55 2. Refraction surveying and its associated straight-line curve, and the delay time curve. The procedure was applied on each line independently. To obtain a map with a homogeneous sampling interval in both cross-line and in-line directions, the delay time curves were interpolated by kriging with an omni-directional variogram model composed of a nugget effect, a cubic structure with a range of 55 m and a long-scale spherical structure with a range of 145 m (Bourges et al., 2012). Finally, a kriging with the model described above, and a filtering of the nugget effect (random acquisition noise) were performed to obtain the filtered delay time map on a grid 2.5 m x 5m (Figure 2.15-a). a b Figure 2.15 Plus – minus method. a) Delay time map, b) Wz depth map. To perform the depth conversion, the velocity of the medium situated above the refractor must be known. Here, it is given by the slope of the direct wave. The medium situated above the refractor is defined as the weathering zone (Wz). In the area, the velocity V2 of the refractor was found to be 3,350 m/s (from interpretation of the t − curves), and the velocity of the Wz to be 850 m/s. On the Wz depth map (Figure 2.15-b), the arrow indicates the direction N 90° which corresponds to the main orientation of fracture corridors. The picked times of the first seismic arrivals on all shots (in-line and cross-line shots), the Wz depth map and the velocity model obtained by the Plus–Minus method are input data for the inversion procedure, called tomography, which can be used to obtain the velocity distribution in depth (Mari and Mendes, 2012). More information on tomography is provided in the following chapter, and readers should also refer to the article by Mari and Mendes published in Near Surface Geophysics (2012).
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