1 12 Frudures and hemispherical projection(4) A normal with an orientation of 051/24 measured represents a
plane which dips towards the southwest at low angle (the orientation of
the plane is 231/66). However, we know that the fractures dip towards
the northeast and are almost vertical, which is clearly not the orientation
just found. To resolve this problem, we must draw the portions of the
circles around the boreholes which are at the west and south sides of the
projection.
(5) Carefully sketch the remaining portions of the circles by counting
'across' the projection. This entails counting towards the periphery along
a great circle, and then counting along the complementary great circle
from the opposite side of the projection by the remaining angle, such
that the total distance is 40". When this is complete for both circles, the
intersection will be found towards the southwest of the projection.
(6) Rotate the tracing paper so that the intersection is on the east-west
line, mark a tick on the periphery and write down the plunge of the
intersection, 08".
(7) Rotate the tracing paper back to north and read off the trend of the
intersection, 228". Thus, the orientation of the normals to the fractures
is 228/08 and hence the dip direction and dip angle of the fractures are
048/82. This accords well with the known orientation of the fractures:
dipping towards the northeast at a steep angle.
(8) The trend of the production holes is that of the trend of the
normals to the fractures, and is either 048" or 228". Two orientations
are possible, because the holes are sub-horizontal and can therefore run
in two directions. In either case, the angle between the normals to the
fractures and the axes of the boreholes will be 8".
47.9 A length of core, from a borehole whose orientation is 143/68,
contains a fracture plane of 204/47. The core has rotated through
a clockwise angle (looking down the borehole) of 140° during re-
trieval. What will be the apparent orientation of the fracture as it
emerges from the borehole?A7.9 To solve this question, we incline the projection so that its centre
coincides with the axis of the borehole. This allows us to apply the rota-
tion, before considering the inclination and determining the orientation
of the fracture.
(1) Mark ticks on the periphery for the plane and the borehole axis.
(2) Rotate each tick mark in turn to the east-west line, and mark the
position of the borehole (BH) and the line of maximum dip of the plane
(D). From this latter position, count across the projection by 90" to plot
the position of the normal to the plane; call this position N.
(3) Rotate the tracing paper so that the borehole axis is on the east-
west line, and incline the borehole to the centre of the projection; this
represents an angular movement of 22".
With the tracing paper in the same position, move the normal to the
plane (i.e. point N) along its small circle in the same direction and by the
same amount (22") as the borehole inclination; call this new position N'.
We have effectively inclined the projection so that the centre now repres-