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MONITORING LONGWALL MINE SUBSIDENCE AND FAR-FIELD DISPLACEMENTS USING MULTI-WAVELENGTH RADAR INTERFEROMETRY Hsing-Chung Chang, Alex Hay-man Ng, Kui Zhang, Yusen Dong, Zhe Hu, Linlin Ge and Chris Rizos Cooperative Research Centre for Spatial Information & School of Surveying and Spatial Information Systems University of New South Wales, Sydney, NSW 2052, Australia [email protected], [email protected], [email protected], [email protected], [email protected], [email protected], [email protected]

ABSTRACT This study uses the spaceborne radar interferometry technique to monitor ground subsidence and far-field displacement due to underground longwall mining in the Southern Coalfields of New South Wales, Australia. The radar images acquired by the European ENVISAT/ASAR and the Japanese ALOS/PALSAR sensors were used to compute the terrain deformation. ASAR and PALSAR sensors use microwave signals in the C- and L-band, respectively. The shorter wavelength of C-band (5.6cm) signal is more sensitive to small scale displacement in the far-field; while the longer wavelength of L-band (23.6cm) signal is suitable for measuring large deformations near the centre of the subsidence bowl. In addition, the radar images acquired in both ascending and descending orbits have to be used to measure the vertical and horizontal displacements. During the radar interferometry processing, orbit errors were corrected for by refining the baseline between the locations of the antenna in space during the radar image acquisitions. The atmospheric effects can be accounted for using a low-pass filter. In addition, corner reflectors on the ground are used to calibrate the radar image. The resulting deformation maps were analysed with the aid of a geographic information system (GIS). Possible horizontal movements due to underground mining were identified outside the normal angle of draw. INTRODUCTION Land subsidence due to underground coal mining activities has the potential to impact on surface or near surface infrastructure. Longwall mining was introduced into Australia from Europe in the early 1960s. The majority of underground coal mining in Australia uses the longwall technique because of its higher productivity, typically from 75% to over 90% of coal recovery. It is the major method of underground coal mining at a depth of 400-480m in the Southern Coalfields of New South Wales. Longwall mining uses a machine shearer moving back and forth along a coal face about 200~400m wide and 4km long. The panels of coal are mined, separated by narrow pillars that act as

H.C. Chang, A. Ng, K. Zhang, Y. Dong, Z. Hu, L. Ge and C. Rizos

supports. After excavation the roof behind the working face is allowed to collapse under the weight of the overlying strata. As a result, the effects of mine subsidence can be discerned at the surface. As the mine face advances, the surface subsidence moves along the direction of operation. At the same time the amplitude of the subsidence increases continuously until it reaches the maximum possible extent which is less than the thickness of the seam being mined. Movement of the ground surface due to underground coal mining activities includes vertical subsidence, horizontal displacement, horizontal strains, curvature and tilt. Vertical subsidence is dominant around the centre of the subsidence basin, while the horizontal displacement is more significant at a distance further away from the centre. Generally, mine subsidence is less than the thickness of the coal seam extracted underground. The angle of draw (Peng 1986) is the angle between the vertical line at the panel edge projected on the surface and the line connecting the panel edge and the edge of the movement basin. The angle of draw depends upon the strength of the strata and the depth of cover to the coal seam, and typically ranges between 10 and 35 degrees from the vertical. In the Coalfields of NSW, if local data is not available, the cut-offpoint is taken as a point on the surface defined by an angle of draw of 26.5 degrees from the edge of the extraction (Pells et al. 2006), i.e. a point on the surface at a distance of half the depth of cover from the goaf edge or pillar. When longwall mining is close to rivers, gorges, dams and streams, an additional protection zone, as a self-regulation measure, is given by using a 35 degree angle of draw from the edge of the structure (Total Environment Centre 2007). It was reported that in the Southern Coalfield of NSW horizontal movements of up to 25mm have occurred, even up to 1.5km from the mining face (Hebblewhite et al. 2000; Pells 2008). These horizontal movements are referred to as “far-field effects”, which is due to the reorientation of the very high horizontal stresses in the underlying layers of rocks (Pells et al. 2006). An example of a lateral movement of about 40mm was observed about 1.5km away from the corresponding longwall panel at a depth of 480m (Pells 2008). That is equivalent to an angle of draw of greater than 72 degree, or 3 times the mining depth. Far-field movement is more difficult to monitor as the extent of movement is less predictable. When mining is close to important civil infrastructure, such as highways, gas pipelines, and optic fibre cables, the tension force caused by the far-field effect may cause damage and even put safety under threat. This paper aims to demonstrate the capability of radar interferometry for monitoring the mine subsidence of a test site in the Southern Coalfields of NSW. Radar interferometry with the aid of geographic information systems (GIS) and GPS can be used as a cost effective monitoring solution, and is in many ways complementary to the conventional ground surveys for monitoring mine subsidence and the possible far-field effects.

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METHODOLOGY Differential Radar Interferometry Repeat-pass spaceborne differential synthetic aperture radar (DInSAR) is utilised in this study to derive the maps of ground displacement due to underground coal mining. The basic theory of radar interferometry can be found in literature such as, e.g., Zebker and Goldstein 1986; Gabriel et al. 1989; Bamler and Hartl 1998; Rosen et al. 2000. In short, two SAR images acquired from two slightly different positions and at different revisit times are compared so that the phase difference, or the so-called interferogram, can be measured. The interferogram contains topographic information, land deformation that has occurred between the two acquisitions, atmospheric disturbances, orbit errors and noise. DInSAR is the process of measuring the phase variation due to land deformation by eliminating or minimising the other phase components. The topographic phase contribution can be simulated by introducing a digital elevation model (DEM). A 25m resolution DEM generated using a stereo-image pair is used in this paper to simulate the topographic phase, so that the topographic phase can be removed from the interferogram. The atmospheric component is primarily due to fluctuations of water vapour in the atmosphere in the path of the radar beam between the satellite and the ground. As the water vapour in the atmosphere varies with low spatial frequency, it is sometimes negligible in applications such as mine subsidence monitoring where the spatial frequency is much higher. The atmospheric delay can be identified using the fact that its fringe structure is independent over several interferograms, or can be modelled by using a local GPS network (Ge et al. 2003). Alternatively, the atmospheric signature can be isolated by applying a low-pass filter to the interferogram, and then eliminated. There is, however, a trade-off or risk that some deformation signals at low spatial frequencies are also filtered out. In a differential interferogram a complete 2π phase change is equivalent to a height displacement of half of the wavelength of the radar signal in the slant range direction, or so-called line of sight (LOS), i.e. in the look direction of the radar. The wavelengths of ENVISAT ASAR (C-band) and ALOS PALSAR (L-band) are 5.6cm and 23.5cm, respectively. As the measured phases in the interferogram are wrapped in modulo of 2π, the height displacement map can be derived by ‘phase unwrapping’ the interferogram. Finally the unwrapped phase can be converted to deformation, ΔR, along the LOS of the radar. If the deformation, D, projected onto the plane of the radar range direction is the composite of both vertical, Dv, and horizontal, DH, displacements in the range direction of the radar, the relationship between D and ΔR is:

D=

ΔR cos(θ − ρ )

Equation (1)

where DH = tan(ρ) x DV; ρ is the angle between D and DV;

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θ is the incident angle of the radar. For pure horizontal movement, i.e. DV = 0: DH = D = For pure vertical movement, i.e. DH = 0: DV = D =

ΔR sin(θ )

Equation (2)

ΔR cos(θ )

Equation (3)

Horizontal Displacement Mapping Due to the lack of variety in current satellite SAR imaging/viewing geometry, DInSAR is more sensitive to vertical surface deformation than in the horizontal direction. DInSAR is especially insensitive to displacement in the north-south direction. By using SAR images acquired in both ascending and descending satellite orbits, the deformation vectors in the vertical and easterly horizontal directions can be estimated by leastsquares fitting the solution for equation (4) with an assumption of negligible displacement along the north-south direction. That is:

[cos(θ ) − sin(θ ) cos(α )] ⎡⎢

DU ⎤ ⎥ = [ΔR ] D E ⎣ ⎦

Equation (4)

where θ is the incident angle of the radar signal; α is the heading angle of the satellite; DU is the vertical displacement; DE is the horizontal displacement along the east-west direction; ΔR is the deformation vector along the slant range direction. Data Calibration The DInSAR results were calibrated and geo-referenced using ground corner reflectors and a high resolution orthorectified aerial photograph with a pixel resolution of 50cm. The ground corner reflectors are simply trihedral metallic devices with a length of 1m at each side. There are six corner reflectors deployed in the test site. The backscattered signals from the corner reflectors are strong, and appear as bright pixels in the radar intensity image. Therefore, the locations of the corner reflectors are easy to identify when the return signal intensity of the surrounding area is relatively low. The coordinates of these corner reflectors were measured using GPS with a horizontal accuracy of a few centimetres. These corner reflectors are used to calibrate the SAR image for geo-referencing. The overall quality of geo-referencing of the radar image is confirmed by comparing the results to the aerial photograph. An example of one of the corner reflectors set up in the test site is shown in Fig. 1.

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(a)

(b)

Fig. 1. Example of a corner reflector established in the test site for calibrating the coordinates of the image. (a) location of the corner reflector in a high resolution (50cm) aerial photograph, and (b) strong backscattered signal from the corner reflector in the radar image.

CASE STUDY – SOUTHERN COALFIELD IN NSW, AUSTRALIA Input Data The spaceborne SAR images acquired by ENVISAT ASAR (C-band) and ALOS PALSAR (L-band) were used in this study. On the basis of the availability of radar images and the constrains of suitable perpendicular and temporal baselines of the image pairs, the selected interferometric pairs are listed in Table 1. The perpendicular and temporal baselines of the pairs are denoted by Bperp and Btemp, respectively. IP is the acronym for ‘interferometric pair’. Table 1. Summary of the interferometric pairs tested in this study. IP

Sensor

Date 1

Date 2

1

ENVISAT ASAR ALOS PALSAR ALOS PALSAR ALOS PALSAR

8/7/2007

2 3 4

Bperp

Btemp

12/8/2007

Incident Orbit Angle Orientation 0 29 Ascending

28m

14/8/2007

29/9/2007

390

Ascending

-506m

5/4/2008

21/5/2008

470

Ascending

-457m

16/4/2008

1/6/2008

470

Descending 171m

35 days 46 days 46 days 46 days

Far-Field Effect: Tahmoor Colliery The geo-referenced DInSAR interferograms derived using IP1 and IP2 were postanalysed in a GIS and overlaid on the underground mine plan. The results shown in Fig.

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2 indicate clear phase changes extending outside the planned longwall panel at the western end. The edge of the most outer phase fringes, which are the concentric circular colour rings, is about 550-670m westwards from the pillar at the side of the longwall. The depth of cover of the Bulli Seam is about 420-470m. Therefore, the angle of draw given by the results was as large as 49 to 58 degrees. That is greater than the typical angle of draw, 27 to 35 degrees, as mentioned earlier in this paper. After phase unwrapping the DInSAR interferogram, the displacement of the extended subsidence basin is 1-5mm over the extended region outside the main subsidence basin. As its location is further away from the centre of the subsidence, the horizontal displacement should be more dominant than the vertical displacement. If we assume the displacement is purely horizontal, using Equation (2) with the incident angle of 290 for ENVISAT data and 390 for ALOS, this is equivalent to a horizontal movement of 16mm towards to the centre of the subsidence basin. Vertical and Horizontal Displacements Estimation: Metropolitan Colliery To resolve the three dimensional land displacement vectors, DInSAR results derived from both ascending and descending satellite path SAR images over the same time span need to be combined. If the deformation is purely vertical, the vertical displacements derived from both orbits should be the same, although the displacements measured along the line-of-sight (LOS) may vary depending on the look angle of radar. Otherwise, the horizontal displacement has a signature. The three dimensional deformation vectors can be calculated from the DInSAR solutions derived from three independent LOS results. This was demonstrated by Sircar et al. (2004) using ascending and descending Radarsat-1 SAR image data. The deformation vector along the LOS signal (DLOS) is a composed of up (DU), easting (DE) and northing (DN) deformation components. The deformation measured in the differential interferogram is the sum of the vertical and horizontal deformation components projected onto the LOS.

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(a)

(b) Fig. 2. Geo-referenced DInSAR interferograms generated using (a) IP1: ENVISAT 8/7/2007 - 12/8/2007, Bperp = 28m, Btemp = 35 days; and (b) IP2: ALOS 14/8/2007 29/9/2007, Bperp = -506m, Btemp = 46 days.

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As mentioned earlier, due to the lack of diverse satellite observation directions, the current ALOS or ENVISAT viewing geometries are only sensitive to vertical movement and horizontal displacement in the easting direction. At least three DLOS observations are needed in order to solve the three unknowns, DU, DE and DN. In this study, the ALOS interferometric pairs IP3 (ascending orbit) and IP4 (descending orbit) were used to estimate the DU and DE by assuming that DN is negligible. The colour-coded DLOS results of these two selected pairs overlaid on an aerial photograph are shown in Fig. 3. The derived DU and DE are shown in Fig. 4.

(a)

(b) Fig. 3. DInSAR land deformation maps along line-of-sight of radar for Metropolitan colliery: (a) ALOS 5/4/2008 - 21/5/2008 and (b) ALOS 16/4/2008 - 1/6/2008.

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(a)

(b)

Fig. 4. Vertical (a) and Easting horizontal (b) displacement maps of the underground mining induced deformation for Metropolitan colliery. The unit of the legends is metres. By assuming zero northing displacement, Fig. 4 indicated the maximum vertical subsidence was over 0.25m and the easting displacement was about 0.1m towards the centre of the subsidence basin. The 3-D deformation analysis can be further improved by having more observations from various directions, and over the same or very similar time span. CONCLUDING REMARKS Civil infrastructures and the natural environment may be damaged by land deformation caused by underground mining activities. Besides vertical subsidence and horizontal movement within the subsidence basin, there are some unexpected but significant horizontal displacements far away from the active longwall in the Southern Coalfields of NSW. These lateral movements are the far-field effects. This study demonstrated the capability of radar interferometry with the aid of GIS as a cost-effective method for mine subsidence, especially for far-field effect monitoring in the NSW Southern Coalfields. There were deformation found outside the normal ‘angle of draw’ at the Tahmoor Colliery. The results suggested possible presence of far-field effect. Also, 3-D mine subsidence mapping was demonstrated using two ALSO DInSAR results derived from images acquired in ascending and descending orbits, with an assumption of negligible northing displacement. The 3-D deformation analysis can be further improved by having more observations from various directions, and over the same or very similar time span. ACKNOWLEDGEMENT This research work has been supported by the Cooperative Research Centre for Spatial Information through Project 4.09, whose activities are funded by the Australian Commonwealth’s Cooperative Research Centres Programme. The NSW Department of Lands has been supporting this research with spatial information data and resources.

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The Australian Coal Association Research Program has also supported research for ground subsidence monitoring using DInSAR. The authors wish to thank the European Space Agency and the Earth Remote Sensing Data Analysis Center (ERSDAC) for providing the ENVISAT ASAR and ALOS PALSAR data, respectively. METI and JAXA retain ownership of the ALOS PALSAR original data. The PALSAR L-1.1 products were produced and distributed to the IAG Consortium for Mining Subsidence Monitoring. The authors also wish to thank BHPBilliton for providing the ground survey data and other relevant spatial data for mining. REFERENCES Bamler, R. and P. Hartl (1998). "Synthetic aperture radar interferometry." Inverse Problems(4): R1-R54. Gabriel, A.K., R.M. Goldstein, et al. (1989). "Mapping small elevation changes over large areas: differential radar interferometry." Journal of Geophysical Research, 94(B7): 9183-9191. Ge, L., H. C. Chang, et al. (2003). The integration of GPS, radar interferometry and GIS for ground deformation monitoring. 2003 Int. Symp. on GPS/GNSS, Tokyo, Japan, 15-18 November, 465-472. Hebblewhite, B., A. Waddington, et al. (2000). "Regional horizontal surface displacements due to mining beneath severe surface topography", in Proc. 19th Int. Conf. on Ground Control in Mining, Morgantown, WVA, USA, 149-157. Pells, P.J.N. (2008). "What happened to the mechanics in rock mechanics and the geology in engineering geology." Journal of the Southern African Institute of Mining and Metallurgy, 108: 309 - 323. Pells, P.J.N., T.D. Sullivan, et al. (2006). "Goulburn River Stone Cottages - Moolarben Coal Project." Report PSM 1067.TR1: 30 Oct 2006. Peng, S.S. (1986). Coal mine ground control. New York, Wiley. Rosen, P.A., S. Hensley, et al. (2000). "Synthetic aperture radar interferometry." Proceedings of the IEEE, 88(3): 333-382. Sircar, S., D. Power, et al. (2004). "Lateral and subsidence movement estimation using InSAR". IGARSS '04, Anchorage, Alaska, 20-24 September, 5, 2991-2994. Total Environment Centre (2007). "Inquiry into NSW Southern Coalfield." 30 July 2007. Zebker, H.A. and R.M. Goldstein (1986). "Topographic mapping from interferometric synthetic aperture radar observations." J. Geophys. Res., 91(B5): 4993-4999. BRIEF BIOGRAPHY OF PRESENTER - Linlin GE Linlin is currently an Associate Professor of remote sensing and GPS in the School of Surveying and Spatial Information Systems at the University of New South Wales (UNSW) and a Project Leader and Senior Research Fellow with the Cooperative Research Centre for Spatial Information. He is also a Visiting Professor of the NSW Department of Lands. He received his BEng (1st Hons) in Optical Engineering from the Wuhan Technical University of Surveying and Mapping (1985), MSc in Crustal Deformation from the Institute of Seismology (1988), and PhD in GPS and remote sensing from the UNSW

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(2001). In 1997-1998, he was a postdoctoral research fellow in the Meteorological Research Institute of the Japan Meteorological Agency sponsored by the Science and Technology Agency of Japan. Linlin is a Representative on the National Committee of the Remote Sensing and Photogrammetry Commission and foundation member of the Spatial Sciences Institute. He is a Member of the IEEE (the Institute of Electrical and Electronics Engineers, Inc.) and a Life Member of the AGU (the American Geophysical Union). He is also the Cochair of Sub-Commission 4.4 “Applications of Satellite and Airborne Imaging Systems” of the International Association of Geodesy (IAG) and a Principal Investigator for satellite missions such as Envisat, ALOS and Radarsat-2.

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