Flatten a reference horizon to a datum..
We routinely do this on correlation panels to show thickness variations across a unit at the time of deposition and to make the correlation of neighbouring markers easier.
How does this compare to Vertical Simple-Shear Restoration ?
Vertical simple shear restoration involves the vertical displacement of vertical slices of a cross section to restore a reference horizon to a datum [1]. Often these sections contain extensional faults. In compressional settings it is more usual to use flexural slip as a restoration method. Anyway, vertical simple shear restoration is effectively what we are doing when we slip logs until they are aligned. We are effectively carrying out the equivalent restoration on a cross-section with faults in an extensional setting like the North Sea.
An example of log slipping..
The example below is from Rick Groshong’s book [2]. Lithology logs from 4 wells are shown. The sandstone reservoir is shown in yellow. Our reference horizon (that we want to restore to/to be flattened) is top of the sandstone (red line on Fig. 1a). The leftmost well has the shallowest penetration of the sandstone top. We restore this reference horizon by slipping the 3 deepest penetrations of the sandstone upwards to the level of the leftmost penetration (well) i.e., to the datum on Fig. 1b.
Interestingly, we can see a fold at the top of the brown limestone unit meaning that there was an anticline structure present at the time that the sandstone was deposited.
Fig. 1. Log slipping/vertical shear restoration (a) deformed state cross section based on lithological logs. Black line is zero elevation/sea level (b) cross section flattened to datum at the top of the sandstone (yellow unit) shows an anticline at the top of the lowermost brown unit. Modified from Groshong (2006), Fig 11.41.
Conclusion..
When we restore well logs to a datum on a correlation panel we are essentially carrying out a vertical simple-shear restoration. Therefore, log slipping is the same type of restoration as a vertical simple-shear restoration on a cross section containing faults.
References
[1]. Verrall, 1982. Structural Interpretation with applications to North Sea Problems. Course Notes No 3. Joint Association of Petroleum Exploration Courses (JAPEC), London, unpaginated.
[2] Groshong. R. 2006. 3D Structural Geology: A Practical Guide to Qunatiitative Surface and Sub surface Map Interpretation. 2nd Ed, P 344 - section 11.7.1.1. Vertical Shear.