40 Seismic Imaging Figure 2.3 GRM method. Figures 2.3-a and b are schematics of the ray paths used in the calculation of both the velocity and the shape of the refractor. For a value of XY such that the forward and reverse rays emerge from nearly the same point on the refractor, i.e. P, R and Q coincide, as shown in Figure 2.3, a result similar to the mean of the migrated forward and reverse delay times is obtained. At the optimum value of XY (P and Q are coincident), accurate velocities can be measured with deep or irregular refractors and the maximum definition of the refractor can be recovered from the travel time data (Palmer, 1986). The Plus-Minus method (a simplified version of the GRM method with XY=0) assumes that first arrivals only originate from critical refraction and laterally continuous refractors with relatively simple velocity distributions. Figure 2.4 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 (Figure 2.4, top). To obtain the velocity of the refractor (top of the reservoir) and its depth, the Plus-Minus method was used. This method requires geophones to be 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 − and the t + curves to obtain the velocity V2 of the
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