Toward accurate seismic flattening: Methods and applications

Abstract

Seismic flattening maps a seismic volume from the original space in depth (or two-way traveltime) to the Wheeler domain in geologic time where all of the seismic reflections are horizontally aligned. It provides an efficient way to interpret a whole volume of horizons all at once by extracting the horizontal slices in the flattened space. Conventional slope-based flattening methods that can locally flatten the seismic reflections, however, often fail to flatten the reflections in a global sense and cannot accurately align the reflections across faults. We have developed an iterative method to improve the flattening by using the slopes and correlations of seismic traces. The local slopes, estimated for each image sample, can locally follow reflections but may fail to track the reflections over a long distance or correlate the reflections across faults. The seismic correlations, computed for randomly and sparsely extracted seismic traces, help to align the corresponding reflections over a long distance and across faults. We compute flattening shifts, one for each image sample, by fitting the seismic slopes and correlating in the least-squares sense. We further apply the shifts to relatively adjust the seismic samples in the vertical direction so that all of the reflections are horizontally aligned. We iteratively flatten the seismic volume by repeating the process multiple times (often less than five times). With the flattening shifts, we can compute a relative geologic time (RGT) volume that implicitly contains all the structure information of the seismic volume. An arbitrary number of horizons then can be extracted as isosurfaces of RGT values. Multiple field examples demonstrate that our method significantly improves the flattening, especially across faults and missing data zones, compared with conventional flattening methods. Our method is especially helpful to interpret subtle stratigraphic features (e.g., vertically thin channels) by providing an accurately flattened seismic volume.