MINING LAND SUBSIDENCE MONITORING USING SENTINEL-1 SAR DATA

In this paper, DInSAR technique was used to monitor land subsidence in mining area. The study area was selected in the coal mine area located in Yuanbaoshan District, Chifeng City, and Sentinel-1 data were used to carry out DInSAR techniqu. We analyzed the interferometric results by Sentinel-1 data from December 2015 to May 2016. Through the comparison of the results of DInSAR technique and the location of the mine on the optical images, it is shown that DInSAR technique can be used to effectively monitor the land subsidence caused by underground mining, and it is an effective tool for law enforcement of over-mining .


INTRODUCTION
Surface subsidence is a very common phenomena around mining area, and it has brought great difficulties to the economic development and environmental protection of the city.In China, the problems of land subsidence are very prominent, which directly endanger the ground construction facilities and the natural environment and affect the human living environment, local economic development, and even human life and property security.Therefore, it is important to monitor the surface subsidence of the mining area which can not only provide decision reference data for the environmental protection department and production management department, but also the disaster assessment and prevention (Perski 1998, Perski 2000).The monitoring of ground subsidence caused by underground mining relies mainly on traditional earth leveling, static GPS measurements or dynamic GPS measurements.However, these methods based on point measurements are difficult to obtain subsidence trends throughout the region and also require extremely high human and financial resources (Hebblewhite et al., 2000).
In recent years, space-to-ground remote sensing technology, especially satellite radar measurement technology has been rapidly developed.A large number of studies and application examples show that Synthetic Aperture Radar Differential Interferometry (DInSAR) can be applied to long-term slow surface deformation monitoring (Rodriguez et al., 1992, Carnec et al., 1996).DInSAR technique uses phase information of SAR images to extract the surface deformation, and its precision can reach the millimeter level, which also has unprecedented continuous space coverage, and highly automated capacity.It provides a new approach for the deformation of the surface automation monitoring (Chang et al., 2005, Colesanti et al., 2005, Liu et al., 2009).Relative to the traditional GPS and leveling measurements, the advantages of monitoring mining land subsidence using DInSAR technology are mainly in the following three points: (1) Compared with GPS, DInSAR technology has the advantages of high sampling density, good spatial continuity and no need to establish ground receiving station.
(2) Compared with other monitoring displacement instruments, DInSAR technology is able to avoid the high cost of measuring instruments and lots of ground control points.It makes the monitoring of large area surface deformation easier.
(3) DInSAR, which is very sensitive to ground deformation, with high-precision ground deformation measurement capability and high spatial resolution detection capability, can be used to accurately determine the ground subsidence.At the same time, the method has unparalleled advantages compared to GPS, that makes it a useful complement to traditional monitoring methods.Moreover, it has a certain degree of continuity, which doesn't have to establish ground monitoring network.
Besides, DInSAR can acquire ground elevation and deformation information all-weather and all-day.In particularly, its measurement accuracy of up to millimeter-level potential and continuous space coverage, make it an useful method to monitoring long-term surface deformation.DInSAR has become a very potential space-to-ground observation technology.Therefore, the study of the application of DInSAR technique and its application in mining land subsidence monitoring has important application value and broad application prospect.

THE SENSITIVITY OF INTERFEROMETRIC PHASE MEASUREMENT AND THE FEASIBILITY OF DEFORMATION DETECTION
The change of interferometric phase depends on the topographic relief and surface deformation (surface motion).Using this principle we can extract ground elevation and detect surface deformation with differential SAR interferometry technology.
Often people are confused of the different accuracies between the extraction of elevation using InSAR and surface deformation using DInSAR.Here, we explore the sensitivity of phase measurement relative to the elevation change and deformation and the relationship with several key parameters.
After taking the derivation of equation of deformation   , we can get the sensitivity of the phase to the terrain change: Where the parameters are shown in the Fig. 1.B is spatial baseline,  is phase difference of 1  and 3  ,  is looking angle.2) is much smaller than the value on the right side of equation (3).Therefore, the accuracy of DEM generated through InSAR technology can only reach a few meters in general.However, when used for detecting surface change, the phase to deformation accuracy can reach the level of centimeter or even millimeter.According to the different sensitivities of phase to terrain and deformation, it clearly illustrates that DInSAR has the ability to detect small surface deformation and movement.

STUDY AREA AND DATA
Yuanbaoshan District located in the eastern Inner Mongolia, with the area 952.14 km 2 , geographical coordinates of longitude 119 °119.5 °, latitude 42 °42.5 °.The elevation of Yuanbaoshan District varies from 500 to 700m.In Yuanbaoshan District geologists have found 14 coal, gold, copper, or iron ore, in which there are 5 medium-sized deposits, 1 small deposits, 5 mining points, 3 mineralization points.Oil and natural gas have considerable reserves.Coal reserves in this area is about 2 billion tons throughout 8 townships, with an annual output of nearly 10 million tons of coal.Currently there are cracks and faults appears on the ground, buildings and railway inYuanbaoshan District.Local government has devoted to the monitoring and recovering of such kind of subsidence, and preliminary results have been achieved.

Sentinel-1 data
The Sentinel-1 system is a radar satellites constellation consisting of two C-band synthetic aperture radar sensors, Sentinel-1A and Sentinel-1B, with different resolution (down to 5 m) and coverage (up to 400 km  2. Sentinel-1 data for processing

External DEM data
The external DEM is SRTM (Shuttle Radar Topography Mission, 30m x 30m ground resolution) downloaded from the seamless data distribution system of the Earth Resources Observation and Science of USGS whose website address is http://srtm.csi.cgiar.org/SELECTION/inputCoord.asp (USGS Earth Resource Observation and Science (EROS) Center, 2016).The data cover the whole processing area in Yuanbaoshan District.

RESULTS AND ANALYSIS
Standard DInSAR technique is applied to every interferometric pairs listed in Tab. 2. Fig. 3 With the development of mine excavation activities, mining subsidence would cause a great impact on the surrounding environment.We can see that the land subsidence of the mining area has spread to the residential area, the road and the railway nearby.In Fig. 5 Red border represents the extent of the land subsidence exceeding 5mm which would affect the safety of buildings, highways and may cause major safety accidents.From the land subsidence on the residential area, the road and the railway around the mine, it can be speculated that underground cross-border excavation occurred in the area.Law enforcement agencies can easily judge whether mining company overlift or not through comparing the ground subsidence results with the scope of mineral rights.

CONCLUSIONS
The mine excavation activities could lead to land subsidence.DInSAR technique can monitor the land subsidence using the phase information of SAR images.By choosing interferometric pairs with small spatial baseline and time baseline in the same season, the coherence can preserve better.The application of DInSAR technique is of great importance for monitoring mine underground excavation.The results of DInSAR technique can be a basis for law enforcement officers when they survey that mining company whether has an underground cross-border excavation or not.

Fig. 1
Fig.1 The basic principle of DInSAR