the latest developments in laser profiling, borehole deviation and laser enhanced videometry

8/9/2019 The Latest Developments in Laser Profiling, Borehole Deviation And Laser Enhanced Videometry

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The Latest Developments in Laser Profiling, Borehole DeviationAnd Laser Enhanced Videometry

By

Bob Wells

Measurement Devices LtdYork, England

Abstract

This paper presents a novel development for the determination of burden inquarrying and mining applications. Combining data from a laser scanner with anintegrated video camera, together with borehole deviation measurements, gives acomprehensive overview of the area of rock face to be blasted. This data is modelledin a unique software package for more accurate evaluation of burden.

Background

Since the development of bench blasting mining techniques in the late 1800s, manysignificant improvements have been made in drilling techniques, explosives chemistryand costs. Until the 1980s however, methods of determining burden relied on suchtechniques as holding a plumb line in over the crest, triangulating in on a few points,use of a burden stick or eyeballing. Early in the 1980s, Measurement Devices Ltd aScottish based company, developed rock profiling, borehole deviation and integratedcomputerised blast geometry design systems. Initially the systems were aimed atimproving blast safety margins particularly with regard to the avoidance of fly-rock.Later changes in the UK mines and Quarries regulations reflected the growingconcern for blasting safety and its direct correlation with blast geometrymeasurement. Elaborating on early deductions that a safe blast is an efficientblast, MDL changed the emphasis of their developments with the aim of specificallyproviding fast, accurate and concise management information for the control ofblasting through in situ geometry measurements, and reporting.

The results from applying this technique at a number of major quarries clearlyindicate dramatic improvements in blast efficiency, costs and safety.


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History of Blasting

The first explosive, gunpowder, was developed in China in the 13th century. This

invention made possible the development of fireworks, rockets and weapons thatpropel high velocity projectiles. The first recorded use of an explosive for mining was

in 1627 by Kasper Weindl at the Royal Mines of Schemnits at Ober-Biberstollen,Hungary. In this application, holes were drilled in rock with hand-held star drills andsledgehammers. Black powder was then loaded into the holes, a fuse was inserted toallow personnel to evacuate the area, and the charge was ignited, fracturing anddisplacing the rock.

The black powder used at this time was a relatively slow-burning mixture containingfuel and oxygen and was highly inefficient by modern standards. However, this newmining method was a significant improvement over the standard pick and shovel.

With the invention of dynamite in 1867 by Alfred Nobel of Sweden, a new age ofexplosives development was begun. Unlike black powder, the nitro-glycerine baseddynamite formulation had high detonation velocity and energy. Many variations onNobels basic formulation were quickly developed which further increased availableenergies and provided other desirable properties such as lower sensitivity and morewater resistance.

The invention of high energy dynamites made modern style bench blastingeconomically viable. These explosives quickly became the workhorse of the miningindustry, both above and below ground. Development of the modern rock drill bitfurther enhanced mining efficiency and modern bench blasting was born.

The subsequent development of ammonium-nitrate based explosives, emulsions,slurries, water gels, and improved drilling equipment further improved benchblasting costs, but did nothing to change the basic technique, which has evolved littlesince the late 1800s.


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Current Blasting Practices

There are a number of factors to consider when deciding on a specific blast design.Some of the major factors are: -

Safety and Environmental Concerns

Placement of holes too close to the free face can result in dangerous fly-rock, asituation where rocks are sometimes propelled great distances, endangering lives andproperty. Excessive noise or airblast can also result, creating public relationsproblems with nearby homes and businesses.

Excessive ground vibration created by the blast can also cause environmentalproblems and, in extreme cases, damage property. While some vibration is created byevery blast, the problem can be greatly exaggerated by subdrilling (excessivedrilling of blastholes below the bottom of the free face), improper detonator timing

selection, and improper blast geometry.

Production Concerns

The desired rock sized distribution from the blast can vary dramatically from oneoperation to the next based on many factors. The major factor to be considered isthe end use of the blasted rock. If the rock itself, or some component of the rock, isthe desired final product, the blasted rock is normally subjected to additionalmechanical crushing steps to further reduce average rock size. Often this secondarybreakage produces a mixture of sizes, which are segregated and sold based on size.

Sometimes the rock is subjected to leaching operations to remove desired minerals(gold or silver for example) so size distribution is important since it directly affectsleaching efficiency. In other operations, such as cement manufacture, all the blastedrock is crushed to a relatively small size for processing.

If the rock or its components are not the final product, different blasting strategiesmay be employed. Examples of this situation are construction blasting and stripmining, which is the removal of a layer of rock to allow recovery of a lower layer of adesired material such as coal. In these situations, the blasted material is to be movedas quickly and inexpensively as possible and no thought is given to any subsequent

processing of the rock. Occasionally, the blast itself is used to move a significant partof the blasted rock, thus reducing mechanical loading and hauling costs.

Thus, the desired size distribution from the blast can vary dramatically depending onthe end use of the rock and the type and size of the equipment used to move andprocess the results of the blast.


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Cost

Another major factor to consider when deciding on a blast design is overall cost ofdrilling and blasting the rock mass. The sum of drilling, explosives and labour costsmust be kept to a minimum to ensure the continued competitive viability of themining venture.

Blast Design

There are a number of variables under the control of the blaster as he designs theblast:

Burden- the distance between the blasthole and the free face. Spacing - the distance between adjacent boreholes. Sub-drilling - the amount of borehole drilled below the bottom of the free face. Hole Diameter- controls the amount of explosive, which can be loaded in a given

borehole. Delay Pattern- the chronological order in which the holes are detonated. Stemming- the amount of inert material loaded in the top of the hole or within

the hole to contain the explosive energy and products during the detonation.

The procedure described above has a number of uncertainties, which can dramaticallyeffect the results of the blast:

Inaccurate estimates of shothole burdens can result in substantial variation fromhole to hole, and even within the same hole, of the amount of rock to be blasted

by each unit weight of explosive. Inaccurate borehole placement and path can also create large variations inexplosive energy distribution within the rock mass.

Uncertainties about rock face height and floor elevation can result in wastedexplosive energy, uneven floor, oversize rock and high vibration.

Incorrect estimates of rock volume and explosive usage, can result in productioncost uncertainties, inventory calculation errors and poor business decisions.


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Survey Methods

Early laser profiling methods consisted of a basic 2 dimensional approach to burdenmeasurement and this technique is still in use and often adequate in many cases.Points are surveyed at relevant points between the crest and toe of the bench. Thisdata is manually recorded or collected in some form of logger and downloaded to

the computer. The proposed drill angle, hole offset from the crest are entered intothe software and a profile of the rock face is created graphically. Normally atabulated text output of depth and burden is produced at the same time.

Example print out from MDL 2D Profiler Software


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Advances in laser design and microchip design have now reduced the early 2Dsystems from a power hungry tripod mounted system to a simple hand-held devicethat requires only 2 AA batteries.

The latest hardware from MDL, the BurdenAce, now has the capability to measureand calculate the burden in front of the shothole directly on the laser itself and the

tabulated information printed out on a thermal printer in the field. This pocket-sizedinstrument is ideal for verification of burden prior to drilling and is ideal accessory forthe drilling contractor and blast designer.

MDL BurdenAce Thermal Printer

However, 2D systems are inevitably subjective, in that the points in the profile are thechoice of the individual operator and potentially dangerous burden can be missed bynot picking up enough points to accurately define the true profile.Another potential error in 2D profiling is not surveying the profile directly in front ofthe shothole, which results in an increased burden calculation.

.

Incorrect

Burden


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2D methods are no substitute for a full 3D survey of the face to be blasted but can be

useful ideal for quick calculations and for re-checking profiles prior to detonation.

The major drawback of purely 2 dimensional systems is that the area of the faceeither side of the profiles is not surveyed and inevitably this part of the face may wellcontain weak burden areas. This limitation has been recognised for many years and3D systems were quickly developed whereby the entire face is surveyed and profilesinterpolated by the software. This approach allows the blast designer to extractprofiles at any given interval and also to vary the shothole positions and recreate theprofiles again.

However the profiles interpolated from the face survey may still not show the areas

of weak burden which may lie either side of the profiles.Methods have been devised to look either side of the profile to see if there is anarea of weaker burden but these methods are still subjective and have problemswhere a corner or loose end is included in the blast

UK Quarry regulations require the weakest area of burden to be detected on theface and MDL has addressed this requirement by introducing a new methodology.

The entire rock face to be blasted is modelled as a complete surface and compareddirectly with the surface as defined by the proposed blast holes or the surveyed drill

holes, which have been measured using one of the MDL BoreTrak deviationinstruments.

Profiles can be extracted to assist with the design but are not the end result, whichwas the previous case. The new method of analysis overcomes the potential areas ofweak burden, which might easily have been missed by a profile based analysis only.

Whilst writing the software for this application it was decided to also incorporate thelatest developments in hardware which includes the integration of a video camerainto laser scanner.

This enhances the data capture and also provides the opportunity to use theinstrument from a remote location and to integrate digital imagery into themodelling techniques.


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Laser Enhanced Videometry (LEV)

The system is the product of integration of a motorised theodolite, a zoomable CCDcamera and a laser range finder. The laser rotates with the theodolite and determines

three dimensional co-ordinates of points using the bearing and distance principle.The camera provides a visual interactive guide for the laser scanning operation andalso captures images to be used for image-based rendering and, where necessary,stereo photogrammetric measurement.

A variety of survey techniques are used to collect data for 3D modelling. The mostoutstanding ones are digital photogrammetry and laser scanning. The former has theunique advantage of providing image information for lifelike rendering whereas thelatter provides direct 3D data with the extent of automation and reliability that theformer will struggle to match for a few more years to come. The combination of the

two techniques provides the best of both worlds and potentially more. Suchcombined systems have recently emerged for airborne platforms demonstrating hugepotential.Measurement Devices Ltd have developed this advanced system, with patent appliedfor. The aim of the project is to fully exploit the concept of laser, camera andtheodolite combination for various terrestrial-surveying applications. The systemdeveloped is designed to provide a surveying tool that eases the territorial surveyingand exhibits virtually the territory realistically and informatively. It will be a usefulmeans for environmental planning, production of 3D games and films, quarry 3D facemodelling, architectural preservation and reconstruction etc.

A zoom lens CCD camera and a laser reflectorless range finder are mounted on amotorised theodolite so that they can pan and tilt panoramically. The zoom CCDcamera used in the system is Sony EVI-371, which is designed for use in camcorders.

The CCD chip is a 1/3 inch interline transfer chip with 752582 image cells and the cellsize is about 6.5(H)6.25(V) microns. The lens can zoom from 5.4 to 64.2mm focallength in 12 optical settings. The frame grabber used for the system is creative BlasterIE500 imaging card. It digitises both fields of the composite video input andgenerates a digital image.A desktop or laptop computer can control the zoom lens and the focal length of thecamera. Live image from the camera can be shown in real-time on the computer

screen. The wide angle focal setting is mainly used for capturing image for imageindexing and user interfacing purposing, and long focal settings are used formeasurement and rendering purposes.


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The laser and theodolite used in the system are based on an existing product theMDL Quarryman ALS (Autoscanning Laser System) designed for rock profiling andscanning. The computer via an RS232 port, can control the laser range finder and thehorizontal and vertical positions of the theodolite. At the same time, the computercan receive range, horizontal and vertical angles. A control program has beencomprehensively implemented to enable the operator to control the instrument and

the camera from the computer.

The automatic calibration of the interior parameters and the exterior parameters ofthe camera is one of the fundamental tasks for the development of the system. Thecalibration precision determines the accuracy of the measurement and the realisticityof 3D rendering on the computer screen. The calibration includes three interiorparameters and six camera-to-theodolite parameters at various focus settings withinthe range of zoom.

A rigorous and precise calibration based on the camera-on-theodolite calibration

method was implemented for the manufacturers use

The Principal Point

It is assumed that the optical axis of the camera is straight so that the principal pointfor all zoom lenses falls at one point on the image. When the camera zooms in, thetargets on the image move towards the centre of the image. The intersection of alltarget paths, while zooming, is considered as the principal point. The controlprogram was implemented to enable the operator to click targets while zooming inand out. The computer calculates the average of the intersections of all target paths,which is considered as the principal point. The principal point needs to be calibrated

several times and the average is taken. For the current camera used in the system, theprincipal point is at (3.1, 13.0) from the centre of the image (y downward is positive)

with image size (800600).

The Principal Distance

The principal distance varies with zoom lenses. At each zoom position, the calibrationstarts with pointing the camera/theodolite so that the central part of the image isfilled with featured scene. This central part of image is shown as a rectangle and it iscalled the interest image in the paper. The angular readings of the theodolite are

recorded in the mean time. A pixel with the most unique surrounding features withinthe interest image is chosen as the target point and its image co-ordinates arerecorded. Normally this point has the most features and relatively easy to bematched. The theodolite is then rotated to 5 positions along four directions: left,right, up and down. At each position, a corresponding image is grabbed and theangular settings of the theodolite are recorded. The interest image is moved toenclose the moved target point by best estimate from previous calibration data. Thetarget point is then searched and located with sub-pixel precision by area basedmatching techniques. A very strict check including back matching etc is performed to


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discard some unreliable matchings. If both horizontal and vertical directions have 4matches discarded, re-calibration is suggested. At least 7 sets of locations of thetarget (including the initial target location) with respect to the angular settings ofthe theodolite can be obtained along the horizontal and vertical directions.

Target

The target and The principle image

)tan( 00 = fxx (1)wherex---- target location in xdirection,x0---- Initial location of the target in xdirection,

f---- Principal distance in pixels

---- Horizontal angle of the theodolite

0---- Initial horizontal angle of the theodolite associated with x

0

x0,

0and fare unknown. A set of xand have been recorded. The least squares

method is used to fit the equation (1) to solve x0,

0and f . Because the value of f(the

principal distance in pixels) depends on the image dimension, the horizontal view

angle His calculated and stored instead,

=

f

HRH

2tan2 1

where, HRis the horizontal dimension of the digital image in pixels. The horizontalview angle can also be obtained from the calculation of the vertical direction. If thecalibration results for both horizontal and vertical directions are valid, the averagevalue is taken. A further check on the reliability of the matching can be conducted on

the basis of the least squares solution.This calibration method can be conducted automatically without the need of settingspecial targets, which enables the user to carry out the procedure at any timenecessary. It is designed for regular instrument check-up purpose. The automationwithout the set of targets greatlyreduces the cost of the calibration and considerablyincreases the ease of the use of the calibration utility.


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GENERATION OF 3D MODELS

An algorithm was developed to detect the laser spot on the live image when firingthe laser by considering the offset of the laser and the camera optical axis and thecalibration result. A method of calculating the angular settings of the theodolite

from pixel co-ordinates on the image was derived. That means whenever theoperator clicks one point on the computer screen, the theodolite can pan and tilt toshoot the target clicked.

An algorithm was also developed to find the laser spot associated with a specificimage. Those algorithms enable the operator to define an irregular polygonal area tobe scanned by using a mouse. Internal points are generated by the computeraccording to the scanning density defined. The instrument will automatically carryout the scanning procedure, grab the image, and collect 3D data with correspondinglocation on the grabbed image.

Algorithms and programs were also developed for the construction of a triangulatedirregular network (TIN) model from the collected 3D points. This model can acceptrandomly scattered points and an irregular polygonal scanning area with convex orconcave boundary.

The 3D triangular wireframe model of an example scan is shown below together withthe video image draped over the scan.

It can clearly be seen that the enhancement of the video draping on to the meshbrings greater definition to the overall model and allows for visual inspection andinterrogation.


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Borehole Deviation

In many ways performing a scan of the rock face is only part of the overall evaluationof a potential blast. Unless the actual boreholes are measured accurately then theburden calculations can only be an estimate of the true value.

Where rock conditions are good and there are no wet holes, in many cases a simpletape measure a torch and an inclinometer can give a reasonable indication of depthand drill angle. The direction or azimuth of the hole is less easy to measure butvarious mechanical methods have been adopted.

Where such simple methods are not appropriate it is important to measure the actualborehole direction and to combine this information with the results of the scan of therock face. This task was addressed by MDL some time ago with the introduction ofthe BoreTrak system.

The principal of this device is based on a pitch and roll probe mounted on the end ofa series of lightweight carbon fibre rods. These rods are of fixed length and measurethe actual hole depth but more importantly will not rotate whilst in the hole. Theprobe is also fitted with a clock and records the pitch and roll at a given time intervalduring the survey.

A logger at the surface also records the time and the number of rods (depth) downthe hole. The rods are located in a rack at the surface, which is aligned along aknown azimuth. In this way the co-ordinates of hole can be calculated throughout itsoverall length.

MDL Rodded BoreTrak System


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Recently a new BoreTrak has been introduced which uses a compass for the azimuthand thus removes the necessity for the rods. Although this makes the overall systemmuch more lightweight, compact and easier to use there are inevitably potentialproblems where there may be ferrous materials in the rock, which can affect thecompass readings.

MDL Cabled BoreTrak

The position of each hole is measured either directly with the laser or as an offset to abase line on the bench. The co-ordinates of the collar positions are then calculatedand the measurements from the BoreTrak equipment applied to determine the truepath of each of the boreholes. The data from all the boreholes is combined to form asecond surface.

3D model showing proposed and surveyed boreholes


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3D Surface Analysis

The two surfaces from the laser scan and the BoreTrak data are compared and anisopachyte (burden map) surface created. This third surface reflects the burden overthe entire face and is represented as a contoured plan with colour enhancement to

show the percentage of the planned burden. It can be seen immediately whetherthere are any potential areas of excessively weak burden without the need for anyfurther processing.

3D face model showing burden map of face

The model can be further interrogated by a series of cross and long sections in order

to verify that the blast design is satisfactory. If the hole positions are only theoreticalthen the software allows for the collar positions, drill angles and proposed burdenvalues to be amended. The burden map is automatically regenerated and theresults displayed for further analysis. Once the blast design is accepted a report isgenerated for the driller and blast engineer to set out the bore hole positions and tocharge the holes accordingly.


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Conclusions

This new approach to face profiling should go a long way to assist the reduction offly rock incidents as the software automatically detects the potential areas of weakerburden. The laser scanning is automated to the extent that the operator does not

need to survey individual points other than perhaps the crest and toe lines and theshot hole positions. As far as the rock face itself is concerned, the only decision to bemade is the spacing of points in the horizontal and vertical axes to determine thescan density.

Of course the software can only model the data that is surveyed and if the density ofpoints chosen is not sufficient then the model generated will not be an accuraterepresentation of the surface. By inclusion of the video the survey can assessed by asupervisor and potential problem areas identified from the 3D image.

Further development of the system will include automatic edge detection from thevideo image so that crest and toe lines can surveyed automatically and the inclusionof photographs from a digital camera alongside the integrated video. Models will bemerged together for composite mine plans for planning purposes.

It can be seen that by the reduction in size that 2D systems will be used for the mostbasic of tasks and for checking. The more advanced 3D systems will be used not onlyfor blast design, but to provide a full 3D visualisation of the entire quarry site withwalk through facilities.

ReferencesStephen L Ball, Managing Director, Measurement Devices Ltd, Quarry Face andBorehole Surveying 1998.

the latest developments in laser profiling, borehole deviation and laser enhanced videometry

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