Introduction

Accurate interpretation of subsurface seismic images is of the utmost importance for oil exploration and production. To achieve this goal it is necessary not only that the images be of good quality, but also that they correspond exclusively to the energy from the primary reflections, that is, those reflected from only one subsurface interface before their recording at the surface of the earth. Any other form of energy is undesirable. Such energy may correspond, for instance, to refractions, surface noise, guided waves and multiple reflections. They are collectively called coherent noise. Multiple reflections, energy that has been reflected at more than one interface, are particularly troublesome for seismic interpretation since they can be easily mistaken as primary reflections. Figure 1 shows a schematic representation of some of the more common forms of multiples. In general, long period multiples are more common in marine data, whereas short period multiples are more common in land data.

 

mul_esq1
Figure 1 Broad multiple classification. Left: long period multiples. Right: short period multiples.

Multiple reflections are particularly difficult to discriminate from primary reflections in land data because they generally lack the familiar periodicity associated with marine data multiples. This has led to the misconception that multiple reflections are only a problem with marine data. Seismic interpretation in areas such as the one illustrated here can be severely jeopardized by the presence of short period multiples which are extremely difficult to identify. Figure 2 shows such a seismic line. It will be shown later that it is plagued with multiple reflections; but just from the seismic section, there is no way to tell immediately which reflections are multiples.

The suppression of multiples in seismic data processing can be achieved by several means of which filtering in the parabolic Radon Transform domain is the most common Hampson (1986). This process will be described in some detail and the results of its application to the seismic line in Figure 2 are presented. It is shown that the suppression of the multiples not only produce a more faithful image of the subsurface, but has the added benefit of allowing for the computation of more accurate NMO-stacking velocities.

 

res2_stack.rot
Figure 2 Stacked seismic section. Which reflections are multiples?