103 4. Near-surface reflection surveying The analysis of the noise profile shows that a seismic acquisition with a minimum offset of 40 m and a geophone interval of 2 m can be implemented to obtain reflected events in the 60 to 150 ms time interval. These parameters were checked in a 3D configuration, with an elementary cross-spread composed of 48 geophones (Figure 4.10-a). The distance between 2 adjacent geophones was 2 m. The source was a weight dropper situated in the middle of the recording, with a lateral offset of 45 m. Figures 4.10 to 4.12 illustrate the step-by-step processing sequence. Step 1: Display of the shot point before and after band pass filtering (Figure 4.10-b). In the upper part, we can see the refracted wave, the high frequency air wave and the low frequency surface wave. In the lower part, after filtering the air waves and surface waves are attenuated. Step 2: Display of the shot point after amplitude recovery and band pass filtering (Figure 4.10-c): refracted waves can be seen in the first arrivals, and a reflected wave can be hypothesized after 0.1s. Step 3: Display of the shot point after amplitude recovery, band pass filtering and deconvolution (Figure 4.10-d). The deconvolution increases the vertical resolution and facilitates wave separation. Step 4: Extraction of refracted waves by SVD filter (Figure 4.11-a). Step 5: Calculation of the first residual section: in the F-K diagram one can see events with wave numbers close to 0, and frequencies ranging between 50 Hz up to 200 Hz (Figure 4.11-b). Events with low frequencies and wave numbers ranging between -0.1 and 0.1 c/m can also be seen. Step 6: Extraction of seismic waves with low apparent velocities by F-K filter (Figure 4.11-c). Step 7: Calculation of the second residual section (Figure 4.11-d): one can mainly see reflected waves. Step 8: Reflected waves after static corrections (Figure 4.12-a) Step 9: Time variant velocity model used for the NMO correction (Figure 4.12-b) Step 10: NMO correction with time variant velocity model. The reflected waves are flattened (Figure 4.12-c). Seismic horizons between 0.04 and 0.12 s can be seen. Step 11: Time to depth conversion of the seismic section (Figure 4.12-d). The upper part of the figure shows the depth versus time law. On the seismic section, a continuous layer can be seen above 60 m. The layers below 60 m are situated in the bedrock.
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