50 Seismic Imaging a b c d Figure 2.10 Chronos method (Coppens, 1985). Coppens showed that picking two traces per shot, i.e. two constant-offset sections on a seismic profile, is sufficient to determine the delay time for each source or receiver location. This method is well adapted to end-on spreads (Figure 2.10). The two constant offsets must be chosen to ensure that the first arrival times on the two selected traces originate from the same refractor as shown in Figure 2.10-a: • Ti,i+n is the arrival time of the refracted wave with the source located at position i and the receiver at position j= i + n, the source receiver distance being n (near offset). • Ti,i+m is the arrival time of the refracted wave with the source located at position i and the receiver at position i + m, the source receiver distance being m (far offset). • Ti+n,i+m is the arrival time of the refracted wave with the source located at position i + n and the receiver at position i + m, the source receiver distance being m -n (near offset). Consequently, the far offset (m=2n) is the double of the near offset (n). • The arrival times Ti,i+n, Ti,i+m and Ti+n,i+m are used to calculate the delay DSi+n at position j= i + n.
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