Seismic Expression of Structural Styles: A Modeling Approach
Martha O. Withjack, Kristian E. Meisling, Jon Reinke-Walter
Synthetic seismic sections created with a two-dimensional, ray-tracing modeling program reveal the seismic characteristics and interpretational pitfalls for several structural styles. Salt structures appear as zones lacking stratal reflections. Surrounding reflection packages change thickness and have synclinal and "bow-tie" patterns associated with folding produced by salt piercement and withdrawal. Interpretational pitfalls are (1) salt bodies appear too large, (2) adjacent structural and stratigraphic features are concealed, and (3) underlying flat-lying strata appear deformed because of velocity pull-up.
Seismic evidence of thrust faulting includes vertical repetition of reflection packages. Fault-surface reflections and aligned terminations of stratal reflections characterize thrust-fault ramps. Folds have anticlinal or synclinal reflection patterns, although deep and/or tight synclines have "bow-tie" patterns on unmigrated sections. Interpretational pitfalls are (1) bedding-parallel thrusts are difficult to recognize, (2) reflections from steep fold limbs are lacking, and (3) velocity pull-ups and push-downs distort structural geometries beneath thrust-fault ramps.
Divergent wrench faults appear as upward-widening zones of reflection terminations and overlapping stratal reflections. Offsets and thickness changes of reflection packages are inconsistent across these zones. Aligned terminations of stratal reflections and anticlinal, synclinal, and "bow-tie" reflection patterns characterize secondary normal faults and folds, respectively. Interpretational pitfalls are (1) wrench fault locations and geometries are difficult to define and (2) velocity pull-ups and push-downs distort structural geometries near wrench faults.
Fault-surface reflections and aligned terminations of stratal reflections characterize normal faults. Many fault-surface reflections are listric-shaped because of velocity pull-up. Interpretational pitfalls are (1) neither terminations of stratal reflections nor fault-surface reflections coincide with actual fault locations on unmigrated sections, (2) normal fault geometries are difficult to define, and (3) velocity push-downs distort structural geometries beneath normal faults.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.