Vertical Shear

Flatten on a horizon represented by well picks..

We do this to show thickness variations across a unit at the time of deposition. By slipping logs, we are carrying out the equivalent to a vertical shear restoration on a cross-section without wells. It involves the differential vertical displacement of vertical slices of a cross section to restore a reference horizon to a datum [1]. There are other angles of shear that we can use (often 60 degrees for oblique shear) for cross section restorations but vertical shear only applies to log slipping.

An example of log slippage..

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) is top of the sandstone (red line on Fig. 1a). The leftmost well has the shallowest penetration of the sandstone top. We restore to this reference horizon by moving/slipping the 3 deepest penetrations of the sandstone upwards to the level of the leftmost penetration (well) 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.