Analysis of velocity spectra in Tertiary sand-shale sequences sometimes shows anomalously high velocity events, with high correlation factors, deep in the section. One source of these events may be diffraction patterns. Figure 1 shows a model where a velocity gather is centered over a diffraction source; the diffraction acts like a normal reflection common depth point. Calculations yield the expected velocity of 7500 ft/sec. In Figure 2, the velocity gather is centered 6000 ft on the ground horizontally from the vertical projection of the diffraction source. Analysis of the data of Figure 2 results in a velocity which is quite a bit too high. Various combinations of velocity, depth, and offset produce varying degrees of error. The velocity computed from a spectrum not centered over a diffraction pattern is always too high, when we assume that the diffraction source is in the plane of the section.

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