84 Geophysics in Geothermal Exploration Refraction seismic method Today, the refraction method is a quick reconnaissance-mapping tool for delineating near-surface velocity structures. It requires only the measurement of arrival times of first arrival waves (direct and refracted waves) to provide a geologic model while the reflection methods require a complete processing of the recorded wavefield. Picking of first arrivals is much easier than identifying and picking of other events. Seismic refraction is currently used in civil engineering and hydrogeology for objective depths less than 300 m (Mari et al., 1997). The method is particularly suited for the following studies: In civil engineering for: • preliminary studies for construction sites, • determination of the near surface structures, • rock mechanics (rippability, Poisson ratio), • search for cavities. In hydrogeology for: • highlighting channels carved in the bed rock, • highlighting fractured areas in the bed rock, • measurement of the water table depth. Refraction-based velocity estimation of the subsurface can be conventionally done by using well-known methods, such as the Hagedoorn’s Plus-Minus method (1959) or the generalized reciprocal method (GRM) proposed by Palmer (1986), which gives simple models of the subsurface defined by refractors with simple geometry and mainly constant velocity distribution. The GRM method, widely used in refraction prospecting requires direct and reverse shots. It assumes that first-arrivals are only originated by critical refraction and lateral continuous refractors with relatively simple velocity distributions. It assumes small lateral variation and it is used to define refractors with simple geometry and mainly constant velocity distribution. Picked times of direct and reverse shot points (Figures 2.22a and 2.22b) give access to the t plus (t+) and t minus (t–) curves which allow the computation of the refractor velocity analysis function, and the generalized time-depth or delay time, respectively. The refractor velocity analysis function tV, at position G (Figure 2.22c), is defined by the equation: V G AY BX AB t t t t t = = − + ( ) − 1 2 1 2 (2.12) This function is computed for each pair of forward and reverse arrival times, tAY and tBX, and the reciprocal time, tAB. The value of the function tV is referenced to G which is midway between X and Y, and it is plotted as a function of the distance AG. Considering a multi-layer model, the tV curve is approximately a linear function (Figure 2.22d), the slope 1/V’n of which gives an apparent velocity V’n which approximates the velocity Vn of the refractor.
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