Engineering Rock Mechanics

438 Appendix 6: Hemispherical projection

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Consider this scenario: a borehole is drilled with a trend of 305" and a
plunge of 65", it intersects a plane with orientation 145/73, but the core has
rotated through 55" clockwise (looking down the hole) during recovery.
What is the apparent orientation of the discontinuity as it emerges from
the hole? The simplest way of solving these sorts of problems is to use the
net to perform the various rotations.

Start by plotting and labelling the

data: here it is BH for the vector

corresponding to the direction of

the borehole, D corresponding to

the line of maximum dip of the

plane, and N for the normal to the

plane.

N

Rotate the tracing paper so that

the borehole is on the E-W line.

Then move BH to the centre of

the net-in this case through an

angle of 90 - 65 = 25". Move the

normal N through the same

angle, but along the small

circle. Label the new points

145i73 BH' and N'. In effect, the net

has been inclined to be perpen-

dicular to the borehole.

BH

305165

Rotate the tracing paper so

that N is on the the E-W line.

Mark its azimuth and mea-

sure its dip. Count round the

rotation (55" in this case), and

put a new tick: call this N.

perimeter the amount of the N' 14sn3