87 2. Surface geophysical methods (a) (b) (c) (d) (e) (f) Figure 2.23 Hydro-geological experimental site in Poitiers; (a) location map, (b) seismic line implementation. Seismic acquisition: (c) 2D in line acquisition geometry, (d) 3D cross line acquisition geometry, (e) example of in line shot gather, (f) example of cross line shot gather with 60 m of lateral offset. After Mari and Mendes (2019). A direct shot and a reverse shot were recorded per receiver line (Figure 2.23c). Three shot points in the crossline direction were fired at distances of 40, 50 and 60 m from the receiver line under consideration (Figure 2.23d). Figure 2.23e shows an example of an in line shot gather and Figure 2.23f a cross line shot gather with a lateral offset of 60 m. The picked times of the first seismic arrivals on all the shots (in line and cross lines 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 is appropriate to obtain the velocity distribution in depth (Mendes, 2009; Mari and Mendes, 2012).
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