Geology Reference
In-Depth Information
Begin either technique by compiling the hard data onto the line of section. The
cross section that shows just the original data will be called the
data
section. The cross-
section interpretation should be done as an overlay on the data section. The interpo-
lation between the control points may change dramatically during the interpretation
process, but the locations of the control points and bedding attitudes should remain
the same. Including the data section along with the final interpretation separates the
data from the interpretation, a fundamental distinction that should always be made.
The data may be subject to revision, of course. For example, inconsistencies in the
cross section may indicate that a geologic contact has been mislocated or that a fault
is required. This is one of the important reasons for constructing a predictive cross
section. In the best scientific procedure, the original data and the interpreted result are
both presented in the final report.
Data to be compiled will include dips and contact locations. If the section is not
perpendicular to the fold axis, all dips shown on the cross section must be the apparent
dips in the plane of the section. Measured attitudes must be converted using Eq. 2.18
or with a tangent diagram or stereogram. On an exaggerated profile (not recommended),
the dip must be the exaggerated dip from Eq. 6.3 or 6.4. When the data have been trans-
ferred to the cross section, the section should again be checked against the information
seen along the line of section on the map. A common mistake is to produce a cross
section that fails to match the map along the line of section. All elevations, geological
contacts, attitudes (apparent dips), and attitude locations must match exactly along the
line of section. Projection of data to the line of section is commonly required and is
discussed in Sect. 6.6.
It is useful to summarize the stratigraphic thicknesses in a “stratigraphic ruler” which
will greatly speed up the drawing of an unexaggerated cross section. Draw the strati-
graphic section at the scale of the cross section on a narrow piece of paper or, in com-
puter drafting, make it a group of its own. This can be used as a ruler to mark off the
stratigraphic units on the cross section and provides a quick check to see if the thickness
of a unit is consistent with its dip. This only works on unexaggerated cross sections
and is one of the important reasons for not using vertical or horizontal exaggeration.
Many structural interpretations are based on seismic reflection profiles which are al-
ready displayed in the form of a cross section. A seismic line that is to be interpreted
structurally should satisfy the same criteria with respect to the choice of the plane of
section and vertical exaggeration as a geological cross section. If geological data are avail-
able, transferring the data from maps to the seismic line will provide constraints that will
help in the construction or validation of the depth interpretation. A successfully depth
-
converted seismic line must follow the same geometric rules as a geologic cross section.
6.4.1
Dip-Domain Style
The dip-domain method is based on the assumption that the beds occur as planar
segments separated by narrow hinges. This method was originally proposed as sec-
tion construction technique by Gill (1953) who called it the method of tangents. The
basis for the technique lies in the relationship between the bedding thickness and the
symmetry of the hinge (discussed previously in Sect. 5.4.2). For constant thickness
