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 uses a diamond bit, which
rotates at the end of drill rod (or pipe) (Figure 16 – 1). The
opening at the end of the diamond bit allows a solid column of rock to
move up into the drill pipe and be recovered at the surface.
Standard core sizes are 7/8 inch (EX), 1 3/16 inch (AX), 1 5/8 inch (BX)
and 2 1/8 inch (NX). Most drill rods are 10 feet long. After
the first 10 feet is drilled, a new section of pipe is screwed into the
top end, so the combination of pipes can be driven another 10 feet into
the ground. The diamond bit is rotated slowly with gentle pressure
while being lubricated with water to prevent overheating. The
drill depth is estimated by keeping count of the number of drill
rods.
Figure 16 – 1. Diamond bit.
The driller listens to the drill very carefully to evaluate the
condition of the drilling below. He will adjust the rotation
speed, pressure and water circulation for different rock types and
drilling conditions so as to avoid problems, such as getting the bit
stuck or overheated. Highly fractured rocks (often encountered
near the surface), in addition to the risk of a stuck bit, allow the
fluids to escape, leading to overheating. The problem is minimized
by injecting “drilling mud” (or sawdust or other materials) into the
drill hole to “plug” the fractures and prevent escape of the fluids.
Inside the drill pipe is a “core tube”, which has a latching mechanism
attached to a cable. At the end of each 10 foot run, the cable is
used to winch the core tube containing the new rock core to the surface
where it can be recovered. The drill core is stored in specially
designed core boxes containing compartments to hold sections of the
core. Standard core boxes are 2.5 feet long and contain four
compartments, so ten feet of core can be stored in each box.
The drill core is first washed and logged by a qualified geologist, and
then split in half to provide a sample for geochemical analysis.
Since so much time, effort and money is expended to obtain the drill
core, it is worthwhile to study and log the core very carefully. A
standardized log form is used to log the core. The form has
columns for each of the types of information which will be recorded,
with tick marks indicating the footage. The information typically
shown includes the recovery %, lithology, alteration, mineralization,
rock quality data (RQD), and structural details. Although the
strike and dip of planar features such as bedding, foliation, faults and
veins is not known, the angle of these features with respect to the axis
of the drill core axis is noted, because it still provides valuable
information about the geometry of the features. Mineral testing
may also be done, including testing for fluorescence (scheelite),
testing for effervescence with dilute HCl (carbonate alteration), or
mineral staining (feldspars or carbonates). Often the core will be
photographed as well. The recovery % is the ratio of the actual
core length compared to the drill interval indicated. Voids and
fracture zones cause poor recovery. For example, if a drill run of
10 feet obtains 8 feet of drill core, the recovery is 80 %. The
drill logs used to construct drill sections (cross sections showing the
drill holes) which illustrate the subsurface geometry of the ore
body. The current trend is to create drill logs in digital or
spreadsheet format, which facilitates the construction of drill sections
by computer.
Core splitting is done using a rock saw or an impact core
splitter. There is always the problem of obtaining a
representative split of the core. Great care must be taken to
avoid this problem. Sometimes the entire core is analyzed to avoid
this problem, but usually only if logging is extremely thorough.
In some cases a series of small chips are collected along the length of
the core to form a “skeleton core” for archiving purposes.
RVC drilling is fundamentally different from
diamond core drilling, both in terms of equipment and sampling.
One major difference is that RVC drilling creates small rock chips
instead of solid core. Other major differences are in the rate of
penetration and cost per foot. RVC drilling is much faster than
diamond core drilling, and also much less expensive.
RVC drilling requires much larger equipment, including a high capacity
air compressor. The compressor forces air down the outer space of
a double wall pipe (Figure 16 - 2). The air circulates back up
through the inner pipe carrying the rock chips, which are recovered at
the surface. The chips travel at such high velocity they must be
slowed down first, using a “cyclone”. The “return pipe” directs
the chips to glance off the inside wall of the cyclone chamber, and then
spiral downward to the bottom of the cyclone, loosing velocity in the
process. The chips are collected continuously as the drill
advances into the ground. Drill pipes used for RVC drilling are
usually either 6” or 8” in diameter and 20 feet in length. Each
pipe is extremely heavy and requires the use of a winch to lift and
position over the drill hole.
Figure 16 - 2. RVC double wall drill pipe (with tricone bit)
showing air path (from NEW ERA Engineering Corp).
RVC drill bits are also completely different from diamond drill
bits. One type is called a “hammer bit”, named for the way it
rapidly pounds and pulverizes the rock interface. This type of bit
works well in dry drilling conditions (ie, above the water table) and in
rock formations which are dense and hard. Below the water table,
the water in the formation actually cushions the bit, making it much
less effective at shattering the rock. Another type of drill bit,
called a “tricone bit”, has three revolving cone-shaped grinders, which
rotate together like the differential gears in a car transmission.
Tricone bits are slower drilling in hard formations, but are very
effective in soft formations and in wet drilling conditions.
Samples of drill cuttings are usually collected over five foot
intervals. The large diameter of the drill hole creates a huge
volume of material for each sample, which is typically “split” into a
reasonable volume to handle and send to the lab for
analysis. In dry drilling conditions (above the water
table), a dry splitter is used (also know as a Jones splitter) (Figure
16 - 3). Usually a split of 1/8 of the total is collected.
The Jones splitter is made up of tiers, each of which splits the sample
in half. After the third tier split, 1/8 of the original total
sample remains, which is collected in a bin or bucket. When the
drill reaches the depth of the water table, rotary “wet” splitter is
used (Figure 16 - ). The wet splitter spins around and
splits the sample using a series of fins, similar to the fins in a
turbine engine. Every other chamber directs material to a pipe
which funnels the material into a bucket.
Figure 16 - 3. Typical dry splitter samping (A) and wet splitter
sampling (B) arrangements.
Small representative samples of the chips are collected continuously
during the sampling process and placed in plastic boxes with
compartments called “chip trays”. These are carefully observed and
logged by a competent geologist. Of course some types of
information, such as structural details, are not possible to obtain in
the absence of solid rock. In spite of this disadvantage, much
valuable information can still be obtained from the rock chips.
For example, the chips are much easier to examine under a
microscope. Testing of fluorescence and effervescence are easily
accomplished.
Drill Sections
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
inches long.
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
orientation.
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
mineralization type.
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).
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).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 inches long.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 orientation.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 mineralization type.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).
Technique of geological core drilling
Drilling rig capable of both diamond and reverse circulation drilling.
Exploration diamond drilling is used in the mining industry to probe the contents of known ore deposits and potential sites. By withdrawing a small diameter core of rock from the orebody, geologists can analyze the core by chemical assay and conduct petrologic, structural, and mineralogical studies of the rock. It is also often used in the geotechnical engineering industry for foundation testing in conjunction with soil sampling methods. The technique is named for the diamond encrusted drill bit used.
History
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Diamond core drill bits
Rodolphe Leschot is often cited as being the inventor of the first core bit in 1863.[1] Early diamond drilling opened up many new areas for mineral mining, and was related to a boom in mineral exploration in remote locations. Before the invention of the portable diamond drill, most mineral prospecting was limited to finding outcrops at the surface and hand digging. In the late 1970s, General Electric pioneered the technology of polycrystalline diamond compacts (PDCs) as a replacement for natural diamonds in drill bits.[2]
Diamond drilling
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Illustration showing drill core
Exploration diamond drilling differs from other geological drilling (such as Reverse Circulation (RC) Drilling[3]) in that a solid core is extracted from depth, for examination on the surface. The key technology of the diamond drill is the actual diamond bit itself.[4] It is composed of industrial diamonds set into a soft metallic matrix. As shown in the figure, the diamonds are scattered throughout the matrix, and the action relies on the matrix to slowly wear during the drilling process, so as to expose more diamonds. The bit is mounted onto a core barrel, that is attached to the drill string, which is connected to a rotary drill rig. Drilling mud is injected into the drill pipe, so as to wash out the rock cuttings produced by the bit and also to reduce the heat produced due to friction which causes less wear and tear of the bits.[5] An actual diamond bit is a complex affair, usually designed for a specific rock type, with many channels for washing.[6]
The drill uses a diamond encrusted drill bit (pictured on the right) to drill through the rock. The drill produces a "core" which is photographed and split longitudinally. Half of the split core is assayed, while the other half is permanently stored for future use and re-assaying if necessary. Although a larger diameter core is the most preferred it is the most expensive. The most common wire line tube diameters and purposes are NQ (47.6mm) and HQ (63.5mm).[7]
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Extraction of core using triple-tube
Merely advancing the drill by rotary action (and washing) causes a core to be extracted inside the barrel as shown. However, at a depth of perhaps 300 m, there must be a way to retrieve the core and take it to the surface. Constantly withdrawing the entire heavy drill pipe is impractical, so wireline drilling methods were developed to pull up the core inside the barrel.[8] If the rock would always be solid granite, and the core would always break at the drill bit, then it would be a simple matter to stop the drilling, and lower a simple grabbing device by a wire and pull up the core. Unfortunately, many applications require an undisturbed core in fractured rock, which calls for elaborate wire-line devices.
The photo shows the extraction of a core, using a triple-tube wire-line system, capable of extracting core under the worst conditions.[9] This is very important when exploring fault zones such as the San Andreas Fault.
Tube sizes
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There are five major "wire line" tube sizes typically used. Larger tubes produce larger diameter rock cores and require more drill power to drive them. The choice of tube size is a trade-off between the rock core diameter desired and the depth that can be drilled with a particular drilling rig motor.
Standard "Q" wire line bit sizes:[10]
Drill cores
Size
Hole (outside)
diameter, mm
Core (inside)
diameter, mm
AQ
48
27
BQ
60
36.5
NQ
75.7
47.6
HQ
96
63.5
HQ3
96
61.1
PQ
122.6
85
PQ3
122.6
83
CHD 76
75.7
43.5
CHD 101
101.3
63.5
CHD 134
134.0
85.0
References
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