Geology Reference
In-Depth Information
Cross Sections, Data Projection and Dip-Domain Mapping
6.1
Introduction
A cross section shows the relationships between different horizons and allows the
information from multiple map horizons to be incorporated into the interpretation.
Cross sections may categorized as illustrative or predictive. The purpose of an illustra-
tive cross section is to illustrate the cross section view of an already-completed map or
3-D interpretation. A slice through a 3-D interpretation is a perfect example. The pur-
pose of a predictive cross section is to assemble scattered information and, utilizing
appropriate rules, predict the geometry between control points. A predictive cross
section can be used to predict the geometry of a horizon for which little or no infor-
mation is available.
Data projection is typically part of the cross-section construction process. Relevant
data commonly lie a significant distance from the line of section. Rather than ignore
this information, it can be projected onto the section plane. The quality of the result
depends on selecting the correct projection direction. This chapter describes how to
select the projection direction and gives several techniques for making the projection
by hand or analytically. Projection within a dip-domain style structure involves defin-
ing the 3-D axial-surface network and so becomes a blend of mapping, data projection,
and section construction.
6.2
Cross-Section Preliminaries
6.2.1
Choosing the Line of Section
Cross sections constructed for the purpose of structural interpretation are usually
oriented perpendicular to the fold axis, perpendicular to a major fault, or parallel to
these trends. The structural trend to use in controlling the direction of the cross sec-
tion is the axis of the largest fold in the map area or the strike of the major fault in
the area. Good reasons may exist for other choices of the basic design parameters.
For example, the cross section may be required in a specific location and direction
for the construction of a road cut or a mine layout. If other choices of the parameters,
such as the direction of the section line or the amount of vertical exaggeration, are
required, it is recommended that a section normal to strike be constructed and vali-
