Comparison among Interferometric, Reduced-time and Kirchhoff Migration of CDP Data

One of the difficulties in seeing beneath salt is that the migration velocity in the salt and above is not well known. This can lead to defocusing of migration images beneath the salt. In this paper we show that reduced-time migration (RTM) and interferometric migration (IM) can mitigate this problem. Reduced-time migration shifts the data with the time difference between the calculated arrival time τ refsg and natural arrival time τ ̃ refsg of a reference reflection, where s and g denote the source and receiver locations on the surface and τ refsg is calculated by raytracing through the salt velocity model. We use the term natural and calculated to represent, respectively, the arrival times that are velocity-independent (times directly picked from the data without knowledge of the velocity model) and velocity-dependent (times calculated by raytracing through a given velocity model). The benefit of RTM is a significant reduc-tion of defocusing errors caused by errors in the migration velocity. Interferometric migration on the other hand requires 1) extrapolation of the surface data below salt using the natural arrival times τ ̃ refsg of the subsalt reference reflector, and 2) migration of the extrapolated data below the salt. The benefit with IM is that no salt veloc-ity model is needed so the model-based defocusing errors are, in theory, eliminated. To reduce computational time and artifacts we implement IM with a semi-natural Green’s function. We use the term semi − natural because the Green’s function is constructed by a combination of model-based (calculated) and picked (natural) traveltimes. Since no explicit data extrapolation is needed, IM with the semi-natural Green’s function approach is more cost efficient than the standard IM. We also tested both RTM and IM with semi-natural Green’s functions on both synthetic and field CDP data sets. Results show that they both can remove the significant kinematic distortions due to the overburden without knowledge of the overburden velocity, and 1 appear to overcome some distortion problems in the migration images obtained by Kirchhoff migration with residual statics