If an upward-propagating wave field is sampled uniformly at a given depth level, it can be accurately continued upwards to an arbitrary point on the earth's surface by a Kirchhoff summation operator. Let P(t,xi,yj,0) be the wave field recorded by an array of unevenly spaced receivers on the surface. We want to resample the wave field with an uniform sampling interval. We consider the wave field recorded on the earth's surface to be the upward continuation of a wave field P(t,xk,yl,z0) that is recorded by an array of uniformly spaced receivers at the depth z0. This relation is described mathematically by
The choice of z0 is determined from two aspects. The aperture of the upward continuation operator for a given maximum time-dip of input data is inversely proportional to z0. Therefore, to achieve high efficiency, we want to use small z0. However, if the trace sampling intervals of the original data are large, using small z0 causes the conjugate operator of the upward continuation to be aliased.
An assumption made by this algorithm is that the velocities above the depth level z0 are known, which is much weaker than the assumption of the full knowledge of the velocity structure made by the least squares migration (Cole, 1992; Ji, 1992). The formulation presented above is well suited for resampling marine data because the depth level z0 can be set within the water layer that has a constant velocity and does not generate any useful events. It should also work for land data recorded in an area of simple near-surface velocity structure. For land data with complicated near surface structures, a similar scheme can be formulated, in which the unevenly sampled traces are modeled as the reverse time propagation of the wave field sampled uniformly at a level above the earth's surface.