Geology Reference
In-Depth Information
Features other than faults may resemble faults. Reflectors are also truncated at an
unconformity (Fig. 7.4a, between the arrows labeled D). The presence of parallel reflec-
tors above the unconformity supports the interpretation that the truncation is indeed an
unconformity, not a fault. Reflections from steeply dipping beds may fail to be recorded
or may not be correctly migrated, leading to zones of disturbed reflectors that can easily
be mistaken for fault zones. The lack of reflector continuity around and above location E
(Fig. 7.4a) is due to the steep limb of a fold (Fig. 7.4b), not to a fault.
Regions of complex structure are commonly associated with faults and may have steep
dips and large lateral velocity changes, either of which can create discontinuities at deeper
levels on the seismic profile (Fig. 7.5). Discontinuities in the otherwise continuous reflec-
tors below 2-s two-way travel time (Fig. 7.5a) might easily be interpreted as being normal
faults (Fig. 7.5b), although the interpreters of the line correctly did not do so (Fig. 7.5c).
This region is below a major thrust fault that places rocks as old as Devonian on top of Cre-
taceous units. The older rocks have significantly higher seismic velocities than the Creta-
ceous rocks. The geological interpretation (Fig. 7.5d), based on well control and the seis-
mic profile, indicates no offset of the Cretaceous and older units below the thrust, but
rather a continuous westward regional dip of the subthrust units. The apparent eastward
dip of the subthrust units is a velocity pull up caused by the high velocities of the rocks in
the thrust sheet. The apparent normal faults in the subthrust sequence are probably caused
by the rapid lateral velocity gradients associated with fault slices within the thrust sheet.
Reflection profiles can be one of the most powerful tools for structural interpreta-
tion. Because the profile itself is an interpretation based on the inferred velocity struc-
ture of the region, the geological interpretations must be tested with all of the same
techniques that are applied to geological data.
7.2.3
Discontinuities on Structure Contour Map
A linear trend of closely spaced structure contours that form a monoclinal fold may
represent an unrecognized fault (F, Fig. 7.6). The monocline could be replaced by a
fault. Linear fold trends are, of course, perfectly reasonable and so independent evi-
Fig. 7.6.
Structure contour map show-
ing the possible locations of
faults (
F
) between the
arrows
.
Contours are in feet
