The principles and assumptions are described by Nunns, 1991 (Structural Restoration of Seismic and Geologic Sections in Extensional Regimes, AAPG Bulletin, V. 75, No. 2). The method seeks to create paleovertical trajectories in the present day seismic or geologic section that correspond to unbroken vertical traces in the restored section. A hybrid approach is used with the following key characteristics.
- Length is conserved along the reference horizon.
- The internal parts of fault blocks are restored using vertical shear.
- A constant amount of slip is removed along each fault, equal to the slip of the reference horizon across that fault.
This approach is robust for moderate amounts of 2-D structural deformation. The accuracy of the method has been calibrated by restoring cross sections of theoretical and physical forward models, where all the intermediate states are known. Some of these results are shown in Nunns (1991). In many circumstances, the accuracy of restored dimensions is roughly proportional to the cosine of the maximum dip on the restoration horizon - about 95 % accurate for dips up to 10 degrees and 90 % accurate for dips up to 20 degrees.
- If the cross-section you are analyzing is not parallel to the principal displacement direction, then you will get erroneous results.
- You should carefully consider whether there have been multiple phases and directions of strain.
- If there are layers of mobile salt or shale in your section, the restoration will be incorrect within and below the mobile layers.
StructureSolver restored sections are not decompacted. Where there is a substantial compaction gradient with depth, the restored thicknesses of beds near the top of the restored section will not be representative of their true thicknesses at the time of deposition of the reference horizon. However, analysis presented in Nunns (1991) showed that the relative restored structural relationships across faults will be fairly accurate provided that there are not rapid changes in compaction gradient either vertically or horizontally. Xiao and Suppe (1992) came to similar conclusions regarding the accuracy of forward modeling without taking compaction into account.
StructureSolver removes a constant amount of slip across faults during creating of paleoverticals for restoration. This creates a challenge near the bottom of faults. StructureSolver has two ways of handling fault tips. See Nunns (1991) for further discussion.
- If there is no other fault beneath the fault in question, then the amount of slip removed remains constant until the end of the fault and then stops abruptly. This approach is designed to handle faults that continue all the way to the base of the image that is being restored.
- If on the other hand, a fault tips out against an underlying fault, then the amount of slip that is removed tapers out over a distance that is equal to the slip itself. This produces a better result for antithetic faults.
If two synthetic faults merge, then you may wish to continue the upper fault parallel to the lower fault - with a small separation between them. This has the effect of adding the slips for the two faults, which in many cases will produce a more accurate relationship between the hangingwall and footwall in the restored section.
Fault Terminations and Junctions