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Reflection points as common implies that all the

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(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. Artefacts such as displaced reflectors, diffraction patterns and ‘bow-ties’, described in Section 3.2 as affecting radar sections, also appear on seismic
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43 imagery, as is shown in Figure 2.8. Refraction surveys are widely used to study the water table and, for engineering purposes, the poorly consolidated layers near the ground surface, and also in determining near-surface corrections for deep reflection traces. Travel times are usually only a few tens of milliseconds and there is little separation between arrivals of different types of wave or of waves that have travelled by different paths. Only the first arrivals, which are always of a P wave, can be ‘picked’ with any confidence. 4.0 Conclusion In recent years, reflection data have also been for identifying lithology, generally from velocity and attenuation characteristics of the transmitted and reflected seismic waves, and for detecting hydrocarbons, primarily gas, directly on the basis of reflection amplitudes and other seismic indicators. The reflection method comes closer than any other prospecting technique to
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