From CMP to CRS - An Overview of Stacking Techniques of Seismic Data

Stacking procedures with the objective of improvement of signal to noise ratio were in use before the introduction of Common reflection point stacking introduced by Mayne in 1962. But, these techniques were problematic, at times, because they used reflections from a portion of reflectors that were too large. Hence, stacking tends to obscure the very detail of reflector which is being sought. Sorting to CMP gather and their stacking after normal moveout correction was devised by Mayne and provided a practical means of increasing multiplicity without losing the detail of the reflector. Mayne quantified the average enhancement of signal to noise ratio and showed that it is proportional to the square root of the number of signals. CMP stacking is a robust enough to handle seismic data from many different part of the world and provide reasonable good image. However, it assumes stratified earth in applying normal moveout correction and that the CMP stack is equivalent to a zero offset section. Dipping events and in regions of complex geology these assumptions are not valid and conventional processing has difficulty in delineating steeply dipping events. For a planar dipping reflector below a homogeneous overburden, reflection point dispersal take place which is also called reflection point smear. The data of a single CMP gather no longer belong to one and the same reflection point and their stacking produces degraded image. The application of dip move out (DMO) correction to the data removes the smearing and corrected CMP gather contains only reflections from a single point. However, as soon as the reflector becomes curved or the medium above the reflector is not homogeneous, the DMO correction is not exact. A residual reflection point smear will remain even after this correction. Pre-stack depth migration is carried out to correct this residual and also positioning the reflector at correct position. A number other techniques like application of higher order NMO correction and anisotropic pre-stack migration was introduced in processing to get reliable sub-surface image and improve S/N ratio. Recent developments in stacking methods include common reflection surface (CRS) stack which uses far more traces than those present in a CMP stack which leads to a better signal to noise ratio and continuity of the reflector in CRS stack. The CRS technique uses larger stacking surfaces rather than relaying on a single CMP stack location in conventional stacking processes. This leads to a larger stacking fold and results in the improvement in resolution in time and depth domain.