Automated Scanning Lasers
Automated Scanning Lasers, or Laser Scanners, are the most recently introduced type of measuring instrument to be deployed by Mine Surveyors. Laser Scanners deliver a range of unique capabilities to mine surveyors and so the development and adaptation of this technology to satisfy survey tasks in mines has been considerable since their introduction to the industry in 1998.
What Laser Scanners Do
The primary capability delivered by Laser scanners is to be able to measure very detailed, very accurate 3D coordinate information across wide areas and to long ranges in relatively short periods of time. It is common for a laser scanning project to result in tens or hundreds of millions of points being recorded within only a few hours. The spatial distribution of points may be as close as several millimetres for some applications. No access to the scene being scanned is required, and additional features such as digital colour rendering and tilt compensation are available on more recent models.
Laser scanners are unique also in their heavy reliance on software processing to enable the delivery of useful results in mine surveying. The rich point clouds produced as raw data from Laser Scanners, while visually impressive, are not immediately applicable as survey deliverables and generally require the use of specialised software algorithms and workflows to be fully effective. This software will generally be supplied with the Laser Scanning instrument.
How Laser Scanners Work
Laser Scanners used in Mine Surveying use a time of flight measuring principle, similar in concept to other electronic distance measurement, but designed and built using technologies allowing measurement at extremely high repetition rates, very long ranges without the use of reflectors, reliable accuracy and accommodating a very different optical design to conventional optical instruments. The rangefinding laser beam is generally pulsed at speeds of more than 20kHz and each pulse is deflected by scanning mechanisms and optics to a different angular coordinate. The range is measured to each point as the scanning proceeds. Each of these points is collated in a scan data set, and appears when visualised on a computer as a “point cloud”.
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Figure 1: typical raw point cloud produced by laser scanners
Usually multiple point clouds, or scans from individual locations are required to complete a given project in order to get adequate coverage of the objects being measured. This presents the challenge of registering multiple scans together in a relative coordinate space. It is also usually required that the entire dataset of multiple scans is accurately located within a mine coordinate system as well. This process is commonly referred to as Registration, and is a critical task in the successful deployment of Laser Scanners.
Individual point clouds as shown above are virtually useless unless they can be transformed to the mine grid coordinate system, both as single scans and accurately as a project. There are various methods available to achieve this, some easier and more effective than others. Generally, these techniques can be divided into two categories based on the following concepts:
- Referencing features in the scan data after the scan has been acquired
- Referencing physical features of the scanner measurement datum to the appropriate coordinate system before the scan is acquired
The first category here includes techniques such as the use of reflectors or targets which will be easily identifiable in the scan data. Almost all Laser Scanners record the intensity of light reflected for each measurement point, and so reflective materials can be used to make targets which are identifiable in the scan data. By locating a minimum number (usually at least 4) of these targets within the scene and measuring their location within the mine grid coordinate system (using a total station or other conventional instrument) a spatial transformation of the entire scan data set can be carried out based on the transformation required to best fit the measured points to the array of targets visible in the scan data. Usually software will be provided to do this transformation, but some setup work is required.
The second method involves a workflow based on the conventional methods for setting up an optical instrument. That is, the measurement datum of the scanner (in all 6 spatial degrees of freedom) is measured and controlled such that the location of the datum is known to a very high degree of accuracy and the data is then measured directly into the mine grid coordinate system. Instruments allowing this are generally more advanced, with additional sensors such as dual axis tilt compensators, adaptors for mounting GPS antennae to the scanner and survey telescopes for alignment within site control.
Surveyors can also choose to combine these methods as required for a given site or project. For example, it is sometimes the case that a GPS measurement of the location of the scanner is used in conjunction with one or more reflective targets in the scan to constrain orientation in each axis. Generally, avoiding reflecting targets will ensure that the ability to access the scene being scanned is minimised or avoided, and allow for the most flexible capability for a mine surveyor.
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Figure 2: Multiple point clouds registered together to give a detailed data set over several kilometres of a mine.
Scanning is now a common practise in many mines and the scanners used for Mine Survey are developed and adapted specifically for the task. As such, peripheral features, user interfaces and instrument design are all generally provided to suit large area topographic survey in harsh environments and with the flexibility required to suit mine survey.
Performance parameters of Laser Scanners used in mines generally include measurement range of greater than 500m without reflectors, high speed scanning (less than 10 minutes to complete full scan), Class 1 Laser Safety and good battery life. A compact, easy to deploy and operate design is important given the rough ground and other hazards inherent in surveying in a mining environment.
Where are Scanners Used in Mines?
Scanners are used in an increasing range of tasks in mine survey. Primarily, these instruments are used for pit and stockpile pickups and volumes, as well as face mapping for geological and geotechnical detail, pre and post blast measurements to determine blast performance, reconciliation of material movement and other detailed volumetric analysis such as truck tray or shovel bucket modelling, and structural measurement of plant items. Laser Scanners are used underground for open stope surveys, development drive mapping and section measurement to compare and confirm against design. Automated Software algorithms developed specifically for Laser Scanners allow users to extract sections, contours, breaklines, geotechnical, volumetric and more information from the detailed scan data much faster than by any other surveying technique. The quality of the results combined with the speed and efficiency with which they are achieved creates great value for the users of laser scanners, and has led to their increasing use in mine surveying globally.
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Figure 3: filtered and scan data, with breaklines, sections and contours all extracted.
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Figure 4: The extremely popular Maptek I-Site 8800 Laser Scanner, generally accepted as the best..
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