151 6. Hybrid seismic imaging layer presents constant velocity V1 = 1,300 m/s over a bedrock with velocity V2 = 3,250 m/s. This model, along with all available information, was then used to generate the best initial model for tomographic inversion. Tomography was applied to refine the velocity-depth model, which has major benefits when dealing with complex geological setups involving lateral variations. Figure 6.3 shows the advantages of processing by tomographic inversion where strong lateral velocity variations add value to the model. The weathering zone is a heterogeneous shaly mudstone over a compacted limestone. Figure 6.3 Tomographic inversion for the 10EST04 profile. Left: Input model V1 = 1,300 m/s and V2 = 3,250 m/s provided by the Plus-Minus method. Right: Velocity model generated by tomographic inversion. The result exhibits velocity with strong lateral variations for the heterogeneous shaly mudstone over a bedrock of compact limestone. Adapted from Mendes et al. (2014). In a second step, only the reflection events were considered for imaging. In this case, processing capable of isolating and enhancing the reflected waves was required, since they derive from data recorded for a refraction survey and the shot-gathers were corrupted by energetic surface waves that arrived simultaneously with the reflected waves. To obtain a single-fold reflectivity section, shot points 1 and 3 (the end-off shots) were processed according to the following standard sequence: • amplitude recovery; • deconvolution by spectrum equalization (12–160 Hz frequency bandwidth); • wave separation by SVD extraction of refracted waves; • wave separation by F-K filter, to extract surface waves and convert refracted waves; • static corrections based on the high-resolution velocity model provided by the tomographic inversion; • CMP sorting, traces gathered in a common shot-gather are sorted in a common midpoint-gather;
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