--> Seismic Modeling and Expression of Common Fold-Thrust Belt Structures

AAPG ACE 2018

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Seismic Modeling and Expression of Common Fold-Thrust Belt Structures

Abstract

Seismic modeling can be used to understand the expression of common fold-thrust structures in seismic time and depth sections, and to avoid pitfalls in the seismic interpretation of natural structures. Modeling of seismic time sections using both post-stack and pre-stack time migration was conducted for fault-bend and fault-propagation folds. Time-migrated and stacked models of fault-bend folds with low angle fault ramps provide a good rendition of the geometry of hanging wall beds for both pre-growth and syngrowth sections. Because of the typically low dips of both the front and back limbs, the beds are well imaged and can be accurately migrated to their correct positions. Footwall beds typically show pull up of reflectors, particularly under the front limb and the crest. The fault ramp and a segment of the upper flat can also appear to be folded. This can result in the erroneous interpretation of these features as subthrust structures, if the velocity effects are not completely corrected in depth sections.

Seismic modeling of fault-propagation folds for models with constant front-limb angles and trishear models results in many more uncertainties. Although the back limb and crest of the structure are typically well imaged, the front limbs are characterized by wide zones with no data. This effect is significantly more pronounced for steep front limb angles for both constant front limb angle models and trishear models with low propagation to slip ratios. Footwall beds are characterized by low amplitude reflectors and exhibit a pronounced pull up. This can result in their interpretation as upturned beds against the fault. Furthermore, poor velocity information at the anticlinal and synclinal bends on the front limb can result in overmigration or undermigration of reflectors. This can result in an incorrect estimation of the extent of fault propagation through the front limb. Trishear models with relatively small slips, on the other hand, exhibit good imaging of some of the upper units, because the front limb dips are relatively low.

Although depth migration can correct for many of the velocity related pitfalls discussed above, the processing is dependent on accurate velocity models. Therefore, an understanding of the key pitfalls observed in the seismic models is critical in developing accurate interpretations of natural structures.