Geology Reference
In-Depth Information
Interpretation begins with the azimuth-depth and dip-depth plots (Fig. 9.23). At shal-
lower elevations in the well, the dip magnitude is consistently low and the azimuth highly
variable. These are the characteristics expected for a low regional dip (c.f., Fig. 9.9). The
next deeper interval appears to be a cusp on the dip-depth diagram. In the context of a
cusp, the azimuth-depth diagram suggests flattening drag (c.f., Fig. 9.18). The lower por-
tion of the well contains no clear structural pattern and may represent stratigraphic noise,
perhaps a unit containing disparate dips like a conglomerate (where the pebble bound-
aries would produce dip readings) or a reef (where individual corals might be producing
the dips).
Having isolated the cusp as an interval of interest, further analysis will be performed
on that part of the well log alone. The
T
and
L
directions are most clearly found on the
tangent diagram (Fig. 9.24a). There is a substantial amount of scatter but the least squares
best-fit line does a good job of locating the
T
direction. Where it can be checked against
other geological data, the best fit line has proved to be remarkably reliable, even where the
scatter is large. If a quadratic best-fit line approximates a hyperbola and fits the data
reasonably well, then the fold is probably conical. Where a quadratic best-fit is not hyper-
bolic, the best fit is linear and the fold is cylindrical. The dip-azimuth diagram (Fig. 9.24b)
shows substantial scatter but can be interpreted with reference to Fig. 9.18 as showing a
regional dip component and a drag-fold component. The results of this stage of the analy-
sis give
T
= 052 and
L
= 322. Additional valuable information is the strike of the fault,
which must be approximately parallel to the
L
direction, 322°.
Fig. 9.23.
Azimuth-depth and dip-depth
diagrams for data in Table 9.2.
a
Azimuth versus depth.
b
Dip
versus depth
