Geology Reference
In-Depth Information
11.7.2
Fault Shape Prediction Techniques
11.7.2.1
Vertical Simple Shear
The vertical simple-shear method for predicting the fault shape from the hangingwall
geometry was developed by Roger Alexander of Chevron and used successfully for many
years in the Chevron Oil Company (Verrall 1982). The technique is sometimes referred
to as the Chevron or Verrall method. If the data are vertically exaggerated, the inferred
bed or fault geometries will have the same vertical exaggeration as the data used, which
may differ from that in the region into which projections are made.
To use vertical simple shear to predict fault shape from the hangingwall fold, three
pieces of geological information are necessary: (1) the shape of the fold in a hangingwall
reference bed, (2) the regional for the hangingwall reference horizon, (3) the block dis-
placement in the direction of the regional ( D ). In order to correctly determine D , the ref-
erence horizon must be correlated across the fault so that its hangingwall and footwall
cutoffs are known. The technique is as follows (refer to Fig. 11.48 for the geometry):
1. Draw a straight line along the regional from footwall to hangingwall at the restored
position of the reference bed.
2. Drop a perpendicular from the regional to the hangingwall cutoff of the reference bed.
3. Measure the fault separation parallel to the regional ( D ). Beginning at the footwall
cutoff, mark equal distances along the regional with spacing equal to D .
4. Drop perpendiculars from the regional at the equal- D points to form working lines.
5. At a working line 1, measure the vertical distance from the regional to the fault.
6. Shift this measured length one D increment in the direction of transport (to line 2)
and drop it down so that the top just touches the key bed in the rollover. The point
marking the bottom of the vertical segment is the position of the fault.
7. Repeat steps 5 and 6, moving sequentially in the direction of transport (from line 2
to 11), until the section is complete.
Fig. 11.48. Vertical simple-shear prediction of fault shape from rollover shape (after Verrall 1982). t 1 in-
dicates the first vertical column that is shifted over D and down to the top of the reference bed. FWC: footwall
cutoff of reference bed; HWC: hangingwall cutoff of reference bed; predicted fault is heavy dashed line
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