Recent changes in underground traversing techniques in western australia

Recent changes in underground traversing techniques in western australia

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  Recent Changes inUnderground Traversing Techniques in Western Australia Andrew Jarosz (1) and Luke Shepherd (2) Curtin University of Technology, Western Australian School of Mines, Mine Surveying Program, Locked Bag 22, Kalgoorlie, WA 6433, Australia, (1) Email: [email protected] (2) Email: [email protected] Abstract: This paper analyses the survey configuration, instrumentation and error propagation in the control survey technique known as “wall station traversing”. It utilises wall mounted survey points as an alternative to roof (backs) mounted points. Recently, this technique has gained widespread acceptance in underground metalliferous mines in Western Australia. The authors analyse and compare error propagation of the “wall stations” technique in relation to classical traversing and derive an optimal survey procedure and configuration for this technique. Key words: mine surveying; survey control; traversing 1. Introduction Survey control points of underground mines are generally located in the roof (backs) of mine workings. Locating points in the floor is not a viable option, for most mining operations, due to heavy traffic and vulnerability to damage. Although, the backs position provides a secure location for control points it does present a number of drawbacks for the surveyor. The most obvious of which is difficulty of installing and accessing such points. To access the backs in modern underground mines usually requires lifting apparatus to reach heights of over 5 metres. By locating survey points in walls installation and access can be much easier, safer and faster. However, it does require a change of sur-veying technique from classical traversing, as a theodolite can no longer be set under a survey point. The resection (free station) technique, with all available distances and horizontal angles measured, can be used to determine the po-sition of the instrument. “Wall station traversing” utilizes observations to wall-mounted points from a temporary in-strument station that is located in the drive. Recently, this technique has become popular in underground mining op-erations in Western Australia and with this growing popularity it is important that surveying professionals have an in-depth understanding of methodology, accuracy and limitations of the technique. The authors of this paper analyse and compare propagation of errors in “wall stations traversing” in relation to a classical underground traverse and derive an optimal survey procedure for the technique. 2. Traversing Methods Two traversing methods were considered in this study, classical traversing with points located in the roof of a mine drive and “wall station traversing” with points located in drive walls and temporary instrument stations. Analy-sis is carried for straight open traverses starting from a fixed 30 m base and characterised by five 30 m legs. A “forced centring” method was assumed for instrument and targets. The accuracy analysis has been restricted to the horizontal position of traverse points. Observation accuracies of distance ±2mm±2ppm (fine mode) and angle ±5” have been used in this study. 2.1. Traverse with Control Points Located in the Roof (Backs) This is the conventional or classical underground traversing method. Point position is determined by means of ob-serving horizontal and vertical angles between a known backsight target and unknown foresight while the survey in-strument is positioned directly under a known point. This method is widely used and understood. Simple calculations are used to obtain position of survey points. It does however present a challenge for the survey team in installing sur-vey points in the roof of drives and in centring the instrument under them. Centring is achieved by means of an opti-cal (zenith) plummet or plumb bob and centring to points 5-6 metres above an instrument can lead to centring errors of the order ±2 mm or more. To eliminate the accumulating influence of centring errors a “forced centring” proced-ure is adopted. 2.2. Traversing with Control Points Located in Sidewalls (Wall Station Method) In this survey technique the transfer of bearing and position is achieved by means of temporary instrument stations that are derived by observing distances and angle to two back targets positioned on sidewalls. (Fig.1). The technique has advantages over the above method in that survey points are easy to establish and access and that instrument cen-tring under a point is not required. Targets are secured to the wall by inserting a target’s stem into a sleeve mounted in a drilled hole (Fig.2). The method requires specially designed target prisms that retain central position thru all ro-tations. Total station instruments must be coaxial, i.e. distances and angles measured along the same line of sight and
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  equipped with aprocessor and software able to determine instrument position from resection observations (utilising a least square best fit calculation method). Figure 1. Observation of wall stations (after B. McCormack, 2002) 3. Error Analyses - Case Studies Figure 2. Reflector inserted into a wall sleeve (after B. McCormack, 2002) The error calculations and analysis for different configurations of a classical traverse and wall station traversing were carried out using survey adjustment software STAR*NET, (Star*Net 1995). 3.1. Classical Traverse A traverse with fixed points 0 and 1 and measured subsequent points 2,3,4 and 5 are used as a case study. The ex-ample traverse involves transferring survey control in a west-east direction from the fixed base 0-1. Points 2,3,4 and 5 are located at intervals of 30 metres. The transverse and longitudinal standard errors of position in this simple traverse can be calculated from the fol-lowing relationships (Kowalczyk, 1968): n n (2 1) − m m L y α = ⋅ ⋅ Transverse component: (1) 6( 1) − n m m n x = l ⋅ Longitudinal component: (2) where: mα = standard deviation of measured horizontal angle, ml = standard deviation of measured distance L = total length of traverse, n = number of traverse sides (legs). Results of the error analysis for the simple five-leg traverse are presented in Figure 3 and Table 1. The error ellip-ses for this case and for all following cases are based on a 95% confidence level.
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  Table 1. Parametersof error ellipses (Fig.3) Station Semi-Major Axis Semi-Minor Axis Azimuth of Major Axis 0 0.00000 0.00000 0-00 1 0.00000 0.00000 0-00 2 0.00489 0.00178 90-00 3 0.00692 0.00398 90-00 4 0.00848 0.00666 90-00 5 0.00979 0.00975 90-00 Figure 3. Configuration of classical traverse with error ellip-ses (scale of ellipses 1000:1) 3.2. Wall Station Traverse The wall station traverse replicates the classical traverse in its extent, determining the position of four points 2, 3, 4 and 5 from a base 0 and 1. Instrument locations (t1, t2. t3 and t4) are determined by observations to adjacent known targets. The location of the subsequent traverse point (wall station) is determined through measurements of angle and distance. This operation is repeated to determine position of consecutive wall stations. The error calcula-tions were performed for a configuration where the instrument was located 3 m from the second wall station and normal to the traverse direction. The result of error analysis, for the five legs “wall station traverse”, is presented in Figure 4 and Table 2. Table 2. Parameters of error ellipses (Fig.4) Station Semi-Major Axis Semi-Minor Axis Azimuth of Major Axis 0 0.00000 0.00000 0-00 1 0.00000 0.00000 0-00 2 0.01012 0.00344 0-39 3 0.02263 0.00487 0-28 4 0.03788 0.00596 0-22 5 0.05546 0.00770 0-19 Figure 4. Configuration of "wall station" traverse with error ellipses (scale of ellipses 1000:1) The calculated results show a significant increase of transverse errors and a review of survey geometry suggests that such errors are related to positions of theodolite stations (t1, t2, t3, t4) in relation to wall stations (1, 2, 3, 4). It is known that the shape of the resection triangle does influence the accuracy of bearing transfer (as in the two wire Weisbach method of direction transfer through a vertical shaft). This data suggests that to achieve optimal directional accuracy in “wall station traverses” a configuration that has acute triangle geometry is necessary. Temporary instru-ment stations located normal or near normal to “wall stations” impact negatively on bearing transfer (Fig.5). 0 1 1 0 t1 t1 Figure 5. Replacement of right-angled triangles with narrow, sharp angled triangles
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  To confirm suchpredictions an error analysis was conducted for a “wall station traverse” configured with acute (narrow, sharp) angled triangles. Instrument stations were located in the middle of traverse legs and 1 m from the drive’s wall. Resection triangles had a pointed angle of ~2.5 deg. The results of the error analysis are presented in Figure 6 and Table 3. The results show a significant improvement in transverse positional accuracy by factor of four (4). This suggests that a linear alignment of wall and instrument stations is critical for preserving accuracy in “wall station traverses”. However, it is not always possible to align wall and instrument stations. In such cases it is suggested to perform additional observations to previous and next instrument stations to add rigidity to the whole survey structure. The “forced centring method” must be utilised when observing consecutive instrument stations. It has to be strongly noted, that the initial orientation triangle must always be acute as presented in Figure 7. 0. 1 2 3 t2 t3 The results of error analysis for “wall station traverse” with additional observations (links) between instrument stations are presented in Figure 8 and Table 4. Table 3. Parameters of error ellipses (Fig.6) Station Semi-Major Axis Semi-Minor Axis Azimuth of Major Axis 0 0.00000 0.00000 0-00 1 0.00000 0.00000 0-00 2 0.00438 0.00286 89-38 3 0.00642 0.00542 6-00 4 0.01316 0.00653 2-41 5 0.01573 0.00814 2-00 Figure 6. "Wall station" traverse with acute angles Figure 7. "Wall Stations" with additional observations between instrument stations
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  Table 4. Parametersof error ellipses (Fig.8) Station Semi-Major Axis Semi-Minor The error results for this modified “wall station” method are very close to results obtained for classical traversing and suggest that “wall station traversing” is a fully viable surveying technique. However, it does require careful con-sideration of its geometry and additional observations between temporary instrument stations. The comparison of transversal errors for traverses analysed above is presented in Figures 9 and 10. The abbreviations used for traverse type are as follows: Traverse = Classical Traverse (Fig.3), WS-Normal = Wall Station Traverse with right angled resection triangles (Fig.4), WS-Acute = Wall Station Traverse with acute resection angles (Fig.6), WS-Modified = Wall Station Traverse with right angled resection triangles and additional observations between instrument stations, WS-Acute/Mod = Wall Station Traverse with initial acute orientation triangle followed by right angled resection triangles and additional observations between instrument stations (Fig.8), Axis Azimuth of Major Axis 0 0.00000 0.00000 0-00 1 0.00000 0.00000 0-00 2 0.00395 0.00337 158-13 3 0.00563 0.00420 173-18 4 0.00796 0.00487 177-01 5 0.01071 0.00689 179-01 Figure 8. Error ellipses for “wall station traverse” with addi-tional observations between instrument stations Figure 9. Transverse standard deviation
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  Figure 10. Longitudinalstandard deviation Conclusions • Utilisation of “wall station traversing” method can significantly increase productivity and safety of surveying operations carried in underground mines. • This method requires usage of coaxial theodolite and reflectors with “zero constant”. • High accuracy distance (±2mm±2ppm) and angular (±5”) measurements are required. • Geometry of survey has paramount impact on accuracy of transferred position and direction. • With linear configuration of wall and instrument stations accuracy of “wall station traverse” is comparable with classical traversing methods. • “Wall station traversing” where acute geometry of resection angles can not be maintained requires “forced cen-tring” and additional observations between temporary instrument stations References [1] McCormack, B., (2002) Wall Stations (Reference Points): The Use of resection to Replace conventional Underground Traversing, Proceedings, National Mine Surveying Conference, Darwin, Australia, 8-12 July 2002. [2] Kowalczyk, Z. (1968) Miernictwo Gornicze, Czesc 1, Pomiary Sytuacyjno-Wysokosciowe Kopaln, (Mine Surveying, Part 1, Horizontal and Elevation Surveys of Mines), Gornictwo Tom XVII, Wydawnictwo “Slask”, Ka-towice, 1968, (Poland). [3] STAR*NET Survey Network Adjustment Program, International Edition, Version 5 (1995), Starplus Software, Inc.,460 Boulevard Way, Oakland, CA. 94610, (510) 653-4836, Email: [email protected]