As shown in Figure 1.13, an undulating topography can produce moveout delays in the CSG that can also be interpreted as undulations in the reflector, even if the reflector is flat. After an elevation statics correction (i.e., time shifts applied to traces) the data appear to have been collected on a flat datum plane. Figure 1.13 shows that static shifts can also be introduced by near-surface velocity anomalies which usually delay the traveltimes, resulting in reflections having a non-hyperbolic moveout curve.
Static shifts introduced by
topographic variations fall under the class of field statics,
and those due to near-surface lithological variations
that occur within a cable length
fall under the class of
Correcting for static
shifts in the traces can make a significant difference in the quality of a migrated or stacked image.
It is easy to determine elevation static corrections, but not so easy to find the residual static
corrections. One means is to determine the near-surface velocity distribution by refraction tomography,
which will be discussed later.