The most “correct” description of ideal seismic illumination is “a large specular reflection aperture at every subsurface image point (not CMP location) is complemented by complete spatial sampling of the recorded wavefield”. If a suitably accurate 3D velocity model can be built, such an appropriately illuminated seismic volume should yield the optimum combination of high resolution, artefact-free imaging.
Several factors impact the quality of seismic target illumination:
• The topography of the surface (in a land survey) or the water bottom (in a marine survey),
• The complex three-dimensional media between the surface/water bottom and the target. In geological terms, vertical and lateral velocity variations occur because of structural, stratigraphic and lithological complexities. High velocity contrasts have the most impact upon ray path geometry - notably basalt/intrusions, salt, chalk and carbonate layers; and
• The topography of the target interface.
As the complexity of the problem increases, the focus must shift towards more sophisticated acquisition solutions – and Multi-Azimuth (combining surveys acquired over the same area but at different directions to one another) or, in the most challenging scenarios, Wide-Azimuth acquisition (collecting data from every direction or azimuth) needs to be considered. Correspondingly, the processing and imaging technologies employed must also improve in sophistication and effort. Initially, time domain processing may suffice, but as the problem becomes increasingly complex different pre-stack depth migration (PSDM) approaches become necessary - but these will only be successful if suitably accurate velocity models can be built.
In the most extreme cases, the overburden effects upon wave propagation are so severe that no coherent target events can be imaged because of the lack of illumination. Data processing cannot generate data that have not been recorded - even with the most sophisticated processing and imaging technologies.
Fortunately, pre-survey planning is quite sophisticated and reliable. Each of the target illumination concepts and challenges discussed here can be evaluated and contrasted in reasonable time frames.