Sometimes we see sections of planes oblique to their strike. In cases like these, the slope of the plane is not its true dip but shallower. In the case of a section parallel to the strike, the plane can appear horizontal. The dip of a plane as seen in an oblique section is called its apparent dip. We might want to know the apparent dip a bed would have in order to draw a cross-section, calculate where it would outcrop in a trench or highway cut, or for a variety of other reasons.
This problem is actually an intersecting-plane problem where one of the planes is vertical. We already know the trend of the intersection -- it's simply the strike of the section plane. All we need to do, then, is find the plunge, which is the apparent dip as seen in the section plane.
+-----------------------------+ +-----------------------------+
|1 Elev. 430 m / /| |2 / /|
| ------- / / | | / / |
| | 36 / / | |--400-------------------/ / |
| / / | | / / |
| / / | | / / |
| / / | | / / |
| / / | |--300---------------/ / |
| / / | | / / |
| / / | | 170 m / / |
| / / | | / / |
| / / | |--200-----------/ / |
| / / | | / / |
| CONTOUR INTERVAL = 100 M | | / / |
| | | / / |
| 0 100 200 M | | 0 100 200 M |
| |----I----| SCALE | | |----I----| SCALE |
+-----------------------------+ +-----------------------------+
1. Find the apparent dip of the 2. Draw the structure contours
bed in the highway cut shown on the bed. Where they inter-
sect the cut we have elevation
points on the intersection line
+-----------------------------+ +-----------------------------+
|3 | |4 / /|
| 170 m 170 m | | / / |
| |--------|--------| | |--400-------------------/ / |
| o + | | | / / |
| 30 + | | | / / |
| + + 100 m | | / / |
| + | | |--300---------------/ / |
| + | | | / / |
| + 200 m | | / / |
| | | / / |
| | |--200-----------/ / Apparent |
| Tan D(app) = 100/170 = .588 | | / / dip = |
| D(app) = 30.5 | | / / 30 SW |
| | | / / |
| 0 100 200 M | | 0 100 200 M |
| |----I----| SCALE | | |----I----| SCALE |
+-----------------------------+ +-----------------------------+
3. Find the plunge of the 4. Final result
intersection using either
trigonometry or by drawing a
cross-section.
Tan (Apparent Dip) = Tan (True Dip) Sin (Angle between strike and cross-section).
In the example above, the true dip = 36 degrees and the angle between the strike and cross-section equals 53 degrees, so we have Tan (Apparent Dip) = Tan (36) Sin (53) = 0.58, and Apparent dip = 30 degrees.
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Created 5 January 1999, Last Update 5 January 1999
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