Where and what is the near surface? According to Butler (2005), the majority of near-surface surveys are shallower than 30 m, occasionally down to 100 m, and very rarely considered for depths beyond 100 m. In Saudi Arabia, the majority of upholes are of 100 m depth, and models of the near surface are deeper than 100 m in order to model to the seismic datum. In metalliferous mining areas, low-grade metamorphosed sediments or igneous rocks can constitute the near surface for mining engineering but are not considered near surface in the search for hydrocarbons. For those studying the earth's crust and who have a different perspective, Sharma (1978) finds “… velocity distributions show a well-defined low-velocity layer at 100–150 km depth …”; in those studies, the whole of the sedimentary section would be included in the near surface.

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