Comparison of L band and X band differential interferometric synthetic aperture radar for mine subsidence monitoring in central Utah International Journal of Mining Science and Technology xxx (2016) x[.]
International Journal of Mining Science and Technology xxx (2016) xxx–xxx Contents lists available at ScienceDirect International Journal of Mining Science and Technology journal homepage: www.elsevier.com/locate/ijmst Comparison of L-band and X-band differential interferometric synthetic aperture radar for mine subsidence monitoring in central Utah Jessica M Wempen ⇑, Michael K McCarter Department of Mining Engineering, University of Utah, Salt Lake City 84112, USA a r t i c l e i n f o Article history: Received July 2016 Received in revised form 18 August 2016 Accepted 20 September 2016 Available online xxxx Keywords: Mine subsidence DInSAR TerraSAR-X ALOS Interferometry a b s t r a c t Differential interferometric synthetic aperture radar (DInSAR), a satellite-based remote sensing technique, has potential application for measuring mine subsidence on a regional scale with high spatial and temporal resolutions However, the characteristics of synthetic aperture radar (SAR) data and the effectiveness of DInSAR for subsidence monitoring depend on the radar band (wavelength) This study evaluates the effectiveness of DInSAR for monitoring subsidence due to longwall mining in central Utah using L-band (24 cm wavelength) SAR data from the advanced land observing satellite (ALOS) and X-band (3 cm wavelength) SAR data from the TerraSAR-X mission In the Wasatch Plateau region of central Utah, which is characterized by steep terrain and variable ground cover conditions, areas affected by longwall mine subsidence are identifiable using both L-band and X-band DInSAR Generally, using L-band data, subsidence magnitudes are measurable Compared to X-band, L-band data are less affected by signal saturation due to large deformation gradients and by temporal decorrelation due to changes in the surface conditions over time The L-band data tend to be stable over relatively long periods (months) Short wavelength X-band data are strongly affected by signal saturation and temporal decorrelation, but regions of subsidence are typically identifiable over short periods (days) Additionally, though subsidence magnitudes are difficult to precisely measure in the central Utah region using X-band data, they can often be reasonably estimated Ó 2016 Published by Elsevier B.V on behalf of China University of Mining & Technology Introduction Differential interferometric synthetic aperture radar (DInSAR) is a satellite-based remote sensing technique that can be used to measure surface displacement over large regions with high spatial resolution Under good conditions, displacements can be measured with centimeter to subcentimeter accuracy [1,2] DInSAR also has high temporal resolution, and imaging periods typically range from 10 to 50 days [3,4] In the last two decades, the application of DInSAR for mine subsidence monitoring has been demonstrated in coal basins in Europe, Australia, China, and the United States Overall, these studies have demonstrated good data resolution, strong relationships between mine development and subsidence, and reasonable agreement between displacements measured by DInSAR and displacements measured by conventional surveys [5–20] In radar interferometry phase measurements from two nearly coincident radar images are used to precisely measure relative distances [21] Surface deformation, topography, and changes in the ⇑ Corresponding author Tel.: +1 801 5853029 E-mail address: jwempen@gmail.com (J.M Wempen) satellite position contribute most significantly to changes in the radar path length In general, changes in the path length due to changes in the satellite position and topography are known or can be estimated, and as a consequence, centimeter-level changes in the path length due to surface deformation can be measured Because phase measurements are used to quantify distance, the wavelength characteristics of the radar are important Synthetic Aperture Radar (SAR) sensors most commonly use either L-band (24 cm wavelength), C-band (6 cm wavelength), or X-band (3 cm wavelength) radar, and the imaging characteristics of the radar bands are different Radar waves tend to interact strongly with structures similar in size to the radar wavelength, and as a result, surfaces appear rougher in images acquired using shorter wavelengths Longer wavelengths tend to have some penetration of vegetation, dry soils, and ice; phase measurements from longer wavelengths tend to be less sensitive to small changes in the surface conditions over time Additionally, the maximum deformation gradients measurable by DInSAR depend significantly on the radar band and on the ground resolution (pixel size) of the image [22] Longer wavelengths are less sensitive to deformation per pixel than shorter wavelengths, and larger deformation gradients are measurable in higher resolution data http://dx.doi.org/10.1016/j.ijmst.2016.11.012 2095-2686/Ó 2016 Published by Elsevier B.V on behalf of China University of Mining & Technology Please cite this article in press as: Wempen JM, McCarter MK Comparison of L-band and X-band differential interferometric synthetic aperture radar for mine subsidence monitoring in central Utah Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.11.012 J.M Wempen, M.K McCarter / International Journal of Mining Science and Technology xxx (2016) xxx–xxx Though DInSAR has significant potential as a method for subsidence monitoring, SAR systems have variable characteristics Using SAR data appropriate for the regional surface characteristics and deformation rates is important for subsidence monitoring This study evaluates the effectiveness of L-band and X-band DInSAR for monitoring subsidence due to longwall mining in the Wasatch Plateau region of central Utah L-band SAR data from the advanced land observing satellite (ALOS) and X-band SAR data from the TerraSAR-X mission are used Location and data The Wasatch Plateau is characterized by rugged topography with flat topped mesas and steeply incised canyons It is geologically complex and exists in a region of transition from the Colorado Plateau to the east and the Basin and Range province to the west [23] Dominant structures include northward trending normal faults and grabens, vertical joints, and vertical strike-slip faults [24] The subalpine region of the Wasatch Plateau is heavily vegetated; grasses, forbs, and low shrubs are dominant Steep northern exposures at higher elevations are heavily forested, and at lower elevations, aspen, pine and tall shrubs are common [25] Fig shows a TerraSAR-X image of a region of the Wasatch Plateau In this image, topographic characteristics of the Wasatch Plateau region are discernable In the study area, coal has been mined from the upper and lower Hiawatha seams These seams are present in the lower 75– 110 m of the Blackhawk Formation (Mesaverde Group), which has a total thickness ranging from 190 to 245 m Prominent nearseam geology includes the castlegate and the star point sandstones, both massive, medium- to course-grained sandstones The castlegate sandstone overlays the Blackhawk Formation and has a thickness ranging from 45 to 150 m The starpoint sandstone, lies beneath the Blackhawk Formation and has a thickness ranging from 25 to 300 m [26] The mining heights range from to 3.8 m and the typical overburden thickness ranges from 305 to 550 m Generally, the maximum vertical subsidence occurs near the center of the longwall panels, with maximum magnitudes from 1.5 to 1.8 m The average reported angle of draw is 15° [27] The L-band SAR data used in this study were imaged by the ALOS satellite ALOS was operated by the Japanese Aerospace Exploration Agency from 2006 to 2011, and acquired data globally with a minimum recurrence cycle of 46 days The data used in this study were imaged in fine-beam mode with both single and dual polarization; only the co-polarized images were used in interferometric processing The images have swath widths of 70 km, azimuth resolutions of 10 m, and ground range resolutions of 10 and 20 m for single and dual polarizations, respectively [3] Imaging dates and characteristics of the ALOS data are given in Table The ALOS data were acquired from a repository of SAR data maintained by the Alaska Satellite Facility The X-band SAR data used in this study were imaged by the TerraSAR-X mission satellites, TSX-1 which launched in 2007 and TDX-1 which launched in 2010 These satellites are operated by the German Aerospace Center, and acquire data with a minimum recurrence cycle of 11 days The data used in this study were imaged in stripmap mode with single horizontal polarization The images have 30 km swath widths and maximum ground resolutions of 3.3 m in azimuth and 1.7 m in range [28] Imaging dates and characteristics of the TerraSAR-X data are given in Table The TerraSAR-X data were acquired from the German Aerospace Center Processing In this study, data processing was performed using SARscapeÒ and ENVIÒ software To generate a subsidence map using DInSAR, first the perpendicular and temporal baselines of paired SAR images are estimated Next, the paired images are co-registered and a differential interferogram is formed The interferogram is then filtered and the data coherence is estimated Next, the interferogram is unwrapped and the absolute interferometric phases are determined Finally, vertical deformation is calculated from the absolute phases [29] Before interferometric processing, SAR images are often multilooked, or spatially averaged Significant amplitude and phase variation of the radar signal from pixel to pixel, caused by variation in the surface characteristics, make the images appear speckled [30] Spatially averaging the data reduces speckle, but it also reduces the spatial resolution Short wavelength X-band data are very sensitive to surface deformation, and deformation gradients on the order of 0.016 m/pixel are measurable However, in areas with large deformation gradients, phases tend to saturate In this study, the TerraSAR-X images were not multi-looked and were processed at full resolution to limit phase saturation Longer wavelength L-band data are less sensitive to deformation, and deformation gradients on the order of 0.118 m/pixel are measurable In this study, the ALOS images were multi-looked to produce images with 20 m by 20 m pixels Results Fig TDX-1 intensity image of the Wasatch Plateau region (October 20, 2015) For the central Utah study region, three interferograms were generated using L-band data in intervals over the period from June 16 to December 17, 2010 The interferometric data parameters are summarized in Table Phase fringes due to subsidence are identifiable in all of the interferograms, but all the interferograms have topographic artifacts and the data quality is variable Coherence is a statistics that quantifies the sameness of the radar signals in two paired images and it ranges from zero, which represents complete decorrelation, to one which represents perfect correlation In general, coherence reflects the quality of the phase measurements [30] The average coherence of the ALOS data ranges from a low of 0.36 for the period from September 16 to December 17, 2010, to a high of 0.63 for the period from August to September 16, 2010 Vertical displacement maps derived from the interferograms for each period are shown in Fig 2, Fig shows accumulated displacement for the 184-day period from June 16 to December 17, 2010 In Figs and subsidence is contoured every 10 cm starting from 10 cm of vertical displacement Figs and show the progression Please cite this article in press as: Wempen JM, McCarter MK Comparison of L-band and X-band differential interferometric synthetic aperture radar for mine subsidence monitoring in central Utah Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.11.012 J.M Wempen, M.K McCarter / International Journal of Mining Science and Technology xxx (2016) xxx–xxx Table ALOS SAR data characteristics Orbit Path Frame Number Pass direction 201 201 201 201 770 770 770 770 23,400 24,071 24,742 26,084 Ascending Ascending Ascending Ascending Acquisition date Polarization Look angle (°) 06/16/10 08/01/10 09/16/10 12/17/10 Dual Dual Dual Single 34.3 34.3 34.3 34.3 Table TerraSAR-X SAR data characteristics Satellite TSX-1 TSX-1 TSX-1 TSX-1 TSX-1 TSX-1 TSX-1 TSX-1 TSX-1 TSX-1 TDX-1 TSX-1 TSX-1 TDX-1 Orbit Number Cycle Pass direction 30 30 30 30 30 30 30 30 30 30 30 30 30 30 266/44,285 267/44,452 270/44,953 271/45,120 272/45,287 273/45,454 274/45,621 275/45,788 276/45,955 277/46,122 278/29,559 279/46,459 280/46,623 281/30,060 Ascending Ascending Ascending Ascending Ascending Ascending Ascending Ascending Ascending Ascending Ascending Ascending Ascending Ascending Beam Acquisition date Incidence angle (°) Min Max 10R 10R 10R 10R 10R 10R 10R 10R 10R 10R 10R 10R 10R 10R 06/10/15 06/21/15 07/24/15 08/04/15 08/15/15 08/26/15 09/06/15 09/17/15 09/28/15 10/09/15 10/20/15 10/31/15 11/11/15 11/22/15 36.1 36.1 36.1 36.1 36.1 36.1 36.1 36.1 36.1 36.1 36.1 36.1 36.1 35.9 38.5 38.5 38.5 38.5 38.5 38.5 38.5 38.5 38.5 38.5 38.5 38.5 38.5 38.6 Table ALOS interferometric data parameters Acquisition date Elapsed time (d) Baseline (m) Average coherence 06/16/2010–08/01/2010 08/01/2010–09/16/2010 09/16/2010–12/17/2010 46 46 92 299 43 575 0.59 0.63 0.36 of subsidence over time along sections AA0 and BB0 (Fig 3) Maximum measured subsidence during the June 16 to December 17, 2010 period is 1.5 m Gaps in the data in all of these figures are due to pixels with low coherence As noted in Fig 2, subsidence is contoured every 10 cm, starting from 10 cm of displacement As shown in Fig 3, subsidence is contoured every 10 cm, starting from 10 cm of displacement Thirteen interferograms were generated using X-band SAR data from TerraSAR-X in intervals over the period from June 10 to November 22, 2015 The interferometric data parameters are summarized in Table The average coherence of the X-band interferograms ranges from 0.54 to 0.66 Phase fringes due to Fig L-band cumulative vertical displacement map for the period from June 16 to December 17, 2010 (184 days) Fig L-band vertical displacement maps for the three periods Please cite this article in press as: Wempen JM, McCarter MK Comparison of L-band and X-band differential interferometric synthetic aperture radar for mine subsidence monitoring in central Utah Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.11.012 J.M Wempen, M.K McCarter / International Journal of Mining Science and Technology xxx (2016) xxx–xxx Fig Time series subsidence profiles of section AA0 from Fig Fig X-band filtered differential interferogram: June 10 to June 21, 2015 (11 days) Fig Time series subsidence profiles of section BB0 from Fig Table TerraSAR-X interferometric data parameters Acquisition date Elapsed time (d) Baseline (m) Average coherence 06/10/2015–06/21/2015 06/21/2015–07/24/2015 07/24/2015–08/04/2015 08/04/2015–08/15/2015 08/15/2015–08/26/2015 08/26/2015–09/06/2015 09/06/2015–09/17/2015 09/17/2015–09/28/2015 09/28/2015–10/09/2015 10/09/2015–10/20/2015 10/20/2015–10/31/2015 10/31/2015–11/11/2015 11/11/2015–11/22/2015 11 33 11 11 11 11 11 11 11 11 11 11 11 88 37 14 16 44 55 142 25 16 282 360 82 90 0.58 0.54 0.58 0.60 0.66 0.60 0.61 0.61 0.60 0.55 0.54 0.60 0.56 surface displacement are identifiable in the majority of the interferograms, but in the areas with the largest magnitude subsidence, the fringes are difficult to interpret Precise evaluation of the maximum subsidence magnitude is not attempted, but the subsidence magnitudes can be reasonably estimated in many of the interferograms Fig shows an example of a filtered differential interferogram for the period from June 10 to June 21, 2015 Phase fringe due to subsidence are outlined in green There are at least seven fringes indicating a maximum vertical displacement of more than 13 cm and a maximum subsidence rate of more than cm per day Discussion In the Wasatch Plateau, regions of subsidence can be identified by both L-band and X-band DInSAR, but the effectiveness of DInSAR for quantifying subsidence is dependent on the radar band Generally, subsidence magnitudes are precisely measurable in the L-band data The X-band data are more affected by signal saturation and by temporal decoration, and precisely measuring the sub- sidence magnitudes using X-band data is more difficult Notably, though the data quality is variable, the L-band data and the Xband data have similar average coherence However, in the Xband data, coherence is spatially dependent: coherence is generally high over the valley floor and low in the vegetated subalpine region Variable surface conditions in the subalpine region contribute to both low coherence and significant phase noise in the X-band data, which make phases in the interferograms more difficult to interpret Spatial variation in the coherence of the L-band data is less apparent However, the L-band data are sensitive to significant changes in the surface conditions, and low coherence does affect the data quality Variable surface characteristics, including snow cover, likely caused low coherence in the L-band interferogram from September 16 to December 17, 2010 As a result of low coherence, the quality of the displacement map for this period of Fig 2c is lower than the quality of the displacement maps for periods when the surface condition were more stable and the coherence was higher in Fig 2a and b Additionally, in all of the L-band data, areas with very large deformation gradients are affected by low coherence due to phase saturation Although it is likely that the imaging period from June to December did not capture the full development of subsidence, phase saturation has the potential to cause subsidence to be underestimated by tens of centimeters in the L-band data The maximum cumulative subsidence reported for this area is on the order of m [27] In the X-band data, the interpretability of the phases is negatively affected by signal saturation as a result of large displacement rates and by temporal decorrelation, but the aerial extent of subsidence is clearly identifiable in most of the data Additionally, though the maximum subsidence magnitudes are difficult to precisely measure, in most of the images the magnitudes of subsidence can be reasonably estimated Because the X-band imaging periods are shorter than L-band imaging periods, the X-band data provide a more timely report of the subsidence extent Consequently, short period X-band data has potential to accurately identify periods when subsidence has ceased or is minimal Acknowledgments Funding for this research was provided by the National Institute for Occupational Health and Safety (NIOSH) The support of NIOSH Please cite this article in press as: Wempen JM, McCarter MK Comparison of L-band and X-band differential interferometric synthetic aperture radar for mine subsidence monitoring in central Utah Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.11.012 J.M Wempen, M.K McCarter / International Journal of Mining Science and Technology xxx (2016) xxx–xxx is thankfully acknowledged; however, the conclusions expressed in this paper are those of the authors and not represent the opinions or policies of NIOSH Data for this research was provided by the Alaska Satellite Facility, the Japan Aerospace Exploration Agency and the German Space Agency The contributions of these organizations are gratefully acknowledged References [1] Buckley SM Radar interferometry measurement of land subsidence Austin (TX): The University of Texas at Austin; 2000 [2] Massonnet D, Feigl KL Radar interferometry and its application to changes in the Earth’s surface Rev Geophys 1998;36(4):441–500 [3] Rosenqvist A, Shimada M, Watanabe M ALOS PALSAR: technical outline and mission concept In: Proceedings of the 4th international symposium on retrieval of bio- and 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McCarter MK Comparison of L- band and X -band differential interferometric synthetic aperture radar for mine subsidence monitoring in central Utah Int J Min Sci Technol (2016), http://dx.doi.org/10.1016/j.ijmst.2016.11.012... of NIOSH Please cite this article in press as: Wempen JM, McCarter MK Comparison of L- band and X -band differential interferometric synthetic aperture radar for mine subsidence monitoring in central. .. X -band DInSAR for monitoring subsidence due to longwall mining in the Wasatch Plateau region of central Utah L- band SAR data from the advanced land observing satellite (ALOS) and X -band SAR data