Geology Reference
In-Depth Information
Fig. 3.18.
Structure contour direction and
spacing from an attitude mea-
surement.
a
Dip vector at
a point having an elevation
of 800.
b
Structure contours
in the vicinity of the point.
For
I
= 100,
H
= 214
Fig. 3.19.
Contour-spacing ruler
3.6.2
Projected and Composite Surfaces
A projected surface is a structure contour map derived entirely by projecting data from
other stratigraphic levels. A projected marker is sometimes called a ghost horizon.
Usually a projected surface is below the lowest control points. A typical use is to project
the subsurface location of an aquifer or an oil reservoir from outcrop or shallow sub-
surface information.
A composite surface is a structure contour map derived using data from multiple
stratigraphic horizons, including the horizon being mapped. One horizon is selected
as the reference surface and data from other stratigraphic horizons are projected up-
ward or downward to this horizon, using the known stratigraphic thicknesses. The
best choice of a reference horizon is one for which there is already a significant amount
of control and that minimizes the projection distance. Usually a reference horizon that
is stratigraphically in the middle of the best-controlled units should be selected. The
data from multiple horizons provide increased control on the interpretation of the shape
of the reference horizon. This type of map is particularly useful in the interpretation
of outcrop data because the locations of all formation boundaries can be used to pro-
vide control points, greatly increasing the areal distribution of data.
An elevation on a marker surface is transformed into an elevation on a projected
surface by adding or subtracting the vertical distance between the two (Fig. 3.20). The
projection is made from a point where the elevation of the marker is known. This may
be the location of a surface outcrop (as in Fig. 3.20) or the elevation of a contact in a
well. The distance to the projected horizon is derived from the thickness of the unit by
