Recent advances in slope monitoring radar for open

Proceedings of Mine Closure Solutions, 2014
April 26−30, 2014, Ouro Preto, Minas Gerais, Brazil
Published by InfoMine, © 2014 InfoMine, ISBN: 978-0-9917905-4-8
Recent advances in slope monitoring
radar for open-pit mines
Paolo Farina, IDS Ingegneria Dei Sistemi SpA, Italy
Niccolò Coli, IDS Ingegneria Dei Sistemi SpA, Italy
Francesco Coppi, IDS Ingegneria Dei Sistemi SpA, Italy
Francesco Babboni, IDS Ingegneria Dei Sistemi SpA, Italy
Lorenzo Leoni, IDS Ingegneria Dei Sistemi SpA, Italy
Thiago Marques, IDS Brasil Engenharia de Sistemas Ltda, Brazil
Felipe Costa, IDS Brasil Engenharia de Sistemas Ltda, Brazil
Abstract
Slope monitoring radar has emerged in the last 10 years as a leading-edge tool for safety-critical
monitoring of pit wall movements in surface mining because of its ability to rapidly measure slope
movements and displacement with sub-millimetric accuracy over wide areas in any weather conditions,
obviating the need to install artificial reflectors.
Thanks to the improved performance of the new generation of slope monitoring radar based on
synthetic aperture radar (SAR) technology, the standard concept of critical monitoring has now reached
a new dimension by integrating multiple SAR units to achieve full monitoring coverage of the entire
pit. The previous generation of real aperture radar (RAR) provided useful information on areas of
concern for slope stability and required constant relocation.
The new full pit monitoring concept (FPM 360), recently developed by IDS, employs the unique
capabilities of SAR and expands the idea of critical monitoring by giving users a universal image of pit
wall stability by covering the entire pit slope scenario with a few radar units, continuously scanning
from semi-permanent installations with scan times of less than three minutes. This unique functionality
is a first for radar monitoring, providing a vast amount of information of the entire pit wall in almost
real time. The capabilities of slope monitoring radar based on SAR can be used effectively in the pit
and waste dumps and tailings of both operating and closed mines.
The main technical features of typical slope monitoring radar based on the SAR technology are
presented in this paper along with recent case studies of successful slope monitoring in open-pit mines.
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Introduction
Controlling the potential for hazardous ground movements or instability in an open-pit mine to within
acceptable limits is essential to eliminate or minimize safety risks. A comprehensive slope monitoring
program aimed at managing potential large-scale instabilities and able to detect local scale movements
should be part of every ground performance monitoring system. In fact, small-scale slope movements,
sometimes limited to few centimeters of total displacement and characterized by temporal evolutions
ranging from several hours to several weeks, usually precede large failures in open-pit mines.
The capability of providing advanced notice over the whole slope of impending instability
conditions through the accurate and timely measurement of precursors to slope collapses clearly
represents an outstanding benefit for the pit staff involved in geotechnical risk management.
The use of ground-based synthetic aperture radar (GBInSAR) in open-pit mines is today a
standard practice for active monitoring of the pit walls. SAR radar units are effectively used to get a
better understanding of the spatial distribution of slope movements and for the provision of alerts in the
event of progressive movements that could potentially lead to slope failure, with the goal of assessing
the safety of workers and increasing mine productivity.
Early detection of ground failure allows mine operators to plan and implement appropriate
actions (evacuation plans) with sufficient notice to minimize the effect of the failure on personnel
safety and mine productivity.
GBInSAR technology offers several advantages: high measurement accuracy, long-range
capabilities, limited impact of atmospheric artifacts on measurement performances, and the possibility
to acquire a simultaneous response over a large area of the slope without affecting the acquisition time
(Antonello et al., 2004; Bozzano et al., 2001; Farina et al., 2011; Farina et al., 2013; Pieraccini, 2013).
Because of their long range and the resulting ability to monitor pit slopes without relocating the
radar during blasting, as often happens with radar characterized by a shorter working range, the long
datasets typically acquired by GBInSAR systems installed in open-pit mines (years of continuous data)
have recently made it possible to measure very slow movements over long time periods. Such a
possibility allows the use of slope monitoring radar not only to measure fast movements potentially
leading to slope failures but also slow movements over years. Advanced data processing algorithms
developed by the authors for GBInSAR systems are able to simultaneously measure a wide range of
displacement rates across four orders of magnitudes, from very fast movements (up to 150 mm/hour) to
extremely low displacements (a few mm/month), with very high accuracy (1/10 mm). These advanced
capabilities open the door to a new concept of radar monitoring, in which radar also becomes a
valuable source of information to better understand slope behavior before movements start interacting
with mining operations. In fact, early recognition of ground movements allows extraction techniques,
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mine design, and ground support to be reviewed before the instability becomes difficult or expensive to
mitigate.
These unique features, coupled with the long working distance, reaching four kilometers from the
target slope, has made it possible to develop a unique 360-degree full-pit monitoring system, able to
provide universal information on slope movements in real time for enhanced safety-critical monitoring.
Those capabilities can be effectively used in both operating mines and closed mines. In operating
mines, such technology allows to productivity to be improved and miner safety to be increased. On the
other hand, in closed mines, slope monitoring may represent a critical component of post-closure site
management, especially when the site is located in proximity to an urban settlement.
Brief review of slope monitoring radar in mining
Slope monitoring radar based on real aperture radar technology (typically large dish antennas used to
scan the observed scenario) were originally introduced into the surface mining industry for near realtime monitoring of specific “critical” areas of the pit, providing alarms in case of fast movements
potentially leading to the failure of a portion of the slope (Noon, 2003).
Because of the constraints induced by the spatial resolution achievable using real aperture radar
(limited working range and significant time required to acquire a single radar image), its use has been
limited to small sectors of the pit in which movements were first detected by conventional monitoring
systems such as total stations or geotechnical sensors.
Since it was introduced to the mining industry in 2010 (Farina et al., 2011), GBInSAR’s longer
working range (resulting in wider spatial coverage) combined with its fast acquisition has made it
possible to revolutionize the typical application of slope monitoring radar, moving from tactical use
focused on critical areas to combined strategic and tactical use. SAR-based radar can work at very long
ranges with high spatial resolution (up to 4 km from the slope), covering wide segments of the pit from
a single position, away from working or blasting areas, thus making it possible to install the radar unit
in semi-permanent locations and reduce operational costs (Figure 1). Currently, GBInSAR systems are,
in fact, effectively used to cover large areas, detecting movements over the entire pit slope every few
minutes, and at the same time to follow fast movements in specific sectors of the pit, providing alarms
in case of fast acceleration.
SAR application to critical slope monitoring in open-pit mining
SAR radar in open-pit mines provides capability for the detection and management of potential largescale instabilities in the overall slope, multiple inter-ramp slope segments, and localized bench scale
monitoring detection at the same time.
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The principle reason for use of SAR is safety-critical monitoring, that is, alarm generation for
progressive movements based on displacement/velocity measurement.
The radar becomes a tool to be combined with other sources of information that aids risk
minimization by identifying risk conditions and supporting the decision-making process.
Figure 1: Example of a coal mine SAR installation at a 2.4 km maximum distance,
and a displacement map draped on a digital terrain model (DTM) of the mine
Additionally, long-term monitoring (months, years) of slope movements over very large portions
of the pit can also be completed, allowing the geotechnical staff to gain a better understanding of the
mechanism of large-scale instabilities and knowledge of the rock mass strength and deformation
properties via calibration of the movement (Figures 2 and 3).
This use of the radar, mainly aimed at developing effective remedial plans, is also facilitated by
the capability to integrate the geo-referenced displacement maps generated by the SAR radar with
other geological/geotechnical layers and import them into mine planning software and GIS. Basic
geomorphological analysis can be carried out by displaying displacement or velocity maps draped over
a DTM of the pit in a 3D view. In addition, detailed monitoring from both spatial and temporal points
of view is a critical source of information for the reliable calibration and validation of stability analysis
models to identify the mode of failure and the triggering mechanisms, and to assess the performances
of the implemented slope design.
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Figure 2: Geo-referenced cumulative displacement map (55 days) from an installation at
Minera Escondida, Chile at a 2.5 km maximum distance
(data courtesy of Minera Escondida Ltd.)
Figure 3: Time-series of displacements (data courtesy of Minera Escondida Ltd.)
Full pit monitoring—360 degrees
Until recently, typical RAR radar usage was one to four systems that independently operated and
displayed data from subsets of the open-pit area. By taking advantage of SAR technology, newly
developed software now extends the capability of critical safety monitoring and allows data from
multiple SAR systems to be integrated and displayed in one display environment (Figure 4), offering
360-degree full pit monitoring with the benefit of continued ability to critically monitor sub-bench to
multi-bench failures.
The early recognition of both large-scale and bench-scale instability over all the pit walls, without
the need for prior knowledge of the moving areas (as may be the case with short-range radar), allows
increased knowledge of the slope behavior.
The concept of 360-degree full pit monitoring based on the unique capabilities of SAR radar
redefines the rules of radar monitoring in mining by providing the following benefits:

24/7 safety-critical and background monitoring over the entire pit for full situation awareness
of impending and potential failures;
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optimization of conventional monitoring system installations, like prisms for total stations, or
extensometers, by detecting new activations in previously-stable areas;

optimization of production planning by developing proper mitigation strategies according to
the long-term information provided by the background monitoring (e.g., dewatering, optimized
blasting techniques, etc.).
SAR’s high spatial resolution and wide capture area also allow full pit monitoring using a
minimal number of systems. Depending on the pit geometry and mine operational requirements,
typically two to four systems can cover the full pit area.
Figure 4: Conceptual example of the full pit monitoring setup
IBIS has also been successfully installed in closed mines and quarries in different part of the
world, from iron ore mines in Minas Gerais, Brazil, to brown coal mines in Germany to limestone
quarries in Italy (Farina et al., 2012). In those cases, radar data were mainly used to map the long-term
evolution of slow-moving portions of the slope in order to identify the presence of instability issues
potentially able to interfere with infrastructures located close to the mine, such as national roads, urban
settlements, and so on. Even though closed mines in some cases do not represent the ideal site for radar
monitoring because of the presence of vegetation on the slope (a typical limitation factor for the use of
radar because of the temporal de-correlation of the signal induced by the vegetation), the results
obtained by the IBIS radar were very useful for understanding the dynamic of the slope while subject
to very slow movements.
The radar can also be used to monitor deformation of tailings and waste dumps, as demonstrated
by the installation done in a brown coal mine in Germany (Figure 5) and in copper mines in Chile.
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Figure 5: Installation of an IBIS unit to monitor the waste dumps of a brown coal mine
(data courtesy of DMT)
Conclusion
The use of slope monitoring radar represents standard practice for the near real-time monitoring of
slope displacements in operating and closed open-pit mines. The development of slope monitoring
radars based on the SAR technique recently marked a step forward in improving radar technology for
monitoring capability. By covering all the scales of slope potential instabilities, from bench scale in
open-pit mines to overall slope instability, SAR can be used effectively for both safety-critical and
background long-term monitoring (Farina et al., 2012).
The advances of SAR systems over the previous generation of RAR units are related to
improvement of spatial resolution, working distance from the slope, acquisition time, atmospheric
correction, fewer moving parts, and lower power consumption. Improvement of these features enables
users to better cover all the typical scales of slope instabilities, from bench scale to overall slope
failures, and to extend the range of monitored deformation rates to include slow movements (Farina et
al., 2011). Further software development leverages the SAR technology capability, which has now
allowed full pit monitoring with integrated systems, providing 360-degree critical slope monitoring
coverage of pit walls. As a result of its around-the-clock safety-critical monitoring and capability of
handling long datasets (months) for background monitoring and geotechnical back-analysis over the
entire pit, the 360-degree coverage is going to redefine the standards of radar monitoring practice in
modern open-pit mines both operating and closed.
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References
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Bibliography
Atzeni, C., Barla, M., Pieraccini, M., and Antolini, F. (2014). Early warning monitoring of natural and engineered slopes with
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