Imaging zero-offset sections using multipath summation

Abstract

The main goal of reflection seismology is to obtain a reliable image of the subsurface. The method widely used to accomplish this task is NMO and CMP stacking. In areas with relatively simple geological structures this robust technique produces a reasonably good image of a zero-offset time section. However, in areas with complex geology, the central problem of reflection data processing becomes to produce a reliable image of zero-offset section, i.e. to obtain a stacked time section which is as close as possible to the true zero-offset section. This image should include all the reflected events that may be related to different geological features. The reflections may come from various depths and have various dips. In complex geological media, more than one dipping reflector can exist at the same reflection time. The NMO-based CMP stacking fails to resolve conflicting dips, because it acts as a dip filter selecting one predominant dip, thus deteriorating the quality of the stacked section. The usual way to overcome the problem is to apply dip moveout (DMO) correction (Hale, 1984; Deregowski, 1986; Yilmaz, 2001). The DMO is able to preserve all the dips existing in the data and thus to produce a much better image of zero-offset section. The DMO is usually implemented as a part of a processing sequence including velocity analysis, NMO, DMO, inverse NMO, velocity analysis, and NMO with new velocities and stack. However, applying the DMO sequence can be time-consuming and not always effective. We propose an alternative way for imaging zero-offset sections. We show that using the multipath summation approach implemented as a single, simple, quick and efficient process (Keydar, 2004; Shtivelman and Keydar, 2004; Landa, 2004), one can achieve the results equivalent to those obtained by the above DMO sequence. The proposed method does not require information on velocities, since the multipath summation is performed for all possible velocity values within a wide specified range. Furthermore, the method is stretch free, since no explicit dynamic correction is involved. An additional feature of the method is that it preserves the signature of the waves for which the summation is performed. Application of the method is illustrated by a synthetic example. The example demonstrates that the section obtained by the multipath summation is very close to the true zero-offset section.