53 2. Refraction surveying during the Cretaceous and Tertiary ages. It is shaped today as hollows and bumps with a relief reaching up to 35 m. Refraction seismic surveying, described in detail by Mari and Porel (2007), has been used to map the irregular shape of the top of the karstic reservoir. a b Figure 2.13 Hydrogeological experimental site in Poitiers. a) location map; b) seismic line implementation. Due to the limitations of the area, the length of the seismic line could not exceed 250 m in the in-line direction. In the cross-line direction, the extension of the area does not exceed 300 m. As a result, 20 receiver lines have been implemented, with a 15 m distance between adjacent lines. Figure 2.13-b shows the map locating the seismic lines. A 48-channel recorder was used for the data acquisition. An explosive source (25 g) was detonated and a single geophone (10 Hz) per trace was deployed. The use of this type of source makes it easy to identify and pick first arrivals. A 5 m distance between two adjacent geophones was selected to avoid spatial aliasing.
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