85 2. Surface geophysical methods The generalized time-depth or Delay, at position G (Figure 2.22c), is defined by: G G AY BX AB n t t t t t V XY = = + − + ( ) ( ) + 1 2 1 2 ’ (2.13) The Plus-Minus method (a simplified version of the GRM method with XY = 0) assumes that first-arrivals are only originated by critical refraction and laterally continuous refractors with relatively simple velocity distributions. Figure 2.22 is an example of a refraction survey. The refraction line is rectilinear. In the acquisition of data, a 48-channel recorder was used. An explosive source (25 g) was detonated and a single geophone (10 Hz) per trace was deployed. Such a source makes it easy to identify and pick first arrivals. The distance between two adjacent geophones was 5 m. A direct shot and a reverse shot were recorded (Figures 2.22a and 2.22b). To obtain the velocity of the refractor (top of the reservoir) and its depth, the Plus– Minus method has been used. It requires recordings where geophones are aligned with shot points. The arrival times of the direct and refracted waves have been picked on the two in line shots. The picked times from the in-line shots (direct and reverse) have been used to compute the t plus and minus curves to obtain the velocity V2 of the refractor and the generalized time-depth curve. The t minus curve (Figure 2.22d) can be approximated by a straight line, the slope of which gives the velocity of the refractor which was found to be 3350 m/s. The slope of the direct wave gives the velocity V1 of the medium situated above the refractor. The medium situated above the refractor is defined as the weathering zone (Wz). Its velocity was found to be 850 m/s. The generalized time-depth, also called Delay time, shows the shape in time of the refractor (Figure 2.22d). The University of Poitiers (France) has developed a Hydrogeological Experimental Site (HES, Figure 2.23a) for the sole purpose of providing facilities to perform long-term monitoring and experiments for a better understanding of fluid flow and transfers in fractured rocks (Bourbiaux et al., 2007). 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 crossline 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 2.23b shows the map locating the seismic lines. In the acquisition of data, a 48-channel recorder was used. An explosive source (25 g) was detonated and a single geophone (10 Hz) per trace was deployed. Such a source makes it easy to identify and pick first arrivals. A 5 m distance between two adjacent geophones was selected to avoid any spatial aliasing.
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