65 3. Seismic tomography tool to generate detailed geophysical models of the subsurface. However, higher spatial resolution requires greater computational effort, therefore, tomograms with high spatial resolution are limited to smaller scale data acquisitions, such as VSP and cross-hole, where the spatial resolution may reach less than 1 m. Classically, depending on the input data, seismic tomographies fall into three main categories: • transmission tomography using P or S first arrivals, i.e., direct, diving and refracted waves; • reflection tomography using P or S reflection waves; • diffraction tomography using P or S scattered waves (e.g., diffractions, reflections, and converted transmissions). In the following sections, we present some field study examples of each tomography category to show the adaptability of the technique to provide subsurface images in different applications. The intention here is not to describe the field cases in detail, but to provide some background information and the main model features identified from the tomography. The authors emphasize that although a different type of acquisition geometry was chosen for almost every tomography category described below, this by no means implies that these geometries are restricted only to these acquisition types. 3.1 Transmission tomography example: surface seismic field data Transmission tomography is an appropriate technique to define: • horizontal layering structures; • regions exhibiting low complexity velocity distributions. This section shows: • how to obtain a P-wave velocity model from first arrival times; • how to evaluate the spatial (horizontal and vertical) resolution for the tomograms. A transmission tomographic technique was used to invert a 3D seismic data set, part of a more comprehensive geophysical survey conducted in a karstified dolostone region, to provide information about the upper epikarst structure. The selected field example comes from Galibert et al. (2014).
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