Application of Repeat-pass DInSAR and GIS for Underground Mine Subsidence Monitoring Hsing-Chung Chang, Linlin Ge and Chris Rizos School of Surveying and Spatial Information Systems University of New South Wales Sydney, Australia
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[email protected] Abstract—This research used both ERS and JERS-1 images to investigate the feasibility of differential radar interferometry (DInSAR) for mine subsidence monitoring in the state of New South Wales, Australia. DInSAR results are analysed and validated with the aid of Geographic Information System (GIS) tools. The drawbacks of using interferometric measurements for mine subsidence monitoring are discussed. The repeat-pass tandem and JERS-1 DInSAR results are presented. Keywords-interferometry; DInSAR; GIS; mine subsidence; ERS-1/2; JERS-1; Envisat
I.
INTRODUCTION
Interferometric synthetic aperture radar (InSAR) systems exploit the phase differences between two SAR images acquired over the same area. Useful topographic information, such as digital elevation model (DEM), can be derived. Differential InSAR (DInSAR) has been further used to measure the deformation of the ground terrain. The feasibility and capability of DInSAR for underground mine subsidence monitoring have been tested in the UK [1], France [2], Germany [3]. In these studies, the images acquired by the two ERS satellites are the only data source. Due to the size of the ERS radar wavelength (5.6cm), the signal is sensitive to changes in the vegetation cover of the ground surface, and therefore the phase noise level is normally higher in rural areas than in urban areas. Furthermore, the short ERS radar wavelength limits the capability of detecting high surface deformation rates over a small spatial extent. Sequentially, for higher deformation rates the phase gradients become too large to be correctly ‘unwrapped’. Nevertheless, the coverage of subsidence can still be indicated by using ERS images. This research used both C- and L-band radar images acquired by the ERS and JERS-1 satellites to investigate the use of radar interferometry for mining-induced subsidence monitoring in New South Wales, Australia. Repeat-pass DInSAR is used to identify and measure the location and amplitude of the ground subsidence. With the aid of Geographic Information System (GIS) tools, the DInSAR results are analysed and validated against other spatial information, such as aerial photographs, mine plans and ‘ground truth’ acquired by various means. The recent SAR
images acquired by ENVISAT over the same test site are also discussed in this paper. II.
REPEAT-PASS DINSAR
Repeat-pass spaceborne DInSAR has been used to derive ground displacement maps. Two SAR images acquired from two slightly different positions, at different revisit times, are used to measure the phase difference, or so-called interferogram, between the two acquisitions. This paper utilised the repeat two-pass DInSAR approach, with an external DEM, to measure the displacement of the surface. As shown in (1), the phase change in the interferogram is the composite of topographic information, φ topo, surface displacement between the two acquisitions, φ disp, atmospheric delay, φ delay, and noise, φ noise. DInSAR requires the removal of the topographic phase contribution, and so isolating the ground displacement component. The φ topo can be simulated and eliminated by introducing DEM information. φ = φ topo + φ disp+ φ delay + φ noise.
(1)
The atmospheric component, φ delay , is primarily due to fluctuations of water content in the atmosphere between the satellite and the ground. The atmospheric delay can be identified using the fact that its fringe structure is independent over several interferograms [4], or can be modelled by using data from a GPS network [5]. It is also possible to reduce the atmospheric disturbance to the displacement phase term by using the ‘interferogram stacking technique’ [6]. The underground mines monitored in this study employ the long-wall mining technique. As a result, the mine subsidence is expected to have a spatial extension of a several hundreds of meters and a vertical displacement of a few tens of centimeters over a few satellite repeat-cycles. The subsidence, therefore, has higher spatial frequencies than the lower frequencies caused by atmospheric delay, and its phase signal would dominate over the error term. Hence, it is reasonable to assume that the atmospheric delay is insignificant and that the phase changes detected across the extraction sites are due to subsidence [7].
The height ambiguity for the displacement phase is given by (2). A complete 2π phase change is equivalent to a height displacement of λ/2 in the range direction (for JERS-1, λ / 2 = 11.75 cm). Since the measured phases in the interferogram are wrapped in modulo of 2π, the height displacement map can be derived by “phase unwrapping” the interferogram. φ disp = - (4 π / λ) δR
(2)
where φ disp = ground surface deformation component δR = height displacement in range direction
impact of decorrelation due to too long a baseline, 28 interferometric pairs were formed and tested with a baseline less than 1400m. The results show that only 11 pairs have sufficient coherence to distinguish the phase caused by subsidence from the noise. For this specific test site, the interferometric pairs with baseline distances less than 900m and with temporal separation up to 3 repeat-cycles (132 days) have the better coherence conservation. The limitations of perpendicular and temporal baselines between the JERS-1 interferometric pairs are shown in Figure 2. Figures 1 and 2 show that the usable perpendicular and/or temporal baselines of the interferometric pairs of L-band images are much greater than C-band at this specific test site.
The DInSAR results are imported a GIS for data interpretation and validation (to visualise, manipulate, analyse and display data collected from various sources). The mine subsidence maps are imported into a GIS and analysed together with other data such as aerial photographs, mine plans and ground survey data (see later section). III.
LIMITS OF INTERFEROMETRIC MEASUREMENTS
The ground surface displacement maps can be derived after phase-unwrapping the interferogram. As mentioned earlier, many factors may contribute phase noise to the interferogram, hence making the phase-unwrapping process difficult, and sometimes leading to a degradation of the results. In this study, the quality of mine subsidence monitoring is mainly constrained by the noise caused by the spatial and temporal decorrelation between the interferometric pair, and also the phase discontinuities in the interferogram.
Figure 1. Baseline limitation at the test site for ERS DInSAR process.
A. Spatial and Temporal Decorrelation This paper used both ERS and JERS-1 images for subsidence mapping. Due to the difference in size of their wavelengths, the C-band ERS images are expected to be more sensitive to vegetation cover. Consequently, the C- and L-band interferometric pairs have different threshold values for the spatial (perpendicular baseline distance) and temporal (number of days between the two acquisitions) baseline distance, as far as conservation of coherence is concerned. There are 5 ERS-1 and 13 ERS-2 image acquisitions over the test sites from September 1995 to May 1996, and October 1995 to October 2001 respectively, including 3 ERS tandem pairs. Considering the impact of decorrelation due to long baselines, 48 interferometric pairs with baseline distance less than 800m were tested using interferometry. The test result shows that only 9 pairs, including 2 tandem pairs, had sufficient coherence levels and the phase fringes (caused by subsidence) could be clearly distinguished. The characteristics of the local vegetation cover make it difficult for C-band images to conserve sufficient coherence over 2 repeat-cycles (70 days) of the satellite. The usable perpendicular baseline and temporal separation between the interferometric pairs for DInSAR processing are shown in Figure 1, where the 9 usable results mentioned earlier are colored in blue circles. For JERS-1, there are 13 images available over the test site from August 1993 to January 1996. Again, by considering the
Figure 2. Baseline limitation at the test site for JERS-1 DInSAR process.
B. Phase Discontinuities The phase-unwrapping process is based on the assumption that the spatial sampling rate of the interferogram is sufficient to avoid aliasing. This means that the phase difference between
any two adjacent pixels in the interferogram should be less than one-half cycle (π) [8]. In practice, due to the noise or large phase gradients in the interferogram, phase discontinuities do occur and consequently degrade the accuracy of the ‘unwrapped’ results. The underground mines in this study area employ the longwall mining technique. With this technique the mine subsidence is constrained by the pillars, and therefore the maximum subsiding region would normally occur along the center of the longwall panel. In the DInSAR interferogram, the mine subsidence appears as fringes in the shape of concentric circles or ellipses. Each longwall panel has a width of about 200m. The ground resolution of ERS and JERS-1 images is approximately 20m. Therefore, it is about 5 pixels from the edge to the center of a longwall panel. Under the assumption of no phase discontinuity, the maximum amplitude of subsidence would be 5 times π which is equivalent to 2 and half cycles of phase change. From (2), the displacement is 7 cm and 29 cm for the wavelength of ERS and JERS-1 satellites respectively, along the slant-range direction.
Figure 3. Interferogram of tandem DInSAR, 29 ~ 30 Oct 1995.
The mine subsidence in the test sites considered in this study have the typical amplitudes of 20cm, and even greater during the period of 1~2 months after the mining process has ceased, and can be up to 1m over a year. Therefore the L-band interferometric pairs are more suitable for mine subsidence monitoring as far as minimising the impact of phase discontinuity due to the repeat-cycle of ERS (35 days) and JERS-1 (44 days) is concerned. IV.
RESULTS
This project initially started with using the SAR images acquired by ERS-1/2 and JERS-1. After the successful launch of ENVISAT, successive acquisitions over the same test site have been requested. A. ERS-1/2 The ERS repeat-pass DInSAR results show the phase fringes in the interferogram representing the ground surface displacement were saturated. This is due to the high phase gradient, as mentioned earlier. It is exceptional, however, for ERS tandem DInSAR results. Normally, ERS tandem data are used to generation DEMs with the assumption that there is no ground displacement, or the amount of displacement is too small (and therefore negligible) during the 24 hours between two successive ERS1/2 image acquisitions. However, if a large ground displacement did occur during the acquisitions, as is the case for mine subsidence, then it could be detected by DInSAR. The interferogram of the tandem DInSAR is shown in Figure 3. High coherence was conserved by the tandem image pair due to the very short temporal separation of 1 day. A phase change can be clearly identified at the center of the interferogram. The location of this surface displacement has been confirmed with the corresponding mine plan.
Figure 4. The subsidence profiles of tandem DInSAR. The profiles across longwall (top) and along longwall (bottom) are the short and long line indicated in Figure 4 respectively.
After phase-unwrapping of the interferogram, Figure 4 shows the subsidence profiles drawn arbitrarily as indicated in Figure 3. The profiles show that the maximum subsidence occurred over a 24 hour period is about 1cm, and the variation caused by phase noise is +/- 3mm. B. JERS-1 Of the 11 JERS-1 DInSAR results, 3 of them are derived from 3 successive image acquisitions from March to June in 1995. The subsidence profiles drawn from these 3 results (March ~ April 95, April ~ Jun 95 and March ~ Jun 95) are shown in Figure 5 as lines (a), (b) and (c). Profiles (a) and (b) show evidence of the movement of mine subsidence, which has been confirmed by the corresponding mine schedule. The profile (c), derived from the DInSAR result across the period of March ~ Jun 95, does not reflect the expected amplitude of subsidence (as given by the summation of line (a) and (b)) indicated in light blue color. It is due to the phase gradient
being too high, which causes a discontinuity at the region near the center of the subsidence. As a result, the subsidence map cannot be phase-unwrapped correctly. Ground Surface Displacement Profiles:
level subsidence in 24 hours in the shallow underground mining region, with a vertical accuracy of +/- 3mm. The JERS1 DInSAR result indicates a RMS error of 1.4cm when compared to ground truth. More ENVISAT image acquisitions have been requested and will be processed in the near future.
10
ACKNOWLEDGMENT
Subsidence (cm
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(a)DInSAR03_04 (b)DInSAR04_06 (c)DInSAR03_06 (a)+(b)
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Figure 5. Example of JERS-1 DInSAR mine subsidence profiles.
An overlap between the DInSAR result and the presurveyed surface leveling line was found. After importing the displacement map derived by DInSAR into the GIS software, the subsidence profile was extracted and plotted against the ground truth, as shown in Figure 6. The computed RMS error of the DInSAR result against ground truth is 1.4cm.
The first author is supported by a PhD scholarship from the CRC for Spatial Information. The Australian Research Council (ARC) has been supporting DInSAR research at UNSW over a number of years. The Australian Coal Authority Research Program (ACARP) has also supported research into ground subsidence monitoring using DInSAR. The authors wish to thank Mr. Andrew Nesbitt of BHPBilliton for providing the GIS data, Prof. Makoto Omura of Kochi Women’s University, Japan, for providing the L-band data, ESA and ACRES (the Australian Centre for Remote Sensing) for providing the Cband SAR images. REFERENCES [1]
[2]
[3]
[4]
[5]
Figure 6. Validation of JERS-1 DInSAR result against ground survey data.
C. Envisat To date, seven ENVISAT images have been acquired near the same site during 29 March 2003 and 10 May 2003. However, the images were acquired from both descending and ascending passes with four different imaging modes. Only one interferometric pair can be formed from these 7 images. Unfortunately, the baseline distance of this pair is over 1500m, and it failed to give sufficient spectral overlap between the two images. More ENVISAT acquisitions have been requested. Differential InSAR results from these data will be reported in the future, and compared to the ERS and JERS-1 results. V.
CONCLUDING REMARKS
Repeat-pass DInSAR with the aid of GIS tools has been used to monitor the mine subsidence in the region southwest of Sydney, Australia. Tandem DInSAR analysis has revealed 1cm
[6]
[7]
[8]
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