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.

References [1] AQM copper Inc, 2013 "Copper fundamentals", http://www.aqmcopper.com/s/Copper

Fundamentals.asp [2] E. Villaescusa, 2004 "Quantifying Open stope performance", Western Australia School of

mines. [3] Mario A Morin 2001 “Underground Hardrock Mine Design and Planning- A System's

Perspective” Queen's University Kingston, Ontari, Canada. [4] Remondino Fabio, "From point clouds to surface", International Archives of the

Photogrametry, Remote sensing and Spatial Information Sciences, Vol XXXIV-5/W10 Contact Kalumba Bwale, Lubambe Copper Mine Limited, kalumbab@lubambe.com