The industry-standard method of redatuming is to perform static shifts of data traces. This is adequate when the near-surface raypaths are nearly vertical or when the shifts are small. The static shift method is commonly applied to correct for near-surface velocity variations due to weathering layers and for topographic effects. In this section I will use simple synthetic examples to show that when static shifts are large and the assumption of vertical raypaths is not valid, the wavefield is distorted by the static shift so that velocity analysis, migration and any other wave equation based processing is severely degraded.

The problem with many land data sets is that the topography distorts the data so much that it is not possible to pick coherent reflection events or to estimate velocity. In these situations, wave-equation datuming is necessary.

If the near-surface velocity structure is known, data can be downward continued through the near-surface velocity structure to some lower datum. In this type of situation, Shtivelman and Canning (1988) show the accuracy limitations of static shifting and the need to apply wave-equation datuming when differences in elevation are significant and the velocity model is complicated. For most land data, the near-surface velocity is not known. One method of eliminating distortions due to topography is to upward continue the data to an arbitrary datum above the highest topography. This is analogous to upward continuing marine data from the water bottom back to the original acquisition surface in marine layer replacement applications. The proper velocity to use for upward continuation should be close to the near-surface velocity. Once the data have been upward continued to a higher datum, conventional velocity analysis can be performed. The result of velocity analysis, stacking, and migration after wave equation-datuming is superior to the result after static shift.

Thus, wave-equation datuming provides a way of extrapolating a distorted wavefield to a planar datum without requiring detailed knowledge of the near-surface velocity. Once the data is at the planar datum, the velocity structure can be determined.

2/12/2001