Growing use of stereo photogrammetry delivers simplified advanced profiling for surface and underground mining operations
By Rick Zettler
The mining, quarry, construction and explosive industries are challenged to find capable equipment and experienced personnel to profile, survey, design and layout blast patterns to meet today’s demanding performance and environmental needs. Companies often employ a variety of profiling and survey technologies.
“Some of these technologies are selected for their simplicity and do not meet the performance level needed to ensure blast safety or performance,” said Robert McClure, president of Robert A. McClure (RAM), Inc. of Powell, Ohio, an international blast and engineering consulting firm. Complexities of other technologies require significant training and make it difficult for the occasional user to maintain proficiency.
Building a Better Technology
For years, manual measurements were taken with burden poles, measuring tapes, abney levels, transits and the blaster’s best guess. With these methods, there are often significant deviations – such as the burden, spacing, bench height, subdrilling and hole azimuth and inclination – between the planned and actual geometric parameters of a blast. This brings about a variation of the explosive energy, resulting in course fragmentation, inconsistent muck-pile uniformity and higher production costs. Recognizing the potential for inaccuracy, the industry adapted better technology.
One of the first to be developed, three-dimensional (3-D) laser systems have been used in the industry since the early 1980s. This technology allowed the operator to profile the face contact free and from a safe location, representing a significant improvement in both safety and accuracy over the existing manual methods of the time. Due to the cost and complexity of the 3-D laser technology, two-dimensional (2-D) systems were subsequently introduced and served as an efficient way to measure face burdens.
However, these 2-D and 3-D technologies had limitations. Both were operator dependent, required attention to detail and, in some cases, demanded an extensive surveying background. They were inadequate in providing a visual means to identify mud seams, loose material, faults and joints. Another issue was the difficulty to accurately profile corners to determine minimum burdens. The 2-D technology required multiple setups to achieve this goal, while 3-D profiling required complex merging of multiple profiles surveyed from different locations.
Rise of Stereo Photogrammetry Profiling
These challenges had the industry exploring for better technologies that deliver the required performance level, ease of use and minimal training. Over the last three years, the industry has been realizing the benefits and performance of 3-D photogrammetric surveying and profiling technology. Advancements in this technology are providing the industry with accurate and efficient profiling and surveying. “It is a non-contact system, so it also improves worker safety,” said McClure.
Stereo photogrammetry involves the measurement of 3-D (spatial) information from two photographic images showing the same object or surface, taken from different angles. Today’s 3-D photogrammetric technology, such as BlastMetriX3D, uses images taken from a calibrated single-lens reflex (SLR) digital camera. These images are then reconstructed and scaled to create an accurate 3-D image. They provide a perspective for blast design and rock mass characterization not previously available without the use of expensive equipment and specialized operators.
Operators are now using these systems in surface and underground operations globally. In surface operations, stereo photogrammetry is being used to characterize the rock mass, design bench blast patterns and provide minimum burden reports for optimized blasting. These surface operations range from small quarry and construction benches to large-scale cast blasts.
For underground, the technology is being used to characterize the rock mass, provide drilling information, benchmark blast performance and design bench blast patterns with minimum burden reports. Over and under breakage at the face, roof, and sidewalls can be easily profiled and measure with the system. Uneven and concaved faces can be quickly surveyed so that corrective action can be taken to determine drill depths. Images are collected using the same rules as in surface operations except the camera must be positioned on a tri-pod and low-light conditions require the use of auxiliary lighting.
Laser systems provided some of these capabilities, but were very complex in their operation and training. Laser systems offered a 3-D survey and borehole survey integration, but they provided limited cross-sectional views of the borehole and did not provide a visual reference to allow for rock mass characterization.
Advancements in photogrammetric 3-D technology addressed these capabilities and provided true minimum burden profiles necessary for blast optimization and mitigating flyrock. Stereo photogrammetric technology has the ability to capture significantly more reference points on the face verses the laser based system. “Laser systems are operator dependent, requiring individual points to be shot on the face, whereas the stereo photogrammetric system utilizes the camera’s pixels as points of reference,” said McClure. Hundreds of thousands of points provide a denser point cloud, which more accurately characterizes the topography of the area being surveyed.
Simple, Yet Sophisticated
The 3-D stereo photogrammetry images are generated by an off-the-shelf digital SLR camera. The camera and the corresponding lenses are calibrated for accurate bench face surveying. This process is done in a lab prior to the camera being sent to the field. After calibration, the camera and lenses provide a range of wide angle and telephoto zoom options.
In addition to the camera, the system uses two sets of targets for reference. Two of the targets, referred to as delimiters, are placed on top of the bench where the existing boreholes are drilled or the proposed shot pattern will be laid out. They will serve as the reference line to orient the shot design, where the borehole collars are to be located.
Two range poles are also positioned within the image viewing area, typically on the floor under the bench to be blasted. All four of these targets must be visible when taking both pictures.
Area coverage for a single 3-D image is determined by the camera’s number of pixels and stereo photogrammetric principles. The typical area coverage for a single 3-D image, with a 12-megapixel camera, ranges up to 80-m (262-ft.) laterally and 55-m (180-ft.) vertically. “For large-scale cast blasting operations with long benches, 20-megapixel cameras are capable of lateral distances exceeding 122 m (400 ft.),” said McClure.
The benefit of the stereo photogrammetric system is that the distances don’t have to be precise. During inclement weather or situations where pit traffic presents a safety issue, images can be shot from a vehicle. Unlike the laser based systems no tripod or accurate leveling is needed to collect the survey data.
The 3-D photogrammetric system is particularly effective when multiple profiles are required around corners and long/high benches. When profiling in these situations, a master image with targets is used to start the multiple image merge. Images to be merged are easily made by simply moving down the face, following some basic rules to determine the base length and overlapping a portion of the master and/or additional images. The overlap merges the image into a single, fully developed 3-D image.
Multiple images can then be added to the new master, building a new merged image with up to six total sections. Once complete, the finished multi-section survey is scaled in the software, so actual field dimensions are reflected in the image.
Once the final merge is completed, the intended or existing blast pattern can be input. The designer will see a scaled 3-D image of the actual blast area with all the blast area anomalies and imperfections. With an accurate visualization of those imperfections, the designer is able to design the blast pattern based on drill bit size, burden and spacing, inclination, and direction of holes. This allows for optimization of the burden with a high level of confidence.
The blast design software also enables the design of the preferred burden through borehole placement based on a mean face burden or manual placement. The burdens can be viewed two ways: a 2-D cross sectional profile view of the face; and a 3-D minimum burden view. The 2-D profile provides a view directly in front of the blast hole with greater accuracy levels than the laser based 2-D profile systems.
The minimum burden view, on the other hand, gives a 360-degree view of the burdens. It reflects the minimum burden, whether it is directly in front of the hole, off to the side, or up from the hole. Once the shot has been drilled, it is then possible to incorporate borehole surveying data to the holes and generate the true minimum burden along the entire length of the borehole, which provides a high level of accuracy for blast optimization and flyrock mitigation.
Geo-referencing the blast to local or global coordinates can be done through the advanced blast design software by surveying the targets and entering their coordinates. Survey coordinates can be obtained by “shooting” the prisms mounted on the targets with a total station or picking up the target locations by accurate GPS. Once the coordinates are matched to the targets in the software, the blast design will be geo-referenced.
During the pattern design stage, borehole placement will reflect the planned coordinate location. “The final blast design, with coordinates of the borehole locations, can then be exported in a .CSV file format to both drill navigation or GPS survey systems and used to position the boreholes on the bench,” said McClure.
During the blast design stage, the actual blast area can be seen as a 3-D digital image, allowing the blast designer to identify areas of concern such as mud seams, loose material, faults and joints. The 3-D view provides an additional line of defense against potential flyrock by being able to identify areas of concern.
When drilling conditions change and boreholes are repositioned from their intended locations, an updated “as drilled” pattern file must be generated either by the drill navigation program or surveying. “The updated .CSV file can be imported into the original ‘as designed’ pattern and updated to reflect the actual borehole positions,” said McClure. The update will represent the final borehole locations and minimum burdens without having to return to the bench to reprofile the repositioned borehole locations, saving a considerable amount of time, travel and money.
In blast optimization, having the capability to strategically position the boreholes in the advanced blast design software allows the designer to optimize burdens and powder factors. “This is critical to an optimized shot design when dealing with challenging conditions such as irregular faces, uneven floors, working around bench corners, multi-sided blasts and toe conditions,” he adds.
When borehole deviations and true minimum burdens are identified as potential problems, a decision can be made to modify the design. This could be accomplished with additional drilling or by adjusting the explosive loads to compensate for the deviations.
This proactive optimized blast design approach allows the designer and blaster in the field to address burden and borehole deviation issues, while maintaining desired powder factors throughout the rock mass. “Applying this technology provides optimized control of the final grade, wall, air overpressure, vibration, fragmentation and downstream economics,” said McClure.