64 Seismic Imaging Figure 3.1 Geometry of a computed tomography scanner apparatus (third-generation). The emission of a large X-ray beam encompasses the entire patient’s width and the array of detectors measures the amount of radiation being absorbed throughout the patient’s body. X-ray tube and detectors were rigidly linked and underwent single rotational motion. Adapted from Goldman (2007). The method has been successfully employed in many other scientific fields, such as biology, astrophysics, materials science and geophysics. In geophysics, seismic tomography is an effective technique for 2D, 3D and 4D reconstructions of the Earth’s subsurface, exploiting the properties of seismic wave energy after it has travelled through the ground. Such properties include travel time, ray paths and amplitude, which are an important key to reveal information about seismic velocity, density, and absorption or the Q-factor attenuation of geological formations (Padina et al., 2006; Brzostowski and McMechan, 1992; Spakman et al., 1993; Witten et al., 1992). Currently, seismic tomography is widely applied on a variety of scales and geometries: • On regional and global scales, tomography inverts the seismic records generated by passive sources, such as natural or induced earthquakes, and those received by the seismograph network located around the world. Historically, seismic tomography was first applied to global scale data to study crustal velocity anomalies (Aki and Lee, 1976). The irregularity in time and space distribution presented by this type of source, along with the incomplete coverage of recording stations, leads to significant gaps in the data and limits the spatial resolution of global tomography to 100 - 200 Km. • On the local scale, tomography is convenient for environmental or civil engineering investigations, economic exploration and archaeological research. The versatility of applying this technique to surface, vertical seismic profile (VSP) or cross-hole data makes it very popular, gaining widespread acceptance as a viable
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