113 4. Near-surface reflection surveying influence of surface waves. The distance between 2 adjacent shot points in the cross-line direction was chosen to be 10 m. Figure 4.19 shows the selected seismic spreads and the map locating the seismic lines. The red points indicate the location of the wells. The processing sequence has been described in detail in several publications (Mari and Porel, 2007; and Mari and Delay, 2011), so it is only briefly explained here. Each shot point was processed independently (both in the cross-line direction and in the in-line direction) to obtain a single-fold section with a sampling interval of 2.5 m (half the distance between 2 adjacent geophones) in the in-line direction. The processing of an in-line direct and reverse shot gather enabled a single-fold section with an in-line extension of 240 m to be obtained (indicated by a blue arrow on the seismic line map, Figure 4.19-b) while a cross-line shot gather has provided a single-fold section with an in-line extension of 120 m (indicated by a red arrow on the seismic lines map, Figure 4.19-b). A 3D seismic refraction tomography (Mari and Mendes, 2012, see also the “Refraction Surveying” chapter) was carried out to map the irregular shape of the top of the karstic reservoir, and to obtain static corrections and a velocity model of the overburden. To add information to the inversion procedure, we used in-line and cross-line cross shots simultaneously, with an offset of 60 m. The shots were selected to ensure that the refracted wave was the first arrival wave, regardless of the source-receiver distance. The picked times of the first seismic arrivals for all shots (in-line and cross-lines shots), the depth map of the top of the reservoir (defined from the wells), and the velocity model obtained by the Plus–Minus method were used as input data for the inversion procedure (see “Refraction surveying” chapter). The inversion results obtained with 3D data emphasize the previously mentioned geological structures, providing a better understanding of their alignments and shape (corridor of fractures). Furthermore, no cavities were detected near the surface. The processing sequence includes: amplitude recovery, deconvolution, wave separation (SVD method for extracting refracted waves and combining the SVD and F-K methods for filtering surface waves), static corrections (obtained by inversion tomography) and NMO corrections. A VSP was recorded in well C1. VSP data were processed to obtain a time versus depth relationship and a velocity model. The velocity model was used to apply the NMO corrections. The VSP time versus depth law was also used to convert the time sections into depth sections with a 0.5 m depth sampling interval. For illustration, the elementary cross-spread corresponding to geophone line 11 with a 60 m lateral source offset is shown in Figure 4.20.
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