70 Seismic Imaging Figure 3.7 Diagrams of travel time residues for shot number 83. For the initial model, the travel time residues show high values (asterisks) and in the case of the final model the travel time residues (crosses) have a significant reduction. The good fit provides assurance that the final model accurately reflects the picked data. Next, for structure interpretation, a horizontal slice analysis was applied to the 3D depth velocity output model. An example of a horizontal slice at 7 m depth, cut through the velocity model is shown in Figure 3.8-a. This shallow slice corresponds to the base of the epikarst, reported in the boreholes available in the region. Clear patterns of elongated high-low velocity anomalies were identified, probably corresponding to a succession of ridges and furrows, which is consistent with the aforementioned 175°N trend. However, the transmission tomography was unable to determine the deeper structure, as shown by the horizontal slice at 28 m depth, corresponding to the water level (Figure 3.8-b). The tomography of first arrival times picked from surface data provides images that are low resolution at depth, due to the limited azimuthal coverage. Therefore, a new tomographic inversion combining the surface data with an additional VSP data set available at the region, proved to be an effective and reliable tool for the detection of deeper structures. For details see Annex 3-A. Time (Milliseconds)
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