Engineering Rock Mechanics

98 Fractures and hemispherical projection

stage of the solution of the example question. When using the hemi-
spherical projection method, a sheet of tracing paper is placed over the
projection and rotated around a drawing pin protruding from under-
neath through both the projection and the tracing paper. The diagrams
are of the working on the tracing paper used over the hemispherical
projection, with the perimeter of the net included for reference.

(a) The orientation of the plane containing the two boreholes

N

Mark the north point, draw tick

marks on the periphery at the

borehole azimuths of 298" and

055", and write down the borehole

orientations.

298/38

n

Rotate the tracing paper so that

the tick for Borehole 1, BH1, is on

the E-W line. Count in 38", mark

and label a cross for this borehole

as BH1.

298/38

Rotate the tracing paper so that

the tick for BH2 is on the E-W

line. Count in 7Z0, mark and label

a cross for BH2.

Rotate the tracing paper so that

the crosses for BH1 and BH2 lie

on the same great circle. Draw

the great circle and a cross at its

maximum dip. Place a tick on the

periphery opposite this cross, and

write next to it the dip angle of the

plane (found by counting from the

periphery to the line of maximum

dip).