Drilling is the culmination of the mineral exploration process where the
third dimension of a prospect, the subsurface geometry, is defined.
Drilling provides most of the information for the final evaluation of a
prospect and will ultimately determine if the prospect is mineable.
Geochemical analyses of the drill samples provide the basis for
determining the average grade of the ore deposit. Careful logging of
the drill samples helps delineate the geometry and calculate the volume of
ore, and provides important structural details. The two principle
types of drilling are diamond core drilling and reverse circulation
drilling (or RVC drilling). Diamond Core Drilling
Drill data is interpreted by constructing “drill sections”, which show the
drill holes in a vertical profile analogous to cross sections.
Construction of the drill section begins in the same manner as a geologic
cross-section, by creating a topographic profile. Then the “collar”
locations (where the drill enters the ground) are plotted along the
topographic profile. A vertical drill hole (plunge = -90 deg) will
plot as a vertical line on the drill section, and angle drill holes are
plotted showing the appropriate inclination. The length of the
line(s) which illustrate the drill hole are determined by the scale of the
drill section. For example, if the drill section scale is 1 inch =
10 feet, then a drill hole with a total depth (TD) of 100 feet will be 10
Drill holes which are not situated exactly along the drill section line
can be “projected” onto the plane of the drill section (within a
reasonable distance) (Figure 16 – 4). The projection is done along a
line perpendicular to the drill section line. If an inclined drill
hole does not plunge directly into the vertical plane of the drill
section, then its inclination on the drill section will appear as an
“apparent dip”. The apparent dip angle is always less than the true
dip. The apparent dip angle is a function of the true dip and the
angle between the drill section line and the drill hole surface trace in a
map view (Table 16 – 1).
If a drill hole intersects a tabular-shaped mineralized zone or rock layer
at a 90 degree angle, then the thickness of the zone or layer seen in the
drill core or recorded in the drill log represents the “true
thickness”. If the drill hole intersects the zone or layer at
any angle less than 90 degrees, then the thickness observed is called
“apparent thickness”. The true thickness of the mineralized zone
must be known in order to calculate the volume of the zone (Volume =
length x width x thickness). If the dip of the mineralized
zone is known, and the inclination of the drill hole is known, then the
true thickness can be calculated using simple trigonometry.
Figure 16 - 4. Map showing the projection of several drill hole
collar locations onto drill section line.
Table 16 – 1. Apparent Dip Angles as function of true dip and strike
Each drill hole on the drill section should show the name of the drill
hole above and the total depth (TD) below (Figure 16 - 5). At this
point, a decision is made as to which information will be shown.
Typically each drill hole shows the intervals containing significant or
ore grade values. Often this is done by highlighting or bracketing
these intervals. Now the geologist may interpret the geometry of the
ore zone by extrapolating between drill holes, which is a matter of
connecting the upper and lower contacts of the zone from one drill hole to
the next. The geology may be interpreted in different ways by
different geologists (Figure 16 - 6). To help with the
interpretation, additional drill sections may be constructed which show
different aspects of the drill data. For example, another drill
section may be constructed which shows a specific alteration or
Figure 16 – 5. Drill section with ore intercepts and geology.
Figure 16 - 6. Two different interpretations of the same drill
section (from SME Mining & Engineering Handbook).