Differential Global Positioning Satellites (dGPS)
What is dGPS?
A differential global positioning system (dGPS) uses satellites to locate points on the Earth. By collecting information from different satellites, we can locate the position of a GPS antenna within a few millimetres. Recreational and automotive GPS devices use this same technology.
dGPS on Turtle Mountain
There are 8 permanent dGPS monitoring stations on the south peak and eastern face of Turtle Mountain. NavStar Geomatics Ltd. of Kelowna, British Columbia, designed and installed these stations in 2006.
The signals from these stations are collected at a central collection point on the mountain and data is sent via wireless radio link to the Blairmore Provincial building and then relayed to our servers in Calgary. The AGS compares these signals to those with base stations on Third Peak and in the valley bottom to achieve an accuracy of about one to two millimetres on the horizontal plane.
Each of these stations has two parts, a GPS antenna mounted on a concrete pillar and an electronics box containing telemetry. The latter is mounted on a separate antenna mast located several metres away to avoid possible radio interference between the two.
Figure 17. dGPS monitoring equipment on Turtle Mountain.
Periodic GPS Reading
In May 2007, the University of Calgary Geomatics Engineering Department collaborated with the AGS to monitor the two areas highlighted by the LiDAR studies. The first area is within the active parts of the saddle zone, between South and North peaks. The second area is the eastern face of the mountain below Third and South peaks.
The monitoring network initially consisted of 14 steel targets and was expanded to 18 targets during the summer of 2008. We believe this coverage will be sufficient to detect and describe the movement patterns in these potentially unstable areas.
Since 2007, we have taken two sets of observations at all 18 targets (one in early summer and another in late autumn) using a dGPS. Future periodic GPS campaigns may be planned during the upcoming field seasons.
Figure 18. Periodic GPS campaign on Turtle Mountain.
Ground-Based InSAR (GB-InSAR)
What is GB-InSAR?
Ground-based interferometric synthetic aperture radar (GB-InSAR) uses radar waves to map ground movement. The technique is fundamentally identical to satellite-based InSAR, but instead of acquiring the images from several hundred kilometres away, the images are acquired by a radar antennae moving on a rail within a couple of kilometres of the area.
Satellite-based InSAR collects images every few weeks, whereas the ground-based InSAR acquires images as often as every five minutes. This allows continuous monitoring of movement ranging from millimetres per year to metres per hour in velocity. The working range is up to four kilometres in line-of-site distance.
Figure 19. LiSAmobile system at the Bellevue pump house station.
GB-InSAR on Turtle Mountain
In 2009, the Geotechnical Engineering Department at the University of Alberta purchased an IBIS GB-InSAR system from IDS with the support of the Natural Sciences and Engineering Research Council of Canada. In mid-September 2009, staff from the AGS and a student from the University of Alberta installed the system on Turtle Mountain. It was able to obtain movement information from most of the unstable upper portion of the mountain, as well as much of the lower slope.
In fall 2013, the IBIS GB-InSAR system was retired due to system failure and calibration issues.
In spring 2014, a similar system known as LiSAmobile was installed at the Bellevue pump house to provide the AGS with near-real-time remote monitoring capabilities. LiSAmobile is a monitoring system supplied by Ellegi Ltd. from Milan, Italy. LiSAmobile is made of two main parts: the radar head and mechanical components.
- The radar head is an active sensor that transmits pulses towards an object and receives a radar signal in return.
- The radar head consists of two microwave antennas: one for transmitting and one for receiving.
- The radar head travels back and forth along a four-metre-long track, scanning the mountain every 8 minutes.
- A radome is fastened flush against an aluminium plate to fully enclose the system from environmental factors.
Figure 20. LiSAmobile GB-InSAR track and radar head.
Figure 21. LiSAmobile with radome with Turtle Mountain in the background.
The LiSAmobile system is connected to the internet and data is sent to both AER and Ellegi servers. The LiSAmobile system obtains raw data measurements from the radar head and both 2D and 3D pixelated images of the mountain face are produced to show ground displacement. Positive values indicate movement away from the radar head and negative values indicate that the movement is towards the radar head.
LiSAmobile Study Area
Initially, LiSAmobile underwent a 24-hour calibration process. After that, nine regions (A to I) were identified as most relevant to the study area. The nine zones were eventually redistributed into seven study zones (A to G).
Figure 22. 3D displacement map (top) measured from June 20 through September 20, 2014, and view of the eastern face of Turtle Mountain (bottom). Letters A to G denote locations of regions.
Within each of the study zones, additional points of interest (POIs) were identified for further, more detailed monitoring. If necessary, additional POIs may be added or removed from the study area. POIs isolate areas of movement for further analysis and interpretation.
Figure 23. 3D displacement map (15 mm) measured from June 20 through September 20, 2014, displaying twelve points of interest found throughout regions A–G.
LiSAmobile has been in continuous operation since installation in June 2014 with no downtime except for an annual 24-hour maintenance each spring. Ellegi provides the AGS with 2D and 3D displacement images on a quarterly basis. These displacement images include results on a 14- to 15-day interval. A quarterly report is also provided to the AGS providing detailed analysis of rock displacement values.
Figure 24. Total displacement from June 20 through September 20, 2014, of Turtle Mountain viewed on line of sight 3D displacement map.
What Does This Mean for You?
The AGS has evaluated the LiSAmobile system and the reports provided by Ellegi analysts and have determined that
- the system is reliable, experiencing almost no downtime since June 20, 2014;
- the service alerts us to movements in enough time to anticipate and prepare for a potential slide; and
- the quarterly reports and displacement images from Ellegi analysts are of consistently high quality and will continue to be reviewed and verified by AGS staff.
Figure 25. LiSAmobile annual maintenance.
What is satellite-Based InSAR?
Satellite-based interferometric synthetic aperture radar (InSAR) is a technique that uses repeat-pass data from polar-orbit radar satellites to map very small ground movement over relatively large areas. The radar satellites constantly shoot beams of radar waves toward the Earth and record the time it takes the waves to reach the ground (wave phase). When the satellite revisits the same spot, the phase of the image should be identical. We combine these images to measure ground changes. From this comparison, we can view and measure ground elevation change within a few millimetres over a very broad area.
Satellite-based InSAR on Turtle Mountain
Initially, we were interested in using InSAR on the rugged, unstable, upper portion of Turtle Mountain. However, because of limitations in the orientation of the available radar satellites, the technique was not useful for mapping movement in the upper portion of the mountain, but was found to be very well suited for mapping deformations on the lower slopes. As the lower slopes and valley bottom are covered with bare-rock debris from the Frank Slide and recent rockfall events, there were thousands of points identified that provided very good quality data for deformation monitoring.
The AGS collects satellite data from April to October, typically when the ground surface is bare. The AGS does not collect data during the winter months due to snow and ice coverage. The AGS collected Radarsat-1 data from 2004 to 2013 and Radarsat-2 data in 2015. Previous collections of satellite-based InSAR have allowed the AGS to detect and characterize mine subsidence at the abandoned Frank and Bellevue coal mines, which were active in the early 1900s.
During operation, miners excavated large coal seams, leaving behind pillars of coal to support the roof. Over time, these pillars weakened, resulting in slow downward movement of the roof until it collapsed, often creating a pit at the surface. There are many examples of pits resulting from this style of mining in the Crowsnest Pass area.
Previous analysis of InSAR data has shown that the ground surface above the Frank coal mine has been settling up to 3.15 mm annually, relative to the reference. Average changes of up to 3.2 mm per year (April 2004 to October 2006), relative to the reference area, were also observed overlying the footprint of the abandoned Bellevue coal mine to the east.
Figure 26. Deformation pattern of the lower part of the Frank Slide. The white stripes indicate underground coal mines; the red line outlines the Frank Slide.