121 5. Contribution of seismic and acoustic methods to the characterization of karstic formations A 5 m distance between two adjacent geophones was selected to avoid any spatial aliasing. A direct shot and a reverse shot were recorded per receiver line (“in-line” shots). Figure 5.2 (top right) shows an example of an in-line direct and reverse shot gather. Three shot points in the cross-line direction were fired at distances of 40 m, 50 m and 60 m from the receiver line under consideration. Figure 5.2 (bottom right) shows an example of a cross-line shot gather. The range of offsets was selected to optimize the quality of the seismic image over the reservoir depth interval, i.e. between 40 m and 130 m. A 40 m minimum offset distance was chosen to reduce the influence of the surface waves. The time sampling interval was 0.25 ms and the recording length was 0.5 s. The processing sequence has been described in detail in several publications (Mari & Porel [1] and Mari & Delay [6]), 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 has 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) 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). A 3D seismic refraction tomography [7] 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 [8] method were used as input data for the inversion procedure. The inversion results obtained with 3D data emphasize the previously mentioned geological structures [9], 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 normal move-out (NMO) corrections. A VSP was recorded in well C1 (Figure 5.2, bottom left). VSP data were processed to obtain a time versus depth relationship and a velocity model. The velocity model has been 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. The singlefold depth sections were merged to create the 3D block. The width of the block in the in-line direction is 240 m, and 300 m in the cross-line direction. In the in-line direction, the abscissa zero indicates the location of the source line. The abscissa of the reflecting points varies between -120 m and 120 m in the in-line direction. The distance between two reflecting points is 2.5 m. In the cross-line
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