From acquisition footprints to true amplitude

Abstract

This paper addresses the issue of the sensitivity of 3‐D prestack depth migration (PSDM) with respect to the acquisition geometry of 3‐D seismic surveys. Using the theoretical framework of PSDM, I show how acquisition‐related imaging artifacts—the acquisition footprints—can arise. I then show how the acquisition footprint can be suppressed in two steps by (1) partitioning the 3‐D survey into minimal data sets, each to be migrated separately, and (2) applying a robust variable‐geometry PSDM quadrature. The validity of the method is demonstrated on synthetic parallel and antiparallel multistreamer data and cross‐spread data. The proposed two‐step solution can play an important role in projects where amplitude integrity and fidelity are paramount, e.g., quantitative interpretation and time‐lapse surveying. The concept of minimal data also fills a gap in understanding the relation between acquisition and imaging.

3D Seismic Survey Design, Second edition

“…The program does not account for geometry effects. Gesbert (2002) shows that amplitude variations would reduce considerably if trueamplitude migration took geometry effects into account. This is illustrated in the horizon slices shown on the right side of Figure 5.14, for parallel and antiparallel acquisition.…”

“…The width of such a data set will generally not be large enough to image a reflector effectively. Gesbert (2002) gives a mathematically more precise definition of MDS. He excludes singlefold data sets that do not extend across the whole survey area because they potentially would generate strong artifacts at the boundaries.…”

“…In the interior of all horizon slices of Figure 10.12, the amplitude varies slightly because a simple algorithm was used. True-amplitude migration (Gesbert, 2002) of the MDSs used for Figure 10.12 would have produced constant amplitudes inside the full-image areas with equal amplitude for all four examples.…”

“…Gesbert (2002) formulates precise criteria for a data set to be a minimal data set. This means the acquisition lines should not be contorted such that the illumination areas of the cross-spreads are more than singlefold in some places.…”

“…Very strong amplitude variations are the result of migrating this collection of data. Using true-amplitude prestack depth migration (PSDM), Gesbert (2002) also analyzes a gather of a small range of absolute offsets. His seismic section ( Figure 10.14a) shows strong migration smiles; the horizon slice (Figure 10.14b) also shows considerable amplitude variation of the horizon of interest.…”