DETECTING COAL FIRES IN CHINA USING DIFFERENTIAL INTERFEROMETRIC SYNTHETIC APERTURE RADAR (INSAR) J¨orn Hoffmann, Achim Roth, Stefan Voigt German Remote Sensing Data Center (DLR-DFD), M¨unchner Str. 20, Oberpfaffenhofen, 82234 Wessling, Germany. (
[email protected])
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ABSTRACT
We investigate the feasibility of detecting fires in subsurface coal deposits through InSAR observations of accompanying surface displacements. Uncontrolled burning of subsurface coal seams have been reported from many locations around the world. In northern China alone, more than 10 Million tons (Mt) of coal are estimated to burn every year. This has massive implications for the regional economy and ecology. In fighting these fires and controlling burning coal seams the timely and reliable detection and mapping of the affected regions is critical. However, this has proven to be extremely difficult in the often remote regions of northern China, where many of the fires have been caused by uncontrolled, small-scale mining operations. Both volume change of the burning coal and thermal effects in the adjacent rock mass are expected to cause measurable surface displacements and numerous reports of collapses of the earth’s surface exist. Unfortunately, reliable data on surface deformation accompanying the fires are not available. Nevertheless, theoretical considerations and individual reports suggest that subsidence mapping using differential InSAR may be a suitable tool to detect burning regions and map the spatial extent of the affected areas. Though topography, temporal decorrelation, and poor data coverage complicate the analysis we have identified several localized areas of subsidence in the region. Here we discuss the potential and limitations of using InSAR for coal-fire detection in northern China.
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INTRODUCTION
Uncontrolled ignition and burning of coal seams have been reported from numerous coal mining areas around the world. These fires and the related environmental problems have become particularly severe in China’s central and western coal mining areas (fig. 1). China is the worlds largest coal-producing country by volume with an estimated annual production exceeding one billion metric tons. An estimated 10 − 200 Mt are lost to uncontrolled coal fires annually [3]. Apart from the obvious economic implications of the waste of valuable coal resources, the burning coal fields have caused extreme environmental damages and health risks in the affected regions. Coal is a highly inflammable material. Spontaneous ignition can occur when oxidation heat raises the in-situ temperature. The coal is exposed to oxygen as a result of mining activities or exposure of the coal surface through natural erosion processes. The air in the shafts provides the oxygen for the oxidation process. In absence of adequate ventilation the oxidation heat accumulates and the temperatures can rise to the point where the coal ignites spontaneously. Inadequate ventilation is a common problem in uncontrolled mining. The resulting coal fires, once ignited, are extremely difficult to extinguish or even control. The Coal Fire Research Initiative has been initiated to understand coal fire phenomena and mitigate their negative impacts. The project is lead by the German Remote Sensing Data Center (DLR-DFD) and funded by the German Federal Ministry for Education and Research. It joins commercial companies, government research institutions and universities from China and Germany. The broad research agenda encompasses all major aspects of coal fires from the ignition process and conditions to fire-fighting strategies and including the mapping and monitoring of known and new coal fires with different remote-sensing techniques and the quantification of environmental effects at different scales from local to global. Namely, the principal goals of the project are to • understand the processes leading to fire ignition, • develop tools to model the fire geometry and spreading, • develop innovative fire-fighting strategies, • understand and quantify the local, regional and possibly global environmental influences of coal fires, and ____________________________________________________________ Proc. of FRINGE 2003 Workshop, Frascati, Italy, 1 – 5 December 2003 (ESA SP-550, June 2004) 60_hoffman
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Figure 1: Location of Coal Fire Research Project focus sites. The results presented here refer to the Wuda region (arrow). Inset shows locations of coal fires in China. • monitor and survey endangered areas over the long term. Three test areas including the Wuda, Gulaben and Ruqigou coal fields and two coal fields in western China have been selected as primary study areas (fig. 1). Our observations discussed here have been made in the Wuda region.
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LAND SUBSIDENCE
Active mining activities frequently result in measurable subsidence of the land surface. The volume of the land subsidence observable at the surface typically amounts to about 50-90% of the material removed during mining activities [2] after the shafts are abandoned. Mining-related subsidence has long been studied with conventional techniques such as leveling. More recently InSAR techniques have also been used successfully to measure mining-induced subsidence signals [1, 4]. However, the applicability of InSAR techniques using available space-based SAR data depends critically on the spatial and temporal characteristics of the surface motion. Only if the subsiding areas are spatially extensive enough to be resolvable at the available image resolution and the motion is slow enough to be sampled adequately in time by the available satellite acquisitions can meaningful observations be made. Furthermore, to preserve a coherent phase signal the land surface itself must be largely undisturbed. Unfortunately, for the observation of shallow coal fires in China’s mining areas these conditions prove very limiting. In many cases, the subsurface burns result in relatively small (meters) surface fissures or sinkholes that significantly remodel the surface (fig. 2) and result in a complete loss of coherence in interferometric images spanning the event. However, more subtle and spatially extensive surface motions are observable using InSAR, as we show in the following section. Where they do not result in catastrophic surface breaks these surface displacement are usually not noticed on the ground. Geodetic measurements using spirit leveling or GPS surveys have not been conducted in the area. Thus, InSAR provides the first recognition of these larger-scale displacements above the burning coal-seams.
Figure 2: Surface breaks caused by subsurface coal fires in Wuda region.
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OBSERVATIONS
Prior to the coal fire research project little research has focused on the remote Chinese regions affected by the burns. Consequently, only relatively scarce ERS SAR data suitable for interferometry exist. As mentioned before, geodetic surveys to quantify or even identify ground subsidence have not been conducted and only comparatively few satellite SAR images have been acquired in the region. We have created five interferograms for the Wuda region from available ERS-1/2 SAR data (fig. 3); four interferograms span times exceeding one month and can be used for motion detection. The corresponding perpendicular baselines range from 10 m to 345 m (fig. 3). The interferometric phase images are shown in figure 4. While the two images spanning shorter times of less than one year (B, C) allow the identification of several subsidence features, the other two images spanning over two years (A, D) are affected by severe temporal decorrelation. Even though the decorrelation is not complete and some of the phase signatures visible in images B and C can be discerned in A and D, it would be exceedingly difficult to identify and interpret these in the presence of the decorrelation noise without the additional confirmation from the clearer signatures in the other images.
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DISCUSSION
The subsidence signatures in the phase images agree very well with the coal fires identified in a field survey in September 2002 (fig. 4). In one case the observed subsidence seems to be caused by subsidence above an active sandstone mine (labeled in fig. 4B) and in another case the observed subsidence corresponds to an industrial coal deposit with large burning coal heaps (fig. 4B). Apart from these two exceptions all subsidence signatures clearly identifiable in the phase
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Figure 4: Interferometric phase for interferograms A-D in fig. 3. Red lines delineate the extent of the coal fires mapped in a field survey in September 2002.
images seem to correspond to active coal fires. However, it is important to note that many fires mapped during the field survey do not show any subsidence in the phase images. This may in part be due to the temporal mismatch between the radar acquisitions (1995-98) and the field survey (2002). But it is also likely that this observation indicates a fundamental limitation of the identification of coal fires from InSAR-observed surface subsidence. Small fires may not cause sufficient surface motion to be apparent in the available InSAR images. In addition, shallow fires in particular are prone to cause extensive surface breaking, potentially resulting in complete decorrelation of any interferometric phase images spanning an event of major subsidence. Unfortunately we could not study this effect in detail as this would have required a much denser temporal sampling than was available from the ERS acquisitions Nevertheless, the fact that observable deformation magnitudes accumulate rapidly enough to be visible even in the 35day interferogram (April to May 1995, fig. 4C) suggests that frequent SAR acquisitions could potentially enable a detailed spatial and temporal analysis of the coal-fire dynamics over a larger region. Furthermore, the very sparsely vegetated surfaces in Wuda – and other similar regions in China – might enable the detection of regions of surface breaking by analysis of interferometric coherence in interferograms covering only short time periods that would usually guarantee a high level of coherence.
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RESULTS
We have found that it is possible to detect surface subsidence related to subsurface coal fires in ERS interferograms covering the Wuda region. Subsidence rates are sufficient to accumulate observable signals in one months time in some locations. Due to the complete lack of geodetic monitoring in the regions affected by coal fires InSAR currently represents the only means to detect and monitor surface subsidence in the region. The characteristics of the subsidence, which typically is accompanied by severe surface fissuring and the development of sinkholes, hamper the application of InSAR techniques. Complex surface breaking at scales smaller than the resolution in ERS-SAR images (tens of meters) causes a loss of image coherence over the area of interest when the subsidence signal is strongest. As surface breaking is particularly pronounced for shallow fires, quantifying the amount of subsidence using InSAR techniques will likely remain elusive for these cases. It may nevertheless be possible to detect coal fires either prior to the development of excessive surface breaks or through an analysis of image coherence in a data set of frequent SAR observations with generally high image coherence. Similarly, it might be possible to monitor the temporal evolution of the spatial extent of the fires using such observations.
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CONCLUSIONS
Despite fundamental limitations of decorrelation due to surface breaking InSAR can be a valuable tool to detect and monitor subsurface coal fires, provided a suitable data set is available. Interferometric radar data can complement other remote sensing data, such as infrared images, and aide a robust classification of new fires based on satellite observations. In this study we have been able to demonstrate the feasibility of using InSAR for coal-fire detection and monitoring in principle, despite problems of surface breaking caused by shallow coal fires. Our results suggest that additional and more frequent data acquisitions would enable additional analysis for more robust detections and temporal monitoring of active fires. However, as the data required for this was not available for this study this needs to be demonstrated in future investigations.
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ACKNOWLEDGEMENTS
We thank Anke Tetzlaff for providing the data from the field surveys and helpful discussions of the InSAR observations in the context of the field observations.
References [1] Claudie Carnec, D. Massonnet, and C. King. Two examples of the application of SAR interferometry to sites of small extent. Geophysical Research Letters, 23(24):3579–3582, December 1996. [2] Helmut Kratzsch. Bergschadenkunde. Deutscher Markscheider-Verein e.V., Bochum, Germany, 3 edition, 1997. ISBN 3-00-001661-9. [3] Simon Walker. Uncontrolled fires in coal and coal wastes. IEA Coal Research ISBN 92-9029-324-1, The Clean Coal Centre, Gemini House, 10-18 Putney Hill, London SW15 6AA, United Kingdom, 1999. [4] Urs Wegm¨uller, Tazio Strozzi, Charles Werner, Andreas Wiesmann, Norbert Benecke, and Volker Spreckels. Monitoring of mining-induced surface deformation in the Ruhrgebiet (Germany) with SAR interferometry. In Proceedings of IGARSS’00, Honolulu, USA, 24-28 July 2000. IEEE International, 2000.
