Publications

When imaging in complex environments, two key issues need to be addressed: multipathing and uneven illumination. Multipathing occurs because rays can be reflected from or refracted through the overburden before or after hitting the target. This creates problems for single path Kirchhoff migration which uses an approximation based on the most significant rays (for example largest amplitude or first arrival). Illumination is an issue because significant lateral velocity variation produces significant laterally varying ray bending that leads to uneven hit densities on the target reflectors. Since a seismic image is built up by interference among a large number of migration “sweeps, if uneven illumination is not properly compensated there will be signal/noise problems above target reflectors due to incorrect interference of the sweeps, as well as incorrect amplitudes on the target reflectors. In most of these settings diffractions are not the key issue, as is evident from the form of a typical velocity model. Thus more computationally intensive approaches such as Wave Equation Migration are not necessarily required.These considerations suggest that an effective solution to these problems is to use multipath raybased imaging. Signal/noise is significantly dependent on reflection angle therefore, an effective imaging strategy is to use Common Reflection Angle Migration (CRAM). This directly addresses the multipathing issue since a dynamic ray tracer can store, and the migration program can use, all relevant parameters and rays whether or not they are reflected from or refracted through the hard rock on their way to or from the target reflectors. In addition, appropriately chosen weight functions introduced into the migration sums can compensate for uneven illumination. These weights are generally not available in conventional single path Kirchhoff migration.