58 Seismic Imaging These events are due to heterogeneities and fractures in the bedrock. The shape index map highlights the geological features with an N90 orientation associated with a corridor of fractures (Mari et al., 2018). Figure 2.18 refraction survey: shots before and after the filtering of direct and surface waves, and a shape index map. The proposed inversion procedure applied to the field data enables: • a complete velocity model of the first 35m to be obtained; • the top of the karstic reservoir to be mapped; • the detection of the main corridor of fractures, highlighted by a shape index map; • the calculation of static corrections. 2.6 Conclusion Seismic refraction can be used for investigations at all depths, but for various technical reasons it is mostly used to study the first 300 meters of the subsurface (spread length, importance of source energy…). Refracted P-waves are currently used to obtain a velocity model of the near surface by combining conventional methods such as the T plus – T minus method, the GRM method, and tomography. The refraction method is currently used in hydrogeology and civil engineering. We presented two applications of the refraction method: • the computation of static corrections, • the characterization of a near surface karstic reservoir.
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