From findings of a research, a team of engineers from Tre-Altamira demonstrate that images gathered through Interferometric Synthetic Aperture Radar (InSAR) can be used to detect unnoticed precursors to catastrophic slope failures. They see this as one of the techniques mines in Africa can use to improve safety in and around Tailings Storage Facilities and in open-cast mines by mitigating the potential impact of ground failure.
The sudden occurrence of ground failure in areas around Tailings Storage Facilities (TSFs) and slopes in open-pit mines without early warning has baffled the mining world for a long time. Now, a team of engineers from Tre-Altamira may have found the answer to this puzzle. This is based on findings of their study, Perspectives on the prediction of catastrophic slope failures from satellite InSAR.
The team demonstrate that numerous cases of unnoticed ground deformation are due to the limitations of in-situ sensor-based monitoring devices to detect precursors to catastrophic slope failures accurately. Alternatively, by using Interferometric Synthetic Aperture Radar (InSAR), multi-interferometric analysis, it is possible to process satellite radar images that indicate accelerating displacement prior to the catastrophic failure on sites.
Hopefully, if applied appropriately, this technique could enable mines to contain costs of monitoring, increase safety (prevent fatalities) and protect some of their movable assets from potential damage.
The relevance of InSAR
The relevance for using InSAR (multi-interferometric analysis) becomes evident when the limitations of in-situ, sensor-based monitoring devices are analysed, and how InSAR is able to fill the gap cost effectively illustrated. Common sensors installed in-situ are extensometers and distometers. Alternatively, sometimes survey stations, prisms and slope radar are also used.
The approach to establish effectiveness of SAR
In order to establish the effectiveness of InSAR, images gathered from three sites where ground failure was experienced and sensor-based monitoring had been used for a record of precursors are analysed.
To process three sets of Sentinel-1 constellation images through multi-interferometric analysis, the team utilise SqueeSAR®, a second-generation InSAR algorithm. Radar datasets are acquired by the Sentinel-1 constellation, which is composed of two satellites equipped with C-band (5.6 cm wavelength) SAR sensors featuring a right-looking acquisition geometry and a revisit time of up to 6 days. Three identified areas of interest where ground failure occurred are: an open-pit mine slope at an unnamed location, a natural rock slope in alpine terrain in China, and a tailings dam embankment in Australia.
Case studies demonstrating failure
Interferometric analysis conducted on InSAR images which are gathered from three different ground failure sites suggest the occurrence of precursory ground deformation.
In 2016, a catastrophic slope failure occurred in a copper open-pit mine which caused the death of 16 mine workers and the termination of the extraction activities. The failure happened without apparent warning signs, as it mostly affected a sector of natural slope above the mine crest and outside of the field of view of the slope monitoring system in place at the site. However, radar targets exhibit precursory ground deformation at the source of failure – up to 30.2 mm of Line Of Sight (LOS) displacement recorded in the period 9 to 15 November 2016.
In China, a landslide in the Xinmo village, Sichuan Province, killed more than 100 people. The presence of tension cracks and the occurrence of minor precursory rockfalls persisted undetected. On the contrary, 45 SAR images recorded up to 4 days prior to the accident indicate the presence of hazards.
There was little notice of in-situ evidence of precarious stability conditions (tension cracks or minor slumps), prior to the failure of a tailings dam embankment at Cadia Gold Mine, New South Wales, Australia. Due to the condition, a slurry of sediments, water, and a low level of benign processing reagents were released in the basin of the southern TSF. Though no fatalities were recorded, however, as in similar abovementioned, the evident underlines the limitations of in-situ sensor-based monitoring devices that were used.
Strong business case
Unquestionably, the abovementioned case studies underline a strong business case for extending the application of satellite InSAR to ground deformation monitoring for accurate slope failure prediction. InSAR can help mine operators to mitigate the impact of ground failure accidents on their assets and employees, as well as surrounding communities.
Davide Colombo, a prominent member of the team that was involved in the study, explains: “With InSar, millimetric measurement accuracy and metric spatial resolution are attained in most atmospheric conditions, with no need to install physical reflectors (in-situ equipment) on the ground. The interferometric products ensure a worldwide coverage and are distributed to the public with cloud-based solution.”
“In general, InSAR can improve risk awareness and provide early warning of impending catastrophic slope failures in vast, inaccessible regions as a significantly lower cost than that required for dedicated monitoring systems,” he adds.
Tre-Altamira is looking forward to providing InSAR monitoring of TSFs and slopes in open-pit mines for potential clients in the African mining sector. Based in Italy, with branches in Spain and Canada, the company provides monitoring solutions to clients in different sectors globally.
InSAR fills the gap
There is no question that InSAR fills the gap in the need for techniques that can detect small, unnoticed signs of ground deformation that could occasion a landslide, considering the limitations of the two methods – in-situ based sensors and spaceborne monitoring systems.
It is difficult to detect their precursory signs with in-situ based sensors due to the following factors: inadequate field of view of the instrument; limited number of measuring points; lack of ancillary data supporting the installation of a monitoring network; unawareness about the presence of ongoing instability phenomena; difficult site accessibility. Over and above this, economical or logistical are the constraints involved.
What could have could been an option, spaceborne monitoring systems, has not been applied, as critical data was not gathered consistently, as and when needed. The following issues have been major hindrances to utilising space borne monitoring systems: the poor revisit capacity of orbiting satellites, the limited data accessibility, and the policy on image acquisition (i.e. background and on demand acquisitions) have so far prevented the use of satellite InSAR as a tool for systematic monitoring of critically unstable slopes.
With satellite InSAR, the integration of more than one satellite working in constellation mode has significantly improved the frequency and regularity of the acquisitions, as well as the ground visibility of the flyovers. Besides, with the newest algorithms for the processing of interferometric data-stacks, it is possible to retrieve a greater amount of radar targets within the sensor swath.