [80]. Anisotropic algorithms have been particularly effective in producing improved images of subsalt exploration targets in the Gulf of Mexico (Figure 5: see [PERSON] et al. [83]), which was a region that was previously considered to have a relatively mild anisotropy.