Historical Monitoring at Turtle Mountain

Historical Monitoring at Turtle Mountain

Various technologies have been applied at Turtle Mountain over the last century since the Frank Slide, from simple tape measurements made by John Allen to differential global positioning system (dGPS) and ground-based interferometric synthetic aperture radar (GB-InSAR) monitoring systems used by the current Turtle Mountain Monitoring Program (TMMP).

Immediately after the Frank Slide, geologists R.G. McConnell and R.W. Brock were dispatched from Ottawa to investigate the two remaining peaks, known as North Peak and South Peak. McConnell and Brock suggested that North Peak was hazardous for the inhabitants of the valley below. Between 1931 and 1933, John A. Allan, the founder of the Alberta Geological Survey, mapped the fissures on the back side of Turtle Mountain. It became apparent that South Peak was in fact the area of instability. Allan reported that many deep fissures surrounded a 5 million cubic metre volume of the peak. Evidence from Allan’s studies shifted the monitoring focus to South Peak.



Figure 1. [http://centennial.eas.ualberta.ca/archive/001/90.jpg, University of Alberta Archives #79-23-1357] Photograph of John Allan at his campsite, southeast of Turtle Mountain.

Historical monitoring methods include the following:

1930s

  • Paint markings were used by John Allan in 1933 to measure large cracks by measuring the distance between two paint reference points.

1980s

  • Geophones
  • Moiré crackmeters
  • Electronic distance measurement systems (EDMs)
  • 24 photogrammetric targets
  • A weather station
  • A weir used to monitor stream flow

Early 2000s

  • 22 crackmeters
  • 10 tiltmeters
  • 6 surface wire extensometers
  • 6-station passive seismic network,
  • 19 reflective prisms
  • 11 dGPS receivers



Figure 2. Historical overview, as of December 2013, of the monitoring network on Turtle Mountain as a whole and South Peak of Turtle Mountain in particular (inset), southwestern Alberta.

Crackmeters on Turtle Mountain

What is a crackmeter?
Crackmeters (crack gauge) are thin steel rods anchored on both sides of a crack wall to measure how the cracks separate when the mountain slope moves. The most sensitive of the different monitoring tools on the mountain is a vibrating-wire crack gauge that can detect changes as small as ten microns (the thickness of a human hair).

Crackmeters on Turtle Mountain
Twenty-two active crackmeters were installed on the western side of South Peak in 2003. Continuously recording crackmeters serve to determine whether the surface fractures open at a constant rate or rapidly in one event. Crackmeters are sensitive to snow and ice loading, which introduces large errors in the readings. Although metal roofs were built to protect the crackmeters from snow and ice, they were unsuccessful due to blowing snow and large accumulations of snow and ice. These instruments have become non-operational due to lightning damage and have not been replaced.



Figure 3. Crackmeters on Turtle Mountain.

Extensometers on Turtle Mountain

What is an extensometer?
Surface extensometers measure rock movements over longer distances.

Extensometers on Turtle Mountain
Five extensometers were installed in 2004, and a sixth extensometer was installed in 2006. The extensometers are anchored into bedrock on either side of a crack. One end (head assembly) contains a weight that connects to the other end (anchor end) by a steel cable. The whole sensor hangs over a transducer. When the rock moves, the suspended weight shifts. The transducer measures this distance due to this shift. This information is then read and stored by a datalogger. The extensometer network is non-operational due to sensor failure and environmental factors.



Figure 4. Extensometer on Turtle Mountain.

Tiltmeters on Turtle Mountain

What is a tiltmeter?
A tiltmeter uses electronics to precisely measure angle changes. Tiltmeters are fixed to the sides of large blocks and measure the rotation of the rock.

Tiltmeters on Turtle Mountain
There were 10 tiltmeters located along the highly fractured summit ridge near South Peak. Tiltmeters are used to show temperature cycles, such as seasonal and daily changes. Seasonal tilt changes in general display an increase in tilt at the beginning of the summer and return to the previous trend at the beginning of autumn. A similar cyclic variation in tilt occurs on a much smaller scale with daily temperature fluctuations. The tiltmeter network is also affected by high humidity inside the instrument enclosure, making the interpretation of small rotation very difficult. The tiltmeter network is largely non-operational, and there is no plan to repair the system.



Figure 5- Tiltmeter on Turtle Mountain

Laser Ranging on Turtle Mountain (Electronic Distance Measurement)

What is Electronic Distance Measurement?
Electronic distance measurement (EDM) (also known as laser ranging) uses lasers and mirrors to measure the mountain’s movement. A base station emits a laser pulse to a mirror prism installed on the rock face to be measured. The base station instrument measures how long it will take the laser beam to travel back from the prism. In theory, as the rock face moves down the mountain slope, the distance between the instrument and the prism will decrease.

EDM Monitoring on Turtle Mountain
In the early 1980s, the Survey Engineering Department at the University of Calgary first used EDM technology on Turtle Mountain. A series of remotely monitored reflective prisms were installed on the west side of South Peak. They measured the distance to these prisms from four base stations on the valley floor, located between three and seven kilometres from the mountain peak.

In 2006, the University of Calgary team took new readings, the first in 20 years. The university compared the prism coordinates to the original 1982 EDM measurements to observe if there were any movements. The analysis indicated the average movement was 0.65 mm per year.

Between 2005 and 2007, an array of prisms was installed on the eastern face of Turtle Mountain. The AGS mounted a computer-controlled, laser-ranging theodolite in a protected area on the valley bottom, located about three kilometres away from the mountain and collected data from 19 prisms on an hourly basis.

The AGS found the EDM technology to have limited value due to reliability problems with the system. Measurements could only be taken if there was a clear line of sight between the instrument and the prisms. Unfortunately, the prisms are often covered in snow and ice during the long winter in the Crowsnest Pass. In 2008 and 2009, the EDM system was deemed non-operational due to inadequate results from previous seasons. The EDM system has been decommissioned; however, the prisms remain on the mountain face.



Figure 6. Location of prisms on Turtle Mountain.

Airborne Light Detection and Ranging (LiDAR)

What is LiDAR?
Airborne LiDAR sensors are becoming an increasingly common and cost-effective tool for projects requiring the characterization of relatively large surface areas and for landslide hazard assessment.

Airborne LiDAR systems consist of a laser mounted beneath an airplane or helicopter that follows a predefined path. The laser scans the ground by emitting tens of thousands of pulses per second and detects the reflected light returned from vegetation, man-made objects, and the ground to create a three-dimensional (3D) point cloud model of the surfaces below. A GPS is used to determine the aircraft’s position and provide a datum to calculate the horizontal coordinates (x, y) and elevation (z) of the cloud points.

As this technology has such a dense coverage, it can distinguish between the ground surface, trees, and buildings. The signal returns from vegetation, and man-made structures can be removed from the point cloud, leaving a high-resolution 3D model of the bare-earth surface. For scientists and engineers in the field of geological hazards, this provides a valuable new tool for mapping and characterizing ground-movement hazards.

Using LiDAR on Turtle Mountain
In 2006, the AGS purchased a license for LiDAR data for a 33 km2 area covering Turtle Mountain and the Frank Slide. Using GIS tools, we created a high-resolution digital elevation model (DEM) from the 3D point cloud and used this to assess unstable areas and to create detailed structural maps to verify landslide mechanisms and potentially unstable rock volumes. These data have been critical in revising interpretations of Turtle Mountain's instabilities and have led to expanding our monitoring network on the mountain.

Analysis of the LiDAR data led to the discovery of new unstable portions of Turtle Mountain. We now know there are three unstable blocks that could be involved in a deep-seated, multiblock failure. Two of the blocks are in the area encompassing South Peak, referred to as the Upper and Lower South Peak instabilities. The third block is in the lower slope below Third Peak.

Based on LiDAR data and movement analysis, we have determined that Upper South Peak represents the main unstable area and it has been the main focus of our monitoring. Upper South Peak can be characterized as being composed of three distinct zones, each with a particular deformation mode:

  • an eastern toppling zone,
  • a central wedge sliding to the northeast, and
  • a rear subsidence zone.

The lower portion of South Peak has abundant rock-fall activity and heavy fracturing. Based on the fracture-network orientation, as inferred from surface mapping and kinematic analyses, we can identify at least six different potential rockslides in this area. All of the zones appear to be moving towards the northeast. We estimate the potentially unstable volumes to be 0.12–5.5 million m3.

In 2006, two significant fractures were discovered in the upper part of Third Peak. These fractures show accumulated movement of about 20 cm to the northeast. Kinematic analyses and measurement displacement vectors of the cracks show that different potential rockslides are possible, including a deep-seated gravitational slope deformation and several superficial unstable masses.

Photogrammetry

What is photogrammetry?
Photogrammetry uses multiple sets of low-level aerial photos to monitor specific spots on the ground. By taking several photographs at different times, we can compare these images and measure how much each target has moved horizontally and vertically.

Photogrammetry on Turtle Mountain
In 1981, 24 artificial targets were installed on Turtle Mountain to give broad coverage around South Peak. To analyze the photogrammetry results, photo co-ordinates were compared from the 2005 images to the 1982 aerial photos. As shown below, movement rates between 1982 and 2005 show that changes of up to 88 mm occurred on the east side of South Peak, with movements ranging between 19 and 42 mm on the larger mass between South Peak.

These rates correspond to average annual deformations of 0.9–3.2 mm over 23 years. When compared to results from other historical and recently installed monitoring points, we believe the overall movement rate is reasonable and provides the best picture of the movement patterns in the South Peak area.

 

Figure 9. Photogrammetric target installed on Turtle Mountain in 1982 and repainted during the summer of 2005.


Figure 10. Photogrammetric target layout on South Peak, Turtle Mountain. The vector plots show the total deformation on the targets between 1982 and 2005. The deformations on Target 20 are from 1984 to 2005.

Seismic Monitoring

What is passive microseismic monitoring?
Passive microseismic monitoring uses a network of sensors called geophones or accelerometers to 'listen' to the mountain to detect small tremors. These sensors can detect extremely small tremors anywhere within the mountain. These tremors may be caused by strain accumulation or the release of stress built up in the rock from movement. These movements may be from regional seismic activity or from within the mountain due to adjustments within the rocks, possibly caused by the gradual collapsing of abandoned mines or other small internal movements, such as landslides. The seismic system can detect tiny earthquakes in the mountain; it is so sensitive it can detect mine blasting in the Elk Valley, British Columbia, and larger earthquakes around the world.

Seismic monitoring at Turtle Mountain
Although a microseismic monitoring system was installed in the 1980s, the University of Calgary installed a modern passive microseismic system in 2003. It had an array of seven stations: six surface and one borehole. The surface stations were over a wide area ranging from the Crowsnest River to South and Third Peaks. The borehole station was near South Peak and had instruments at 24 and 38 metre depths. The seismic data was radioed continuously to the Frank Slide Interpretive Centre.

The microseismic network was installed to help locate the movements within the mountain. Unfortunately, this has not been successful due to a number of factors, including the highly fractured bedrock.

Most of the seismicity seen on Turtle Mountain is minor; however, we have seen a clear connection between increased seismicity and rock temperatures passing through the freezing point.

On February 16, 2006, the South Peak seismic station detected a significant localized seismic event. It was very sharp, but not large enough to register at any other station. Based on the arrivals, we believe it came from the northeast quadrant relative to South Peak, most likely within a few hundred metres. As of winter 2008, the passive microseismic system was no longer operational. The AGS archived and documented all triggered event data from 2003 to 2007. The seismic monitoring network will remain non-operational until further notice.



Figure 12. Maintenance of radios on Turtle Mountain used for seismic monitoring.

Thermistors

What is a thermistor?
A thermistor is an electrical probe that measures temperature. It is made of various semiconductor materials whose electrical resistance varies as a function of temperature. Thermistors can be arranged in a series on a cable to measure profiles of temperature within the ground.

Thermistors on Turtle Mountain
To complement the weather station data with subsurface information, we installed a thermistor string in the South Peak borehole in early October 2004. Because we were unsuccessful at installing a thermistor string 55 metres deep into the borehole, we installed a shorter string with sensors at depths of 2.1, 5.2, 8.2, 11.3, 14.3, and 17.3 metres.

A seventh sensor was located aboveground in the protective conduit connecting the thermistor cable to the data-gathering equipment in the borehole enclosure.

The datalogger in the South Peak borehole enclosure recorded hourly temperature measurements at each of the thermistors and then transmitted the measurements to the provincial building at Blairmore. The information was then sent to the Frank Slide Interpretive Centre through radio telemetry.

Rock temperature shows the same general trend as air temperature but is more subdued (lower maximum and higher minimum readings), with a time lag of about 12 hours relative to significant changes in air temperature.

Currently the borehole thermistors have been retired and will not be used in upcoming field seasons to collect data.



Figure 13. Thermistor borehole drilling in 2004.

Water Flow (Weir)

What is flow monitoring?
Water flow is measured to look at seasonal changes in flow patterns and short-term changes due to rain and snow. Typically, water flow is measured on a small stream where a monitoring station called a weir can be installed to measure flow.

Flow monitoring at Turtle Mountain
The AGS installed a water-level and water-outflow monitoring system near the Frank mine entrance at the base of Turtle Mountain in 2004. The site was chosen due to the location relation between precipitation on top of the mountain and the amount and rate of water coming out of the base of the mountain. In theory, this system should give a relative indication of the time it takes water to pass through the mountain after it rains or snows.

The weir was retired in 2010, and water flow data is no longer collected or analysed.



Figure 14. Weir at the base of Turtle Mountain measuring water flow.

Weather Station

A small, automatic weather station was installed west of the ridge crest about 100 metres south of South Peak. This station was installed to enhance interpretation of data collected from other instruments.

The weather station became non-operational during a long, hard winter in the Crowsnest Pass in 2013/14. Multiple sensors stopped collecting data and were unable to be brought back online after multiple attempts remotely.

During the 2015 field visit, AGS staff members noted damage to the weather station presumed to be from environmental conditions such as wind, snow, ice, and lightning. Other possibilities of the station damage may be due to vandalism.

At this time, there are no future plans to replace the weather station sensors and data loggers.



Figure 15. Turtle Mountain weather station 1980s.



Figure 16. Modern weather station on Turtle Mountain.