Geology Reference
In-Depth Information
Fig. 9.20.
Structure contour map, cross section, and SCAT plots for a normal fault striking oblique to
regional dip with drag that flattens the regional dip.
L:
regional strike;
T:
regional down-dip direction;
T':
regional up-dip direction;
L*:
fault strike;
T*:
normal to fault strike;
CP:
crestal plane;
TP:
trough
plane. (After Bengtson 1981a)
diagram. For a normal fault, the cusp points in the direction of the fault dip. For a
reverse fault, the cusp points opposite to the direction of the fault dip. Note that the
cusp on the dip vs. depth plot always points in the same direction because the dips are
not plotted according to direction.
A drag fold may be present on one side of the fault but absent on the other, resulting
in a half-cusp pattern. As indicated by Fig. 9.21a, this geometry may be present at the map
scale as well as at the drag-fold scale. Folds of this type produce a half-cusp pattern on the
transverse dip component plot (Fig. 9.21b). In association with a reverse-fault, the dips
may increase to vertical and then become overturned. On the
T
-component plot (Fig. 9.21b),
the half cusp curves smoothly to the left to a 90° dip, then reappears where dips are 90°
to the right. The isolated group of dips near 90° on the right represents overturned beds,
providing a method for recognizing overturning from the dip sequence alone.
A synthetic example of a dipmeter run across a normal fault will serve to illustrate
the method. The example also illustrates SCAT analysis using a spreadsheet. The tra-
ditional paper-copy dipmeter (Fig. 9.22) is a graph of dip versus depth in a well. The
“tadpole” heads indicate the amount of dip and the tails the direction of dip. Solid
heads represent the best data and open heads the worst. In the case of a four-armed
dipmeter a solid head represents a dip based on correlation of all four arms and an
open head means three of the four arms can be correlated. If only two arms can be
correlated, the dip cannot be calculated and no point is plotted. The numerical data set
is given in Table 9.2.
