Orthographic Projection In

Structural Geology

Any plane is defined in space by three parameters:1- the direction of strike .2- the amount of true dip.3- the direction of true dip (as a general trend).

example: strike = N30ᴼE , true dip = 60ᴼNW …..(strike is in the bearing format)strike = 030ᴼ , true dip = 60ᴼNW …..(strike is in the azimuth format)

Or by two parameters:1- the amount of true dip.2- the direction of true dip (as exact trend).

example: true dip = 60ᴼ , in a direction = N60ᴼW …..(dip direction is in the bearing format)true dip = 60ᴼ , in a direction = 300ᴼ …...(dip direction is in the azimuth format)

Strike of a plane: is the direction of a horizontal line on the plane. True dip of a plane: is the angle between the plane and a horizontal plane,

• measured in a vertical plane.• normal to the strike.

Apparent dip of a plane: is the angle between the plane and a horizontal plane, • measured in a vertical plane• not normal to the strike.

For any plane the value of apparent dip is always smaller than the true dip, The amount of apparent dip ranges between 0 and the value of true dip.

Any line is defined in space by two parameters::1- the trend of the line.2- the angle of plunge (angle of inclination of the line).Example: The line direction (trend or bearing) = S40ᴼW, plunge = 60ᴼ ……(bearing format).The line direction (trend or bearing) = 220ᴼ, plunge = 60ᴼ ..…(azimuth format).

the angle of plunge of a line: is the angle between the line and a horizontal line measured in a vertical plane.

the angle of pitch or rake of a line: is the angle between the line and a horizontal line measured in a plane not vertical.

The orthographic projection of a line:

The projection of a line: Bearing = N40ᴼE Plunge = 30ᴼ

The orthographic projection of a plane:The projection of a plane: Strike = N30ᴼW Dip = 45ᴼNE

Determination of strike and true dip of a plane given two apparent dips

illustration

Given: Two apparent dips measured on a bedding plane as: 30ᴼ on a bearing N40ᴼE. 40ᴼ on a bearing S10ᴼE.

Find: the direction of strike and the amount of true dip of that plane.

Answer:Strike = N20ᴼ ETrue dip = 58 ᴼ SE

Determination of strike and true dip of a plane given two apparent dips:

Determination of apparent dip in a certain trend, given strike and true dip

Given: A bedding plane has: Strike = N30ᴼW. true dip = 45ᴼNE.

Find: The apparent dip in a direction = S50ᴼE.

Answer:App. dip = 19ᴼ

The line of intersection of two planes

Determination of the attitude of the line of intersection of two planesGiven: Two planes:

Find: The trend and plunge of the line of intersection of the two planes.Answer: Trend = N59ᴼE. Plunge = 31ᴼ.

the first: Strikes N60ᴼW and dips 35ᴼNE. the second: strikes N80ᴼE and dips 60ᴼNW.

Exercise:1- Given: Strike and dip of N50ᴼE, 40ᴼSE.

Find: Apparent dip angle in a vertical cliff section trending S70ᴼE .ANSWER: 36ᴼ

2- Two apparent dips are measured on a planar coal seam as: • 25ᴼ on a bearing 130ᴼ (S50ᴼE).• 35ᴼ on a bearing 200ᴼ (S20ᴼW).

Find: Strike and true dip of coal seam.ANSWER: strike = 091ᴼ (S89E), dip = 37ᴼ S.

3- strike and dip measurements on 2 non-parallel planes: • strike = N40ᴼE, dip = 30ᴼSE.• strike = N70ᴼW, dip = 60ᴼNE.

Find: plunge and bearing of the line of intersection. ANSWER: bearing = 094ᴼ (S86ᴼE) , plunge = 25ᴼ