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Satellite-based monitoring of an open-pit mining site using
Sentinel-1 advanced radar interferometry: a case study of theDecember 21, 2020, landslide in Toledo City, Philippines
Ryan Ramirez1,2,4, Rajiv Eldon Abdullah2, Woojae Jang1, Shin-Kyu Choi3, and Tae-Hyuk Kwon1*
1Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea
2University of Santo Tomas (UST), Manila 1015, Republic of the Philippines
3Korea Electric Power Research Institute (KEPRI), Daejeon 34056, Republic of Korea
4National Research Council of the Philippines (NRCP), Taguig 1631, Republic of the Philippines

Abstract. Understanding the causal factors and mechanisms behind catastrophic landslides and debris
flows is crucial for accurate forecasting and disaster risk reduction. Synthetic aperture radar (SAR) data andinterferometric SAR (InSAR) technologies provide valuable information for early warning systems’
landslide and debris flow detection and monitoring strategies. This paper applied the Sentinel-1 Persistent
Scatterer InSAR (PSInSAR) technique to detect and monitor precursory slope movements over the open-pit
mining site in Toledo City, Cebu Island, in the Philippines. The results show that the slope showed
instabilities between December 30, 2019, to December 12, 2020, before the actual failure on December 21,
2020. The landslide initiation zone moved with velocities exceeding –10 mm/yr and reaching –90 mm/yr.
The topography and morphology of the mining site contributed to the cut slope instabilities. The mining
operations and hydrometeorological conditions during the analysis period aggravated the situation, leading
to the cut slope failure. Overall, with favorable slope geometry, surface characteristics, and SAR data
availability, the Sentinel-1 PSInSAR technique can serve as a landslide early warning system tool and aid
decision-making in an actively operating open-pit mine and other landscapes.

1 Introduction
Landslides and debris flows have become more frequentglobally due to climate change. The topography,
morphology, and climatic conditions contribute tofrequent landslides and debris flows, resulting inmonetary losses or even loss of life. Understanding thetriggers and mechanisms behind these catastrophic
events is crucial for accurate forecasting, risk
mitigation, and management plans. With the higher
frequency of extreme weather events combined withanthropic factors, landslide and debris flow monitoringand early warning systems are a possible cost-effective
approach for disaster risk reduction [1–3].
Freely available synthetic aperture radar (SAR) data
provide valuable information to complement groundbased instruments to understand the occurrence oflandslides and debris flows. Advanced differential
interferometric SAR (A-DInSAR) technology has been
extensively applied in regional scales to improvelandslide and debris flow susceptibility and hazard
detection, mapping, and monitoring. Precisely, ADInSAR technology has been used to update landslideand debris flow inventories and state activity [4–6] andestimate landslide and debris flow magnitude (i.e., areaand volume of the source area) [7–9]. A-DInSAR
technology has been exploited successfully to detect andmonitor slope movements in open-pit mines [10–14]. ADInSAR technology offers a high density of monitoring
points over vast, dangerous, and inaccessible areas
compared to conventional surveying techniques.
This paper presents how satellite remote sensing
data, specifically the free-to-use Sentinel-1 SAR data,
and the A-DInSAR technology, can detect and monitor
slope movements to improve records and provide
further information about the landslide and debris flowcauses and triggers. This information can predictlandslides and debris flows and warn when people andproperties are at risk from landslides, potentially
undergoing rapid accelerations and evolving into
catastrophic debris flows. The case study presented inthis paper focuses on applying the Sentinel-1 Persistent
Scatterer InSAR (PSInSAR) [15] technique to monitor
precursory slope movement over an open-pit mining site
in Cebu Island in the Philippines.

2 Materials and method
2.1 Study area
The Toledo Copper Mine in Toledo City in Cebu Islandin the Philippines is between the North Barot andCantabaco faults. The site is sustained by a basement
complex of Cretaceous volcanic and metavolcanic rocks* Corresponding author: t.kwon@kaist.ac.kr© The Authors, published by EDP Sciences. This is an open access article distributed under the terms of the Creative Commons Attribution License 4.0(https://creativecommons.org/licenses/by/4.0/).

E3S Web of Conferences 415, 05020 (2023)
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cloaked with Oligocene to the recent sedimentary unit
[16–17]. Disseminated deposits, predominantly
chalcopyrite, bornite, and covellite, mainly compose theprimary copper mineralization of the porphyritic
intrusive rocks characteristics of the Cretaceous
Lutopan Stock.
The site has three open pits: Carmen, Lutopan, andBiga. On December 21, 2020, a landslide occurred at thenorth wall of the Carmen pit. It resulted in eight injured
personnel, four confirmed deceased personnel, six
missing personnel, and eight missing equipment. The
landslide footprint is plotted against the landslide hazard
map (Figure 1), showing that the pit is an active
landslide zone. On December 18, 2020, Toledo City wasaffected by the rain brought by Tropical Storm Krovanh.

Fig. 2. Sentinel-1 synthetic aperture radar (SAR) dataset andinterferometric pairs.

2.3 PSInSAR analysis
Two main processing stages were implemented for
Sentinel-1 PSInSAR analysis.
2.3.1 Pre-processing using SNAPThe first stage was accomplished using the SeNtinel
Application Platform (SNAP) for the master image
selection (i.e., InSAR Stack Overview) and thesnap2stamps package tool for the single master-slave
images coregistration, interferogram formation, andtopographic phase removal utilizing the Shuttle Radar
Topography Mission digital elevation model (SRTMDEM) 1 arcsec data in the SNAP Library [18].
2.3.2 Time series analysis using StaMPS
The second stage was accomplished using the Stanford
Method for Persistent Scatterers (StaMPS) software forthe time series InSAR analysis [19]. An amplitude
dispersion index (ADI) of 0.35 was used to recognize
introductory PS candidates (PSCs), launching theStaMPS processing. Afterward, the PSCs phase noiselevel was estimated iteratively. Based on the noise level,PSs were selected, and the strident ones were dropped.
The remaining PSs in the wrapped phase were thenamended for spatially uncorrelated look angle error.
Afterward,
theStatistical-cost,Network-flow
Algorithm for Phase Unwrapping (SNAPHU) was usedfor phase unwrapping [20]. Then, the spatiallycorrelated look angle error (i.e., the error in the DEM
itself and the imprecise aligning of the DEM into radarcoordinates) was approximated and removed from theunwrapped dataset. Lastly, atmospheric effects weremitigated using spatiotemporal filters.

Fig. 1. Location of the study area, landslide footprint, andhazard map over the open-pit mining site in Toledo City, Cebu
Island, Philippines.

2.2 Sentinel-1 SAR dataset
The twin Sentinel-1 satellites provide SAR data in allweather and day-and-night conditions for land andocean services under the Copernicus joint initiative of
the European Space Agency (ESA) and the EuropeanCommission (EC). Sentinel-1 satellites are equippedwith C-band having a wavelength of 5.6 cm. In thisstudy, 31 SAR images were collected from December
30, 2019, to December 12, 2020. Due to the Sentinel-1
satellites’ orbit being sun-synchronous and right-sidelooking viewing geometry, the line-of-sight (LOS)
displacement measurements are less sensitive in thenorth-south direction than in the east-west direction. As
the pit slope is south-south-westerly facing, the dataset
acquired by the descending Sentinel-1 mission wasused. The data acquired on June 15, 2020, was set as themaster image (Figure 2) to mitigate the effect of
temporal decorrelation.

3 Results and analysis
3.1 Pre-failure mean annual velocity map
Figure 3 displays the pre-failure LOS mean annual
velocity map of the Toledo landslide retrieved from theSentinel-1 dataset. PSs were detected over the initiation
zone due to the absence of vegetation cover and2E3S Web of Conferences 415, 05020 (2023)
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probably lesser mining operations during the analysis
period. In contrast, there was a lack of PSs at lower
elevations where most mining activities wereundertaken. Red variant dots represent ground targets
moving away from the satellite sensor. Blue variant dots
indicate motion toward the satellite sensor. Most PSs
show movement away from the sensor when the satellite
descends north to south, and the pit slope is south-southwesterly facing. PSs within the initiation zone showed
displacement velocities exceeding –10 mm/yr andreaching –90 mm/yr.

rainfall and the landslide event, which can be attributed
to infiltration and percolation time before groundsaturation, thereby plausibly triggering the mass
movement. At the end of the slope monitoring period,
the cumulative LOS displacements of PSs D1, D2, D3,and D4 were –26.16 mm, –50.70 mm, –69.80 mm, and–90.05 mm, respectively. Although this may be theleading cause of the landslide, other contributing factors
must be further explored and accounted for.

Fig. 3. Line-of-sight (LOS) annual mean velocity map over theToledo Copper Mine site.

3.2 Pre-failure displacement time series
Figure 4a shows the cumulative LOS displacement timeseries of four representative PSs within the initiation
zone. These four PSs are marked as D1–D4 in Fig. 3.
The daily and accumulated rainfall values are alsoplotted in Fig. 4b. The rainfall data were acquired fromthe Philippine Atmospheric, Geophysical andAstronomical Services Administration (PAGASA)
Mactan Station (10°19'18.15"N, 123°58'33.7"E). Figure4b also shows significant hydrometeorological eventsand hazards, particularly indicating the recorded landfallof Tropical Storm Krovanh and the day when thelandslide occurred. Although previous powerful tropical
cyclones (namely Typhoon Molave and Typhoon Goni)
and a super typhoon (namely Typhoon Vamco) may nothave made landfall on Cebu Island, the precipitationfrom these events led to the waterlogging of the groundon the open-pit site. The waterlog effect is evident in thedisplacement time series of PSs D1–D4, where a
progressive movement was initiated in October 2020.
Likewise, the significant accumulated rainfall inFebruary 2020 also coincided when the displacement
time series of PSs D1–D3 showed declining linear
trends until an earthquake hit Cebu Island in July 2020.
These trends seemed to shift after the earthquake until
October 2020, before a series of typhoons hit Cebu
Island from October 2020. Based on the accumulated
precipitation, the landslide occurred 24–48 hours after
the landfall of Tropical Storm Krovanh on the island.
This observation suggests a lag time between peakFig. 4. (a) Representative displacement time series of PSs in
the initiation zone and (b) hydrometeorological and hazard
events.

3.3 Landslide causal factors
Earthquakes, hydrometeorological, and anthropic
factors are hypothesized as potential triggers of theToledo landslide. No seismicity was reported
immediately before or during the landslide. However, aMs 4.2 tectonic earthquake occurred on July 17, 2020, in
Cebu Island, with the epicenter at 10.28°N, 123.80°E,
and an estimated focus depth of 7 km. Ground shaking
was perceived about 11 km southwest of Cebu City,
classified as seismic intensity IV according to thePhilippine Institute of Volcanology and Seismology(PHIVOLCS) Earthquake Intensity Scale (PEIS). The
epicenter of this earthquake is about 10 km from theCarmen pit. Ground ruptures were not reported in theToledo copper mine. It is noted that the Barot fault, aminor fault line, traverses the Carmen pit where thelandslide occurred, and this earthquake might havecontributed to the initiation of the fault movement,
eventually triggering the slope instability.
The landslide occurred after a day or two after thedownpour from Tropical Storm Krovanh. This rainfall
event, along with the days of similarly significant

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References

rainfall in Toledo City preceding it, might have
weakened the contact of the limestone and mudstonelayer in the site’s rock base, as there were alreadywaterlogged. The waterlog caused an excess pore waterpressure in the soil medium, which might haveovercome the shear strength of the soil.
Moreover, according to Biga village residents,
ground cracks appeared in residential areas,
approximately 300 m from the mining site, in November
2020. Blasting activities before the landslide have beenreported to cause ground vibrations leading to sustained
cracks on the floors and walls of nearby structures andhouses. Blasting operations were done twice daily andconducted about 350 m away from the Biga village.
Residents have also reported that the groundwater
spring that served as their water source has dried up.

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4 Discussion and outlook
This paper highlights the application of the Sentinel-1
PSInSAR technique for detecting and monitoring
precursory slope movement over an open-pit mining site
in the city of Toledo, Cebu Island, in the Philippines.
The topography, morphology, and hydrometeorologicalconditions combined with the anthropic activities overthe mining site contributed to the catastrophic landslideon December 21, 2020. Mining slope instabilities
indicating the active movements in the initiation zone
were successfully detected and monitored using free
SAR data and open-source software packages. Thus,
rapid and reliable information can be extracted for
decision-making, risk monitoring, and early warning formines and other landscapes.
Sentinel-1 SAR data, leveraged in this case study,offer broader area coverage, encompassing the whole
Toledo Copper Mine. Covering a broader area implies
that even artificial hills, consisting of open-pit mining
wastes accumulated on specific dump sites, can bemonitored regularly in near-real time. Usually, these
artificial hills comprise tens to hundreds of cubic metersof unconsolidated, loose earthen materials. These
artificial hills also can reach a height of a hundred
meters. With a suitable rainfall-duration threshold
during an extreme downpour or at the onset of an intenseearthquake event, collapses and debris flows areinevitable. Thus, slope stability monitoring using theSentinel-1 PSInSAR technique can include these openpit waste dump sites besides cut slopes during and after
the entire open-pit mining process.
This work was supported by the "Ministry of the Interior andSafety" R&D program (20018265) and also financially
supported by the Korea Ministry of Land, Infrastructure andTransport (MOLIT) as an "Innovative Talent Education
Program for Smart City". This work was also supported by theNational Research Council of the Philippines (NRCP) throughthe Support to Research Dissemination in Local andInternational Platforms (RDLIP) component of the Basic
Research Information Translation for Empowerment in theRegions (BRITER) Program.

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