Structural Analysis and Synthesis. Stephen M. Rowland

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Figure 2.19 Determining stratigraphic thickness t on slopes. (a) Lengths h and v and dip angle δ are needed to derive t. (b) Geometry of derivation.

Determining Stratigraphic Thickness by Orthographic Projection

      In some situations, the preceding trigonometric techniques for determining stratigraphic thickness cannot be used. In Bree Creek the 400‐ft contour interval does not allow the difference in elevation from the base to the top of a unit to be precisely determined. In such cases, orthographic projection can be used to determine stratigraphic thickness.

To determine the thickness of the Conglomerate (Tg) in the Bree Creek area, begin by finding two points of equal elevation at the same stratigraphic level. A line between such points defines the strike. In Figure 2.21a, a line is drawn through the top of Tg at 4800 ft., and another is drawn through the top of Tg at 4400 ft. The objective of this construction is to draw a vertical cross section perpendicular to strike.

      Line AB is drawn perpendicular to the two strike lines (Figure 2.21a). This represents the 4800‐ft elevation line in the orthographic projection. A second line, CD, is drawn, perpendicular to the two strike lines. Line CD represents the 4400‐ft elevation line. The distance between lines AB and CD is taken directly off the map. Next line AD is drawn, which represents the eastward‐dipping top of Tg in orthographic projection.

Repeating this procedure with the bottom contact of Tg results in points W, X, Y, and Z (Figure 2.21b). Line WZ represents the base of Tg in orthographic projection, and the thickness can be measured directly off the diagram. The precision is primarily limited by the scale of the map. In this example, Tg can be measured to be about 100 ft thick.

Figure 2.20 Three combinations of sloping topography and dipping layers, with the appropriate formula for each.

Figure 2.21 Technique for determining stratigraphic thickness by orthographic projection. (a) Plotting top surface. (b) Plotting bottom surface and deriving the thickness.

Figure P2.1 Map of an area showing contacts and topographic contours to be used with Problem 2.1.

Figure P2.2 Cutout to produce a block model of the area in Figure P2.1 to visualize the three‐dimensional geology for Problem 2.1.

Figure P2.3 Base map showing locations of wells to be used in Problem 2.2.

Figure P2.4 Topographic contour map to be used in Problem 2.3 showing locations of wells A, B, and C, leaking industrial landfill, and traverse where cross section (X–X′) should be drawn.

Figure P2.5 Topographic base map with outcrop locations to be used in the solution of Problem 2.4.

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