Civil Engineering Reference
In-Depth Information
such as bedding, which can be used to orient the
core and measure the orientation of the other
discontinuities. However, minor, and unknown
variations in the orientation of the marker bed
are likely to lead to errors. Therefore, it is usu-
ally preferable to use one of the following three
methods to determine the top-of-the-core.
The clay impression method to orient core
involves fabricating a wireline core barrel with
one side weighted so that the barrel can rotate
and position the weight at the bottom of the
hole (Figure 3.14) (Call
et al
., 1982). A piece
of clay is placed at the lower end of the barrel
such that it protrudes past the drill bit when the
core barrel is lowered down the rods and locked
into place. A light pressure is then applied to the
rods so that the clay takes an impression of the
rock surface at the end of the hole. The core bar-
rel is then removed and the clay impression, with
top-of-core reference line, is retrieved. The next
drill run proceeds normally. When this length of
core is removed, the top-of-the-core is matched
with the clay impression and the top-of-core line
is transferred from the clay to the core run. The
discontinuities in the core run are then oriented
relative to the top-of-core line using the method
shown in Figure 3.13.
The advantages of the clay impression core ori-
entation method are its simplicity and low equip-
ment cost. However, the time required to take an
impression on each drill run slows drilling, and
the method can only be used in holes inclined
at angles flatter than about 70
◦
. Also, the ori-
entation line will be lost at any place where the
core is broken and it is not possible to extend the
top-of-the-core line past the break.
A more sophisticated core orientation tool is
the Christiensen-Hugel device that scribes a con-
tinuous line down the core during drilling; the
orientation of the scribed line is determined by
taking photographs of a compass in the head of
the core barrel. The advantage of this method
Eccentrically loaded
inner core tube
Plate welded across upper
end of lifter case
Core lifter with core spring
removed and packed with
modelling clay which
protrudes 10 mm
Diamond drill bit
Clay to take inprint
of core stub
Core stub left at the end
of the previous drilling run
Figure 3.14
Clay impression core barrel to orient drill
core in an inclined hole (Call
et al
., 1982).
(a)
W
S
(b)
N
S
W
N
E
S
E
Discontinuity
Point P on
unwrapped
image
Intersection line of
discontinuity on
borehole wall
Figure 3.15
Orientation of
discontinuity in borehole using image
from 360
◦
scanning video camera: (a)
core-like image of drill hole wall
showing elliptical intersection between
discontinuity and core; (b)
“unwrapped” view of borehole wall
with discontinuity displayed as a sine
wave (Colog Inc., 1999).
Trace of
intersection
line on
unwrapped
image
Point P on borehole
wall
