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A dipping reflector

Consider the zero-offset seismic survey shown in Figure 5.1. This survey uses one source-receiver pair, and the receiver is always at the same location as the source. At each position, denoted by $S_1, S_2, \mbox{and} S_3$ in the figure, the source emits waves and the receiver records the echoes as a single seismic trace. After each trace is recorded, the source-receiver pair is moved a small distance and the experiment is repeated.

reflexpt
Figure 1.
Raypaths and wavefronts for a zero-offset seismic line shot above a dipping reflector. The earth's propagation velocity is constant.
reflexpt
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As shown in the figure, the source at $S_2$ emits a spherically-spreading wave that bounces off the reflector and then returns to the receiver at $S_2$. The raypaths drawn between $S_i$ and $R_i$ are orthogonal to the reflector and hence are called normal rays. These rays reveal how the zero-offset section misrepresents the truth. For example, the trace recorded at $S_2$ is dominated by the reflectivity near reflection point $R_2$, where the normal ray from $S_2$ hits the reflector. If the zero-offset section corresponding to Figure 5.1 is displayed, the reflectivity at $R_2$ will be falsely displayed as though it were directly beneath $S_2$, which it certainly is not. This lateral mispositioning is the first part of the illusion. The second part is vertical: if converted to depth, the zero-offset section will show $R_2$ to be deeper than it really is. The reason is that the slant path of the normal ray is longer than a vertical shaft drilled from the surface down to $R_2$.


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Next: Dipping-reflector shifts Up: MIGRATION DEFINED Previous: MIGRATION DEFINED

2009-03-16