Strike and Dip Exercise

The orientation of a rock unit (locally a plane in space, even if regionally deformed by folding and/or faulting) can be defined by two characteristics: strike and dip.  

The strike is the orientation of the line marking the intersection of the plane of the rock unit with a horizontal plane. Strike is bidirectional and is by convention reported in the northern hemisphere (e.g., N33W, or 325 azimuth). 

Dip is perpendicular to strike (thus can have only two directions once strike is known) and is by convention reported by the cardinal compass direction and the angle (e.g., 23 to the E).

Strike and dip can be written in many ways: Quadrant, Azimuth (right hand rule), Dip and dip direction, and Dip direction dip. The following table is an exercise to write strike and dip in these different ways.

When using the Quadrant method you give the direction of strike as the acute angle from North. In this case it is necessary to state the general direction of dip since it could be to either side of the strike. For example N10E, 50E compared to N10E, 50W. The dips are in opposite directions but the strike direction is the same.

When using the Azimuth direction for strike the right hand rule is normally used. This distinguishes a unique direction of the otherwise bidirectional strike. If you place your right hand on the plane with your right thumb in the direction of strike and your fingers in the direction of dip then the Aximuth direction is the direction in which your thumb is pointing. For example 10,50 would be a strike of N10E and dip 50E where as 190,50 would be a strike of N10E, and dip 50W.

Dip and dip direction (or Dip direction and dip) are basically the same thing but written in a different order. You need to know which method is used otherwise the results can be very wrong. Often it is obvious since dip can never exceed 90 degrees and dip direction is the azimuth direction in with the plane dips and can be between 0 and 360 degrees. It is obvious if 95/85 is stated (95 is the direction of the dip or dip direction and the dip is 85). However if 30/50 is stated it is impossible to know if the dip direction (or the dip) is 30 or 50.

Determine the quadrant, azimuth, dip and dip direction, dip direction and dip where missing. Also, sketch the approximately oriented strike and dip symbol (North is up, parallel to the long edge of the paper).

Quadrant

Azimuth - Right Hand Rule

Dip and dip direction

Dip direction and dip

Sketch (symbol)

N25W, 34NE

 

 

 

 

 

000,63

 

 

 

 

 

23/135

 

 

N52W, 33NE

 

 

 

 

 

 

44/242

 

 

N13W, 20SE

 

 

 

 

 

330,15

 

 

 

 

 

3/183

 

 

Three Point Problems

We can uniquely define the attitude of a plane if we have three known spots on the plane, provided that we assume a constant dip value. In many geological situations this might mean knowing its depth beneath the surface at three localities. Imagine that we have three drill holes (A, B and C) down to a geologic bedding plane (tan) from a horizontal ground surface (green).

A three-point problem requires three points, of known location and elevation, which are ON THE SAME PLANE.

To solve a three-point problem:

  1. The two points with the highest and lowest elevations should be joined with a light, thin pencil line (Line 1). (If two of the points have the same elevation, this step and the following one are not necessary.)
  2. The elevation on that line which matches that of the third point must be interpolated (by calculating the relief between the two points, determining the elevation of the intermediate point as a percentage of that relief, and scaling that percentage of the distance onto the line) and marked on the line (4734). [On a rock unit, this is strike.]
  3. The line connecting the interpolated point with the third point (Line 2) is therefore horizontal, and the perpendicular line from Line 2 to either of the first two points (Line 3) defines the slope direction.
  4. The slope of Line 3, or dip angle of a rock unit, can be calculated from the relief between Line 2 and the selected point and the length of line 3. [Dip direction is downhill, by convention.] Tan 68/200

The map shows three drill hole sites. The ground level is flat (all start at the same altitude). Fill in the answers in the table below the map.

  1. Case 1: The numbers indicates the drill distance to the top of a bed. What is the strike and dip of the bed?
  2. Case 2: The numbers on the map indicate the altitude of the top of the bed. What is the strike and dip of the bed?

 

Quadrant

Azimuth - Right Hand Rule

Dip and dip direction

Dip direction and dip

Sketch (symbol)

Case 1

 

 

 

 

 

Case 2