Imaging and Partial Subsurface Illumination
In Chapter 8, I analyzed how the spatial sampling rate influences image quality. If data sampling is not sufficiently dense, the seismic image may lose resolution and/or it may be affected by artifacts. Unfortunately, however, density of spatial sampling is not the only problem encountered with realistic 3D acquisition geometries. An even more common problem is irregularity of the spatial sampling. Often, irregular sampling in space is a product of practical constraints, examples of which include cable feathering in marine acquisition and surface obstacles in land acquisition. In other cases (e.g., with button-patch geometries), irregular sampling geometry might be inherent in the survey design.
The main effect of irregular sampling geometries is either uneven illumination or incomplete illumination of the subsurface. Such partial illumination causes distortions in the image. In milder cases, distortions are limited to the image amplitudes, and they are clearly visible in depth or time slices. Those distortions often are called acquisition footprint. Figure 1 shows an example of acquisition footprints in a migrated depth slice taken from a marine data set. On the right-hand side, horizontal striping is clearly visible, superimposed over the image of a complex turbidite system with crossing channels. The horizontal striping is not linked to geology; it is along the direction of the sailing lines of the recording vessel.
When subsurface illumination is not only uneven but is also incomplete, the phase of the image is distorted, and strong artifacts are created. At the limit, when the acquisition