38 Seismic Imaging The direct and reverse shots allow the differentiation between anomalies due to the topography and anomalies associated with the refractors, as shown in Figure 2.2. In the synthetic example, the model is a single layer over a substratum. The waves are refracted by the substratum. The topography is flat with a small rectangular horst. The substratum is flat, with a vertical fault. This Figure shows, from top to bottom: • Time-distance curves for the arrival times of the waves refracted from the substratum. The red curve corresponds to a direct shot located on the left; the blue curve corresponds to a reverse shot located on the right. It can be seen that the anomaly due to the topography is located on the T-X curves at the same abscissas X. • The geological model. The raypaths associated with the direct and reverse shots (red and blue lines) are shown within the model. • T-X/V curves. On the T-X/V curves, the anomalies due to the topography are in phase; the anomalies due to the substratum are shifted laterally in distance. The distance between the anomalies of the 2 curves is the double offset or critical distance (Xc = 2h. tg(ic)). The two T-X/V curves must be shifted in opposite directions towards their associated shot point to put in phase the anomalies due to the substratum (fault) at the location of the geological feature (fault). The distance of the shift is the single offset (half the critical distance). Figure 2.2 Seismic anomalies on T-X curves.
RkJQdWJsaXNoZXIy NjA3NzQ=