AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Integration of Coherence and Volumetric Curvature Attributes
(1) Arcis Corporation, Calgary, AB, Canada.
(2) University of Oklahoma, Norman, OK.
Volumetric curvature is a well-established interpretational tool that allows us to image subtle faults, folds, incised channels, differential compaction, and a wide range of other stratigraphic features. In general, curvature is an excellent measure of paleo deformation. With an appropriate tectonic deformation model, a good structural geologist can predict where fractures were formed. However, since their formation, such fractures may have been cemented, filled with overlying sediments or diagenetically altered. Furthermore, the present-day direction of minimum horizontal stress may have rotated from the direction at the time of deformation, such that previously open fractures are now closed, while previously closed fractures may now be open. For this reason, prediction of open fractures requires not only images of faults and flexures provided by coherence and curvature coupled with an appropriate model of deformation, but also measures of present day stress provided by breakouts seen in image lots and seismic anisotropy measures.
The maximum and minimum curvatures (and two principal curvatures), kmax and kmin, define the eigenvalues of a quadratic surface, while the azimuth of minimum curvature, ψmin, defines the eigenvectors projected onto the horizontal plane. By definition (and based on eigenstructure analysis), the maximum curvature is defined as the principal curvature that has the larger absolute value. However, we find that the principal curvatures k1 and k2, where k1≥k2, provide the simplicity of interpretation seen in kpos and kneg, but retain the robustness of kmax and kmin in the presence of steep dip. In the case of faults and folds, a cursory look at the horizon slices through the most-positive curvature, kpos, and the most-positive principal curvature k1 show longer, more continuous folds and flexures, which continue even where their absolute value is less than that of kneg or k2. For this reason, many authors favor these displays when mapping stratigraphic features as well as subtle faults and fractures in the presence of gentle dip. However, in areas of folding in the presence of significant dip, the crest and trough of a fold defined as the highest and lowest points on a vertical section no longer correspond to the locations of the tightest folding. We will illustrate these conclusions with real data examples.