3D laser scanning and DTH hole surveys in stope design and quality monitoring by Kalumba Bwale, Lubambe Copper Mine Zambia Abstract Since the first functional laser being built and tested in 1960, various applications for the laser have been developed over the years. The spatial coherence of lasers makes them ideal measuring tools in situations where measurements need to be taken in inaccessible points and areas. This property gave rise to cavity monitoring systems (CMS) and laser scanners. Technology, that enables huge volumes of spatial data to be collected within a remarkably short period of time. Recent advances in laser scanning coupled with the exponential increase in processing power have greatly improved the methods used to estimate stope tonnages extracted from massive inaccessible stopes. At Lubambe Copper Mine in Zambia, the survey department uses a Faro Laser Scanner or an Optech V500 CMS to collect a dense dataset that is used to create a point cloud of geometric points inside the inaccessible stopes. The collected dataset is then used to construct digital, three dimensional models of the various stope constituents called wireframes or solids. The procedures described in this paper are mainly concerned with the collection and geo- referencing of the data and the intersections of the various solid models to provide meaningful data for production control and quality monitoring processes. The basis for these calculations is an accurate geological model for comparison with the stope parameters. Modern survey techniques are also necessary to locate all geological information to within 0,1 m in 3D space. The ability to affect a mine’s bottom line can be significantly affected by the application of these technologies.
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3D laser scanning and DTH hole surveys in stope design and quality monitoring by Kalumba Bwale, Lubambe Copper Mine Zambia Abstract Since the first functional laser being built and tested in 1960, various applications for the laser
have been developed over the years. The spatial coherence of lasers makes them ideal measuring
tools in situations where measurements need to be taken in inaccessible points and areas. This
property gave rise to cavity monitoring systems (CMS) and laser scanners. Technology, that
enables huge volumes of spatial data to be collected within a remarkably short period of time.
Recent advances in laser scanning coupled with the exponential increase in processing power have
greatly improved the methods used to estimate stope tonnages extracted from massive inaccessible
stopes. At Lubambe Copper Mine in Zambia, the survey department uses a Faro Laser Scanner or
an Optech V500 CMS to collect a dense dataset that is used to create a point cloud of geometric
points inside the inaccessible stopes. The collected dataset is then used to construct digital, three
dimensional models of the various stope constituents called wireframes or solids.
The procedures described in this paper are mainly concerned with the collection and geo-
referencing of the data and the intersections of the various solid models to provide meaningful data
for production control and quality monitoring processes. The basis for these calculations is an
accurate geological model for comparison with the stope parameters. Modern survey techniques
are also necessary to locate all geological information to within 0,1 m in 3D space.
The ability to affect a mine’s bottom line can be significantly affected by the application of these
technologies.
Introduction
Copper is used in general, in the industries that are shaping the future (AQM 2013), as such the
demand curve for the commodity has been stimulated positively. The demand is such that
progressively lower grades of copper ore are being mined profitably and as can be anticipated, a
good proportion of the high grade deposits that have been mined are either depleted or nearly so.
Discoveries of high grade deposits are becoming less frequent due to a decline in exploration
funding and the obvious exploitation of the existing resources. The Metals Economics Group
estimates that exploration spending plummeted 42% to $7,7-billion in 2009.
The average grade at which copper ore is being mined has had a significant impact on what
activities can be considered cost effective or efficient. What would have been hailed as cost
effective methods fifty years ago would cause todays investors to cringe in fear. Rising labour costs,
oil prices, energy prices and various fiscal policies have made things even more challenging. The
overall result is an ever intensive search for more effective and economic means of extracting
copper ore from its in situ state.
From a mineral resource management (MRM) perspective, strict control on the quality of material
that is delivered to the concentrator mill will have a profound effect on the process costs involved in
the final output.
Mine surveyors are uniquely positioned to provide the required information needed to maximise
extraction and reduce dilution to the production teams, regardless of the commodity being mined in
all cases the starting place is the geological model which has to be an accurate one. This is
especially true if the distinction between ore and waste is a difficult one to make visually.
Intensive and precise geological modelling combined with high precision geospatial control and
monitoring enable profitable ore extraction from the thin, low grade ore bodies that were avoided or
abandoned by earlier generations of miners. The introduction of lasers in the mapping of
underground cavities that were (mostly still are) inaccessible brought with it an impressive array of
capabilities that was held back only by the processing power of computers that were available. Point
cloud processing is one of the few avenues of technology where the development of hardware has
been faster than software. Moore’s law of computing finds a place for its application in this regard.
Moore’s law is a term that originated in the seventies which in simple terms describes processing
speeds or overall processing power of computers as doubling every two years. CMS units had been
developed to quite an advanced level before computer had an equally advanced ability to process
the point clouds generated effectively. In time the tables have turned and the ability to process the
data has increased as Moore’s Law rightly predicted.
Over the years this ability to process high density point data has improved and has subsequently led
to applications in mining that were previously untried. These applications have contributed to the
overall productivity of modern mines, the Lubambe Copper ore body on the Zambian Copperbelt,
which is classified as narrow (5,5 m) and low grade (2,3% Cu) is one mine that utilises this
technology.
Control over the stoping operations needs to be of a very high standard in order to generate quality
tons, quality tons will in turn lower the production cost, lower production costs generally equates to
improved profitability and an overall improvement to a company’s bottom line. Accurate drill hole
and stope measurement is central to this control.
Process overview
The major factors in a copper mine’s productivity, generally revolve around operating costs,
production volumes, ore quality and technologies that emphasise the benefits derived from the
economies of scale. Large capacity dump trucks and conveyor belts are very often the solutions of
choice when it comes to moving enough material to turn a profit. The alternative is to reduce the
overall volume of material generated and ensure that the focus is on extracting the contained metal
by minimising external dilution.
The whole process begins with the ore body delineation and rock mass characterisation. Delineation
drilling from the footwall drives provides an ore intersection approximately every 25 m on strike
and on dip. At Lubambe mine ore development generally does not expose the assay hangingwall,
the location is established by drilling a 10 m long hole into the hangingwall at a predetermined
interval. Delineation drilling data is combined with the assay footwall model to generate the assay
hangingwall wire-frame model. This component is handled by the geologists in conjunction with
the survey department; Boreholes are drilled, geo-referenced, logged, and analysed, the data is then
used in the construction of the ore body wireframe model that defines the “ore envelope”. The geo-
referencing consists of high precision collar surveys which are combined with subsequent “down
the hole” borehole surveys. This gives a 3D description of the surveyed borehole.
In addition to the above the geologists mark out the assay footwall contact in the developed ore
drives with yellow spray paint. The surveyors perform an accurate detail survey on this assay
contact line and generate a string file in a digital format that is used by the geologist to construct the
bottom or footwall wireframe of the ore envelope.
This ore envelope is the ore body as defined by the geologist’s wireframe model. A need exists for
routine geological mapping and timely interpretations to keep the ore envelope wireframe current to
ensure that the stope design is based on the data set that has the highest confidence levels (E.
Villaescusa, 2004).
The data used in the generation of the monthly extraction analysis report depends on the three
sources afore mentioned; the ore body model, the designed stope (which is a derivative of the ore
body model) and the as-built wireframes whose construction will be described below.
QA/QC modus operandi
In terms of quality very close coordination is required between all facets of the MRM department
(planning, survey and geology) and the mining and production department. The geologists guide the
development of the ore drives by ensuring that the drive is mined at an economically optimised
location in relation to the ore body. This method of directional control exposes the footwall assay
contact, which is in turn sampled with an x-ray fluorescence spectrometer to identify the location of
the cut-off grade along strike. The position of the cut-off grade is marked out with a very
conspicuous yellow paint for subsequent survey. This painted line is what is referred to as the assay
contact line.
Surveyors equipped with reflectorless total stations survey the assay contact line to 0,01 m accuracy
and provide the geologists and planners with the precise geospatial location of this line in the form
of a CAD string file. The geologist uses this to update the footwall of the ore body wireframe model
and the planners use it to optimise the design location of drill holes. The surveyors also perform a
laser scan of the ore drive and generate a 3D wireframe model which is used in the stope design.
This wireframe is called the prestope wireframe.
Fig. 1: Prestope wireframe.
Based on the geological model and the prestope wireframe provided by the surveyors, the mining
engineers in the planning department design a stope which would provide the most economic
extraction volume for the ore at that location. Complete with machine rigging positions and ring
orientation, the design is passed on to the survey department for staking out underground. A drilling
pattern that corresponds with the rigging positions is issued to the production crew.
Fig. 2: Stope design wireframe.
Once the production stope rings are drilled the survey department performs down-the-hole (which is
actually upwards) camera surveys to verify the drilling quality. The collar coordinates, directions of
the holes, and hole-depths are surveyed and measured with a PeeWee. Once these are compared
with the design, the rings are either recommended for blasting or are re-drilled depending on the
quality of drilling. The print generated by the surveyors is then used by the operational officials to
decide on what is to follow. In order to ensure compliance, disciplinary action would be taken
against a mining official blasting a production stope without taking this necessary prerequisite.
This is a critical aspect that ensures compliance to stope design before the point of no return, i.e. the
point at which the blast is taken and nothing can be done to correct any errors in the blast process.
It ensures that all the holes in each ring are drilled correctly. The tool of choice
for this is the PeeWee.
This tool was originally designed to map and track holes drilled using
directional core drilling holes. This system enables geologists to drill through
a fairly large volume of host rock from a single drilling site. This provided
many benefits included limited environmental degradation resulting from the
establishment of multiple drilling sites, reduced drilling costs due to the
reduced number of setups and reduced exploration timeline. It also reduces
the total number of required drilling meters as the designed deviations can be
executed at almost any depth in the hole.
This is where the drilling method distinguishes itself from drilling
multiple holes at steep inclinations in several directions from the same
site.
The Peewee is a tool that combines a clock, gyroscope, inclinometer and
compass to provide accurate position partials for a given point in time.
The time is used to “isolate” the readings needed to graphically reproduce
the geometric properties of the hole.
From the beginning of the survey to the moment the survey is complete,
the surveyor synchronises the depth and reading by taking note on the moment the PeeWee was
held still in the production drill hole. The data collected can be thought of like a 3D open traverse,
commencing vertically at the collar of the drill hole and ending at the toe.
This data is compiled and shared with the blasting engineer who optimised the timing and charging
up sequences of the respective holes. In situations where the drilling quality is sub optimal the
instruction to re-drill the holes is issued. This includes short holes, which may be the source of
underbreak, holes deviating into the hanging or footwalls creating dilution, or holes too long which
might compromise pillar integrity.
Drill rigs have been made which automatically drill the
production pattern uploaded onto a memory stick which
then follows up to download the actual drilled lengths, dips
and hole directions. What this equipment doesn’t generally
do is determine the amount of down-the-hole deviation resulting
from varying rock densities and such.
Once the holes meet the minimum requirements they are authorised for blasting. And this is where
the Lubambe Mine laser scanning regime differs slightly from most operational mines. The Faro
laser scanners and Optech CMS are used on a very regular basis to conduct a ring by ring
comparative analysis.
The quality controls checks at this point shift from being a hole by hole inspection. The blast profile
is inspected for deviations from the design, dilution, overbreak, underbreak and rib/ crown pillar
integrity. Every eight rings or approximately 17 m, the stope is scanned and the point cloud is
checked for creeping or unplanned breaches into upper levels. The regular scans are compiled into a
single file that is used to generate the progressive stope volume at the end of the month, which
incidentally is the single use that most mines purchase these units for.
The Faro laser scanner and the Optech CMS.
Laser scanning
The point data is collected using a FARO Focus3D high-speed terrestrial laser scanner (TLS) and
the Optech V500 CMS. This scanner offers one the most efficient methods for 3D measurement and
3D image documentation. In only a few minutes, this 3D laser scanner produces dense point clouds
containing millions of points that provide incredibly detailed 3D images of large scale geometries.
Multiple scans from different positions can then be compiled to create a cohesive point cloud,
resembling an exact measureable copy of even the most complex and large structures.
In the case of the stopes at Lubambe, full entrance into and under the stopes is unsafe, since by
design the stopes are not supported, the scanner is mounted on a dolly which in turn is mounted on a
horizontal open lattice truss attached to a monocycle. This contraption is pushed into the stope and
the rear end is stabilised by a heavy duty survey tripod. This enables the approximate levelling of
the scanner. The scanner has a tilt correction capability that will compensate for vertical axis errors
of up to 5°. The scanner is reeled into the stope by rope and pulley and is activated by remote
control. (Any Android tablet with Wi-Fi and flash player works for this).
The surveyor coordinates a mini-prism by total station and swaps this with a registration sphere.
The coordinates are then used to geo-reference the scan which would have otherwise been in a
localised coordinate system with the scanner location defining the origin. The laser scanner also
provides the option of setting the desired scan resolution and measurement quality at this stage.
The scan resolution on the scanner is defined as the distance between two successive measurements
10 m away from the scanners position. Due to the radial nature of the measurements taken, the point
density increases with reduced distance and reduces the further away the scan subject is from the
scanners location. This holds true for both the Faro and the Optech units.
The scan quality is determined by the level of confidence ascribed to a measurement, typically a
single measurement will have the lowest confidence and in the case of the Faro laser scanner, four
independent measurements to the same point will have the highest confidence levels. This in turn
has a direct effect on scanning time.
The higher the resolution, the longer the scan takes complete. At 976 000 points per second the
typical scan takes under two minutes to complete. The survey that enables the georeferencing takes
a much larger portion of the time required to execute the whole operation.
The lowest scan resolution is still adequately sufficient to collect the spatial points necessary to
extrapolate the stope geometry but the drawback is that the minimum distance to the registration
spheres needs to be reduced in order to retain a fair number of points on the spheres. The Optech
CMS does not have this drawback as the data collected is independent of the georeferencing
procedure. The scan is oriented by surveying two points mounted on the scanner and the dolly
which in turn allows a real time transfer of the surveyed coordinate values.
The registration sphere, for the Faro is a sphere of known diameter which is placed in close
proximity to the scanners position. The surface of the sphere needs to have a sufficient number of
scan point measurements on it to ensure the extrapolation of a theoretical circumscribed sphere
whose vertices are the measured scan points. The mini-prism and the registration sphere are
concentric therefore this sphere's centre shares the same XYZ coordinates as the mini-prism that is
used in the subsequent georeferencing. A minimum number of points are required in order to have a
large enough sample from which the theoretical sphere is derived. A maximum of about 46% of the
sphere can be scanned from a single scanner location; the software recognises this and using the
points measured on the sphere calculates the arbitrary coordinates of the center of the circumscribed
sphere.
Pre-processing
The point data as collected by the Faro scanner, is of such high resolution that even at the lowest
settings, the direct conversion of the scan data to .dxf or .dwg formats makes the resultant file too
large to handle in a manner that is technically efficient.
Once the georeferencing process is complete in the generic software an XYZ coordinate list of
points 0,2 m apart is generated. This equates to discarding well over 80% of the measured data, as
scan points are measured as densely as 1 mm apart in areas close to the scanner. This density
renders most CAD packages useless in the subsequent processing hence the need for the filtering.
The scan data is run through software called a scan filter which is capable of reading the output
from the scanner and generating the coordinate list.
The point data is used for two purposes. The first is the periodical quality control and the other is
the progressive volume measurement.
The above diagram is a typical progressive scan report for a production stope this provides for a
means of adjusting charging patterns and timing which creates and opportunity to recover ore which
would have been lost or prevent dilution which would have been inevitable.
Wireframe modelling process
Data is collected underground using the Optech V500
CMS, which is reduced to point data as shown in
Fig. 3. A sequential inspection at 2 m intervals provides
the basis for generating the strings used in the wire
framing process. This also represents the first and only
major loss of fidelity in the measurements taken and derived. However the effect is deemed
negligible as the method provides the most accurate volume, when the constraint of time is taken
Fig. 3: Closed strings for wireframe.
into consideration. The net effect is similar to the output expected when Simpsons or trapezoidal
rule is employed with a 2 m inter-area interval.
A view is taken through a 0,2 m thick cross section of the
sampled point data. This in turn provides the data set
required to generate cross sections of the stope. Closed
strings of the cross sections are created by digitising along
the points displayed in the section. This process serves the
double role of eliminating noise and other undesirable data that is inadvertently collected during the
scan. A process typically referred to as “cleaning”, this removes the measurements taken on people,
mobile equipment, vent-ducts and other items present in the measurement range that is very likely
to distort the outcome.
Software to do this automatically has since been developed but as Morin (2001) identified in his
thesis, software for underground operations is not cheap, nor does the incentive to invest heavily in
its development exist on a large enough scale to lower costs.
The other operation that takes place at this stage is the interpolation of probable edges. These edges
are usually missing datasets that occur as a result of “shadows”. Shadows are formed when an
obstacle, desired or otherwise obstructs the laser beams from the point source and prevents
measurements to a desired face or sidewall from being taken. Shadows are easily overcome by
relocating the scanner; however in the open stopes this isn’t always possible due to accessibility
issues and other associated hazards.
The outline of the stope as mined is generated every 2 m. This series of closed strings is used to
generate the base wireframe model. The base wireframe is the digital representation of the mined
Fig. 4: Complete base wireframe model.
out stope. Two other wireframes are used in the comprehensive stope analysis process; the design
wireframe and the geological ore body model. The generated stope sections are shown.
Using a process called string linking in Datamine a wireframe is generated using the closed strings.
This wireframe then provides a solid model whose various properties can then be queried. At this
point the volume of the stope as mined is immediately available.
At this point the similarity with most stope reporting procedures ceases. The design of the stope is
also converted to a wireframe.
A comparative analysis between the two models enables the volumes of the under-break and, or
over-break to be established with remarkable accuracy for time it takes to do so. Built-in Boolean
operations in Datamine enable the creation of difference models. These are wireframes created by
subtracting one wireframe from the other. The difference between the stope and the design usually
Fig. 5: Stope data sets.
yields the under-break wire frame model. The following sketches show typical sections through a
difference operation.
In generic laser scan usage in tunneling and mining purposes, the typical application is the point by
point comparison that enables engineers to identify areas that might either be too tight or may
require concrete back fill. The Lubambe survey team took this aspect of the scan analysis and
defined the various parameters of the stopes that were non-compliant to the design and derived a
series of wireframe models from the stope. Although this software was developed specifically to
process geological ore body models, the functionality is perfectly suited to processing stopes.
Point cloud
The point cloud is a raw data set that consists of a multitude of points measured in rapid succession and in close proximity to each other. The end product of a fully processed scan is a geometric representation of the measured object or cavity. This data is in the form of XYZ coordinates in the coordinate system determined by the scan references. It serves as a basis for the generation of various wireframe (solids in other software speak) for stope specific analysis.
Stope wireframe (or solid)
The wire frame is a direct-scale model of the excavation under scrutiny. The wireframes are created in Datamine using a series of sections perpendicular to the direction of ore mining. Due to the dangerous nature of the unsupported stope backs. The scanners are pushed into the stopes on an alloy frame and activated remotely.
Prestope /development wireframe
This product is the other critical data source which is used as an input into the stope design. The drill rigs operating parameters and production requirements are assessed against this wireframe. The geological ore body model is superimposed on this and thus provides an as-built basis for stope design parameters.
Stope design wireframe
This is the final product from the design team in as far as production stoping is concerned. The stope wireframe (actuals) is compared to the design wireframe and this is evaluated for extraction, dilution, lock-up as well as additional hanging wall fallout (which usually consists of a skin of ore).
Stope underbreak
The wireframe shows the design without the ore that has been mined out, this represents the very first production losses incurred by the mine as what is left behind cannot be recovered. This happens to be true for Lubambe copper mine where the mining method does not permit the recovery of this kind of ore.
Total dilution This shows the amount of waste introduced into the ore stream from this specific stope. This shows both hangingwall and footwall dilution mined together with the ore. In best case scenarios these volumes are zero. This is material that pushes up the mining cost due to the volumes to be moved, milled and processed but pay absolutely nothing. The next two images show the respective splits between hanging wall and footwall dilution.
Conclusion
The technologies discussed in this paper have been around for a number of years, however the
application particularly for laser scanning has been limited to the calculation of the stoped out
volumes for month-end payments or reconciliation. Where the laser scanners or CMS units are
applied to quality control measures they earn their value back to the operation within a short period
of time by enabling well designed tweaks and balances to the operation.
Where an operation has more than one type of drill rig dedicated to production drilling, these
systems facilitate optimised drill design which provides for an exact match between drill rig and
drill site. This ensures that the drill rig is capable of fitting the drill site without having to physically
test the setup capabilities in that area. An accurate scan is critical to this design aspect.
The correct application of the technologies in this paper will lead to minimised dilution and
reduction in associated product cost build up. Excessive dilution directly affects the tramming and
energy cost of an operation. Downstream processing costs are also held at bay. Every single ton of
material that is run through the mill and floatation tanks will add costs to the concentrator’s
recovery process.
When the production team is perplexed by a drop in the production head grade, the scans help to
pinpoint the exact source of dilution even in areas where this may not be so easy to discern. By
conducting a comparative analysis pre and post dilution event.
The bulk of a mine’s operational capital cost goes towards the acquisition of tramming equipment.
Loaders, dump trucks, conveyor belts and skips for underground operations, and shovels and trucks
for open pit mines. An overall reduction in production cost and increase in quality volumes would
reduce the necessary future investments in these especially where capital planning factors a
standard dilution percentage.
The cost of exploration and infill drilling is sometimes difficult to relate to the final blasted product.
Fuller utilisation of geological information is accomplished by continuous comparative analysis
between production drill holes and ore body model, the blast outline and the ore body model and
stope design.
Sequential scans and continuous measurements ensure that “shadows” are eliminated and the stope
status is updated with a progressively updated geometric shape which leads to more accurate
extraction tonnage estimates, measurements and mining reconciliation.
Recommendations
The equipment described in this paper is very often quite expensive and since it’s naturally viewed
as a measurement device of one kind or another, the potential gain is immediately lost. This
perspective needs to be altered for anyone who is not a surveyor but understands the impact of
changing a mine’s cost structure by controlling what comes out of the ground.
Surveyors ought to take advantage of their understanding of geometric analysis and get involved in
improving a mine’s bottom line. As surveyors we know more than we know that we know. This is
simply because on the positional analysis that comes by default. Surveyors need to be known for
more than just lines, grades and the occasional measurement.
The cost of this equipment could go down as people embrace the technology and apply it to their
own situations and reduce the projected production volumes which are influenced by excess ground
and maximise the extraction of the commodity sought after.