Page 246 - Introduction to Mineral Exploration

10: EVALUATION TECHNIQUES 229

the final cash payment is made. Consequently, or using a hydraulically driven downhole
it is important for this representative to be motor, which acts as a directional control
thoroughly familiar with the contract and the device (Cooper & Sternberg 1988, Dawson &
problems that can arise at a drilling site, par- Tokle 1999). This relatively new directional-
ticularly if in a remote location. drilling tool was originally designed for the oil
The client (i.e. the company) is at liberty to industry. The downhole motor is powered by
fix specifications such as, say, a plus 90% core the circulating fluids passed down the center
recovery, a vertical hole with less than 5 of the rod string, which drives the bit with-
degrees deviation. It is then at the discretion out drill-rod rotation. A spring deflection shoe
of the drilling company as to whether or not located just above the bit acts as a directional
these requirements are accepted, but if they are control device and delivers a constant side pres-
and the conditions are not met the failure is sure to the drillhole wall forcing the bit to
remedied at the expense of the contractor. move (and deflect) in the opposite direction.
The amount of spring loading in the shoe is pre-
set before the unit is lowered into the hole and,
10.3.5 Borehole surveying
obviously, progress is monitored by closely
Drills are usually unpredictable wanderers. spaced surveys. The deflection unit is a special
In section holes drilled at a low angle to rock tool and is not used in normal drilling.
structures tend to follow that structure while
holes at a higher angle tend to become perpen-
dicular to the feature. A similar behavior can 10.3.6 Drillhole patterns
be expected from alternations of hard and soft The pattern of drilling in an exploration pro-
rock (e.g. layers of chert in chalk). Steep holes gram depends primarily upon the intended
are likely to flatten and all holes tend to spiral use of the data. In reconnaissance work where
counter clockwise with the rotation of the drill the geology is poorly known the first holes
rods. However, wandering also depends upon may be isolated from each other and drilled for
the pressure on the drilling bit and its rate of geological orientation purposes. In exploration
revolution, so that two drillholes in similar for sedimentary uranium deposits, coal and
geological situations may take different paths. borates, holes may be drilled up to 10 km apart
Holes with a length of 1000 m have been found to locate sedimentary formations of interest
to be several 100 m off course – while indi- and obtain structural data. Later holes may be a
vidual lengths of drill rod are essentially inflex- few kilometers apart whilst drilling a specific
ible, a drill string several hundred meters in target calls for “fences” or lines of holes, at a
length will bend and curve. Since drillholes are close spacing of 100 –200 m. However, the loca-
used for sampling at depth, if the orientation of tion and spacing of holes in the last eventual-
the hole is not known the location of the sam- ity are guided by detailed geological mapping,
ple is similarly unknown, and geological pro- geochemical, geophysical, and geostatistical
jections based on these samples will be in error. results (see sections 5.2 and 10.4.1).
Borehole surveys are conducted as a matter Generally, a systematic grid of drillholes
of routine in all holes and there are a variety (and samples) taken normal to the expected
of instruments for this purpose. They most mineralized zone is a preferred pattern. This
commonly indicate the direction (azimuth) and provides a good statistical coverage and geo-
inclination of the hole at selected intervals, logical cross sections can be made with a
commonly every 100 m, using a small mag- minimum of projection. Such fences may
netic or gyroscopic compass. The calculation well be 200–400 m apart with individual
of corrected positions at successive depths is holes spaced at 100–200 m intervals, which
a straightforward mathematical procedure, if allows room for systematic infill drilling if
one knows the location of the top of the hole the results justify extra work. The inclina-
(X, Y, and Z co-ordinates) and the initial hole tion of individual holes is obviously import-
inclination. ant and generally they should be drilled at
Deliberate deflection can be achieved either right angles to the expected average dip of the
by wedging holes (say, 1 degree per wedge) mineralisation.