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The pythagoras relationship can be applied to the

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equality of all the lengths marked ‘y’. The Pythagoras relationship can be applied to the triangle SG ˡ P, and the times can be obtained by dividing the distances by V . Thus, T o = 2 d/V and T = SG ˡ /V . The geometry of a CMP shoot differs from that for single-fold coverage, and the effect of dip is therefore different (Figure 2.7). If the interface dips at an angle α , the velocity deduced from a CMP stack is equal to V /cos α and the ‘depth’ is equal to the length of the normal incidence ray from the common mid-point to the interface. In contrast to the single-fold gather of Section 2.1.4, the minimum time is associated with the normal incidence ray. The aim of stacking is to produce a noise-reduced seismic trace that approximates to the normal incidence trace, i.e. to the trace that would have been produced had the source and detector been coincident. The initials CMP replaced an earlier acronym, CDP ( common depth point ) used for the same method. Referring to depth points (reflection points) as ‘common’ implies that all the reflections in a gather have come from the same point on the subsurface interface, which is true only for horizontal interfaces. 3.5 Depth conversion Reflection events are recorded not in depth but in two-way time (TWT). Velocities are needed to convert times into depths, but the Dix velocities (Section 2.1.3) obtained from NMO curves may be 10–20% in error, even
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42 Figure 2.8 Geometric distortion on seismic sections. The image is of a small graben structure beneath an unconformity. The position of the true fault plane BB (indicated by the dashed line) can be estimated from the positions of the terminations of the sub-horizontal reflectors representing the sediment fill within the graben (although care must be exercised because many of the deeper sub-horizontal events are multiples). The event AA is the seismic image of BB. It is displaced because the techniques used to display the data assume that reflections are generated from points vertically beneath the surface points, whereas they are actually generated by normal-incidence rays that are inclined to the vertical if reflected from dipping interfaces. The reflections from the fault and the opposite side of the graben cross over near the lower symbol ‘A’, forming a ‘bow-tie’. Convex-upward reflections near point C are diffraction patterns generated by faulting . for horizontal reflectors. Interpretations should be calibrated against borehole data wherever possible, and field crews should always be on the lookout for opportunities to measure vertical velocities directly. 3.6 Geometric distortion Seismic reflection data are normally presented as sections prepared by playing out, next to each other and vertically down the sheet of paper, the traces from adjacent CMP gathers. Such sections are subject to geometric distortion.
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