Making Data Speak Volumes
June 1, 2010
In Pueblo Viejo in the Dominican Republic, about 100 kilometers northwest of the capital city of Santo Domingo, Barrick Gold Corp. staff and contractors are working around the clock to build an open-pit mining operation that is expected to process 24,000 metric tons of ore per day at full capacity. Seven hundred hectares are directly affected by construction, with 14 kilometers of haul roads and more than 20 kilometers of support roads. Activities at the site include blasting and mining a limestone quarry for stockpiling and constructing metallurgical quality stockpiles, building dam walls, construction batch plants for civil works and providing bulk fill for road building and landfills. Then there is the infrastructure development, such as public route upgrades for transportation of the four 1,000-ton autoclave assemblies, power lines, roads, bridges, workshops, offices and accommodation, topsoil and overburden removal, as well as contaminated material removal from previous mining operations.
The goal is to get the mine into production by the last quarter of 2011. Achieving this objective requires accurate survey data; up-to-date progress maps and material volumes are essential to ensure that all decisions are based on accurate and reliable information.
The intensive construction phase at Pueblo Viejo brings unique challenges to the Barrick surveying crew. The large number of contractors on site and the amount of material being moved means that speed and accuracy are of the essence. But with 13 to 15 different areas under construction, keeping up with mapping changes to the landscape using traditional survey methods is virtually impossible. Safety is also a huge factor. Large front-end loaders, 195-ton haul trucks, 100-ton water trucks, D10 and D9 track-dozers, wheel graders, blast hole drill rigs and a large assortment of contractor trucks and earth-moving equipment share the same road space with lighter passenger vehicles, which presents a potentially hazardous environment.
In late 2009, the surveyors began using a new survey tool--a Maptek I-Site 4400LR 3D laser scanning system. The equipment has changed the way the survey crew operates and is providing ongoing benefits to the mine.
The laser scanning instrument arrived onsite with a survey-grade tripod and I-Site Studio software for processing, modeling and analyzing point cloud data. Barrick also purchased an optional vehicle mounting frame to facilitate scanning across all of the active mine sites.
A two-person survey crew immediately began collecting data throughout the mine, using the vehicle mount whenever possible and resorting to the tripod mount for more precise work and less accessible areas. Being able to conduct surveys from the safety of the vehicle cab has substantially minimized operator risk and sped up data collection. “I estimate the amount of time saved in tripod setup is about 50 percent. This is a major benefit for us,” says Sean Jefferys, chief surveyor for the mine.
The scanner design makes it easy to switch between the stationary and mobile mount. A built-in “quick-connect” power and LAN adapter is combined with a quick-release mechanism so the scanner can be moved easily. With the vehicle mounting frame attached to the roof of the truck cab, the surveyors simply lift the scanner head onto the frame, clamp the quick-release bracket and connect the cables for transport and scanning.
Each day, the crew drives to a specified location, stops the vehicle and begins scanning at a range of 500 to 700 meters, depending on the area being scanned. The scanner is connected to a dash-mounted field tablet with an Ethernet cable. The surveyors use a stylus to indicate the scan location, extent and resolution--high or standard. The standard setting is typically chosen because it delivers the required data in less time.
One scan and two GPS points are recorded for each location. For the vehicle-mounted scans, one GPS receiver attached directly to the scanner and another on the hood of the truck help ensure scan alignment. The GPS system coordinates the position and orientation of the scanner, eliminating the need to establish control for scans through other means. It also eliminates the need for time-consuming registration because the scans are imported in a preregistered format straight from the cab-mounted controller. Scans can be previewed on the field tablet to ensure that all areas are covered and avoid returning to the field to record missing data.
Each scan takes approximately five minutes. The stop-and-go scanning continues for two days during month-end measurements across the entire site. The route changes daily as different areas are affected by construction activity; with the scanner mounted on the vehicle, it is easy to move to new locations “on the fly.” According to Jefferys, such comprehensive site coverage would not be possible with conventional surveying techniques. “With I-Site we are able to track volumes more often than ever possible,” he says. “With traditional methods, we would be struggling to track volumes four times a year. We are now doing it 24 times a year with ease.”
An Accurate Decision-Making Tool
Once all the scans have been collected, they are transferred directly into I-Site Studio version 3.3 software for processing. Although the scans come into the software partially registered due to the use of GPS positioning and orientation methods, the short alignment or backsight orientation prevents them from being precisely aligned. The data processing manager runs a one-step global registration tool on each scan and tightens up the rotation by pulling in overlapping data from all the other scans. Completing this process takes about 10 minutes and ensures the data are accurately registered in a coordinate system. “Being a survey traditionalist, I was initially skeptical of the global registration process,” Jefferys says. “But with time, this process has been tested and proven. It works and is a huge time saver. Human error is virtually excluded.”
Following the registration process, the scans are ready for cleanup, filtering and modeling. The data processing manager uses the topography and range filtering options in I-Site Studio to edit out vehicles and other unwanted objects. The topography filter works by keeping the lowest “z” value point in a specified cell and removing overlapping data and redundant points. The process reduces the dataset to a manageable size and also filters out vegetation and other small objects that aren’t needed for modeling and analysis. According to Jefferys, it’s a simple step to generate triangulations for calculating volumes for each of the active areas that have been surveyed earlier in the day.
The final step is to export the data as .dwg and .00t files for use by the engineers using CAD and mine planning software. “The data have not only helped us get accurate and timely progress maps, but they also have helped the engineers who can now make decisions with data that are realistic,” Jefferys says.
In addition to gathering volume data on the mine’s current construction areas, the Barrick Pueblo Viejo crew is also beginning to apply the scanner to other types of field work. “Because we can go out and get accurate data so quickly and safely, other application areas are being investigated such as geotechnical mapping and slope stability monitoring,” Jefferys says. “Other tasks, such as surveying volumes removed from an old slime dam are unable to be surveyed traditionally, since no one can walk on the surface. The laser scanner now makes it possible to record and monitor such areas.”