Map Into the Void

By Anthony Chamblin • April 8th, 2020

Figure 1. Carlson C-ALS being deployed downhole via a mechanical wench

For many mine sites around the world, the juxtaposition of a modern, open pit site with historic underground workings raises concerns of a potentially catastrophic hazard such as ground instability caused by the presence of old cavities underneath current mining operations. It is often the case that very little reliable mapping data survives from historic underground mining operations. The original mapping may have been of a poor standard, maps may have been lost or in some regions, unauthorized/private mining was not mapped at all. Even if reliable maps were made, and survived, hidden void collapses and void migration can mean that modern operations may have little confidence in the underground information left behind.

With blasting operations, heavy machinery and a fleet of vehicles all working intensively above a network of unmapped voids, there is an ever-present danger of subsidence and ground collapse at older mine sites. Careful management and monitoring of this situation are essential to ensure a safe environment for open pit operations. Some mine sites have established dedicated Voids departments which deal exclusively with ongoing mapping and mitigation to ensure continued safety and productivity.

At such a site, typically no operations in the open pit mine can proceed without authorization from the site’s Voids department. Vehicles and activities are limited or completely prohibited in areas which have not been surveyed and cleared, or where there are ongoing investigations.

Figure 2. Carlson Scan allows operators to see the scan in real-time and manage custom scan settings

One method for mapping out the inaccessible underground voids involves drilling a grid of investigatory boreholes to provide information about the underground environment. The feedback from each drilled hole can determine whether a void has been hit, and the depth at which this happens. Depending on the density of the grid of boreholes, the layout of the voids can be interpolated and interpreted. However, this method is time-consuming, expensive and not particularly accurate. Gravity measurements, seismic readings and Ground Penetrating Radar (GPR) can also contribute to void detection but will often give limited, imprecise information with much room for interpretation.

For over a decade, Voids departments on many mines have made extensive use of the Carlson Cavity Auto-scanning Laser System (C-ALS®) to enable an efficient and accurate picture of the often-unknown underground situation. Investigations must begin with some knowledge of the whereabouts of the voids. This information will come from a combination of historical documents, maps, research and data from site geologists. Other evidence of voids may be from clues on the surface such as thermal imagery detecting ‘hotspots’ in the open pit.

With an idea of the presence and rough location of a void, a borehole is drilled to intersect the cavity. It is through this entry point that the Carlson C-ALS is deployed.

The C-ALS is an underground investigation tool, designed to allow operators to measure underground voids which would normally be completely inaccessible or extremely dangerous to enter. The C-ALS consists of a 50mm diameter, 2m long probe which is deployed via boreholes -as small as 70mm diameter – that have been drilled into the suspected void under investigation.

Once in the void, the C-ALS conducts a laser scan to generate a 3-D model showing the size, shape and position of the void.

Figure 3. Carlson Scan Software Showing the C-ALS starting a Scan of a Void

Most deployments of the C-ALS are down near-vertical boreholes, commonly 30-40 m long, but some can reach up to 200 m and others may be steeply inclined, depending on the best route to reach the voids to be mapped.

It is very important for mine engineers that the resulting survey model from the C-ALS is correctly geo-referenced. This ensures it can be compared alongside existing survey data from the mine, both above and below ground.
Initially, the coordinates of the borehole collar will be accurately surveyed by the mine survey team, usually using a Real-time Kinematic (RTK) GNSS system. This coordinate is the starting point of the C-ALS survey. C-ALS operators from the Voids department will then deploy the C-ALS through the borehole. This can be done by manually lowering the probe on its own umbilical cable, or by using a powered winch which can take the weight of the system and make it much easier to recover, especially for longer deployments. At all times during the deployment, real-time control of the system is maintained through the Carlson Scan software running on a Windows-based tablet computer. As the C-ALS is lowered, Carlson Scan allows the operator to monitor the C-ALS’s integrated borehole camera, to check for obstructions along the borehole, and to determine the point at which the C-ALS emerges into the void.

The most recent model of the C-ALS also incorporates an advanced navigation package consisting of an internal gyro and accelerometers. The gyro, installed and calibrated inside the C-ALS, is crucial in allowing the system – and thus the data it produces – to be positioned and orientated more accurately than ever before. The gyro ensures that the orientation of the C-ALS is tracked as the probe ‘twists’ on its cable during the deployment. At a regular, user-defined interval along the borehole, a reading is taken of the navigation sensors in the probe. As all this positional data is collected, the probe is tracked through a process of dead-reckoning. When the probe finally emerges into the void, its position and orientation are known relative to the borehole collar and to the mine’s coordinate reference system on the surface.

With the C-ALS secured in place and now protruding into the void, the operator can then start a laser scan survey of the void. This takes anywhere from a few minutes to a few dozen minutes, depending on the density of data required. The C-ALS scanning head is articulated horizontally and vertically, giving it the ability to move mechanically and produce a full 360° scan of the void within which it is located.

Figure 4. The C-ALS Gyro features a nosecone camera that allows operators to monitor the deployment in real-time from the Carlson Scan software

As the scan progresses, a real-time view of the collected data appears in Carlson Scan. The scan is displayed as a ‘point cloud’: a series of thousands of individual data points from the C-ALS laser. These points can also be viewed in real-time as a solid 3D model which gives a more intuitive and realistic depiction of the underground environment.

Existing underground and surface survey data from previous C-ALS surveys, design data or other sources can be imported into the software. Thus, as the scan builds up, it is immediately seen in the full context of the mine’s database of surveyed information.

The C-ALS laser can only see along a straight line-of-sight. Any areas of the void that are obscured due to the shape of the rock mass will need to be observed from a different position within the void. In this case, another borehole must be drilled to allow the C-ALS to be deployed into a new location with a different vantage point. By using the original geo-referenced C-ALS scan data which has already been collected, the precise coordinates of the new borehole collar can be determined. The survey department can use the RTK GPS to set out this new drill location.

In this way, the C-ALS can be used to determine each subsequent drill hole, from which additional scan information can be collected. The mapping of the underground voids can thus progress in an efficient manner, avoiding the need to carpet the entire site with a grid of speculative boreholes.

Drawing up engineering solutions to deal with underground voids, from blasting the problematic area to simply avoiding a dangerous location, cannot be contemplated without accurate information about the extent of the voids. The C-ALS provides Voids departments with the ability to compile this information. Efforts to avoid the danger of unmapped voids can, therefore, be approached with the precise geographic information recorded from the C-ALS. Throughout the operation, personnel are kept clear of any perilous situations, with all data collected remotely from the surface.

Written by Anthony Comber, Carlson Software LMD

Originally appeared in Global Mining Review March/April 2020
Volume 3, Issue 2

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