Multiple suppression with land data: A case study

Some of the most important methods for multiple suppression are based on the moveout difference between the hyperbolas corresponding to primary and multiple reflections in a CMP gather. This moveout difference is exploited by means of the Parabolic Radon Transform. In this case study I review the methodology and show the result of its application to a 2-D land seismic line are. Of particular importance are the results that show that without the suppression of multiples a distorted image is obtained of the Paleozoic and its stratigraphic terminations against basement, which constitute the exploratory objective in the area. This is partly due to the improved stacking velocities afforded by the suppression of the multiples. 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. 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. 1email: [email protected] 1