In order to improve the signal-to-noise ratio of noisy seismic reflection data before velocity determination by computer, it's common practice to sum several traces having the same source-to-geophone (SGD) separation but different CDP points. We say we form a vertical stack. When reflectors are horizontal planes, vertical stacking simply reduces the noise without distorting the reflection waveforms. However, when a reflector is a dipping plane, the individual reflections from that plane may be identical; but they arrive at slightly different times. As a result, the sum reflection is distorted: vertical stacking acts as a filter.

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