Andrew Sowter

The application of the Intermittent SBAS (ISBAS) InSAR method to the South Wales Coalfield, UK

Satellite radar interferometry is a well-documented technique for the characterisation of ground motions over large spatial areas. However, the measurement density is often constrained by the land use, with best results obtained over urban and semi urban areas. We use an implementation of the Small Baseline Subset (SBAS) methodology, whereby areas exhibiting intermittent coherence are considered alongside those displaying full coherence, in the final result, to characterise the ground motion over the South Wales Coalfield, United Kingdom. 55 ERS-1/2 Synthetic Aperture Radar (SAR) C-band images for the period between 1992 and 1999 are processed using the ISBAS (Intermittent Small BAseline Subset) technique, which provides 3.4 times more targets, with associated measurements than a standard SBAS implementation. The dominant feature of the observed motions is a relatively large spatial area of uplift. Uplift rates are as much as 1 cm/yr. and are centred on the part of the coalfield which was most recently exploited. Geological interpretation reveals that this uplift is most likely a result of mine water rebound. Collieries in this part of the coalfield required a ground water to be pumped to enable safe coal extraction; following their closure pumping activity ceased allowing the water levels to return to equilibrium. The ISBAS technique offers significant improvements in measurement density ensuring an increase in detection of surface motions and enabling easier interpretation.

DInSAR estimation of land motion using intermittent coherence with application to the South Derbyshire and Leicestershire coalfields

Differential interferometric synthetic aperture radar (DInSAR) is a recognized remote-sensing method for measuring the land motion occurring between two satellite radar acquisitions. Advanced DInSAR techniques such as persistent scatterers and small baseline methods are excellent over urban and rocky environments but generally poor over more rural and natural terrain where the signal can be intermittently good and bad. Here, we describe the Intermittent Small Baseline Subset (ISBAS) method, which appears to improve results over natural, woodland and agricultural terrain. This technique uses a multi-looked, low-resolution approach, which is particularly suitable for deriving the linear components of subsidence for large-scale deformations. Application of the ISBAS method over a coal mining area in the UK indicates that it is able to significantly improve upon a standard small baseline approach.

Mexico City land subsidence in 2014–2015 with Sentinel-1 IW TOPS: Results using the Intermittent SBAS (ISBAS) technique

Differential Interferometric Synthetic Aperture Radar (DInSAR) can be considered as an efficient and cost effective technique for monitoring land subsidence due to its large spatial coverage and high accuracy provided. The recent commissioning of the first Sentinel-1 satellite offers improved support to operational surveys using DInSAR due to regular observations from a wide-area product. In this paper we show the results of an intermittent small-baseline subset (ISBAS) time-series analysis of 18 Interferometric Wide swath (IW) products of a 39,000 km2 area of Mexico acquired between 3 October 2014 and 7 May 2015 using the Terrain Observation with Progressive Scans in azimuth (TOPS) imaging mode. The ISBAS processing was based upon the analysis of 143 small-baseline differential interferograms. After the debursting, merging and deramping steps necessary to process Sentinel-1 IW roducts, the method followed a standard approach to the DInSAR analysis. The Sentinel-1 ISBAS results confirm the magnitude and extent of the deformation that was observed in Mexico City, Chalco, Ciudad Nezahualcoyotl and Iztapalapa by other C-band and L-band DInSAR studies during the 1990s and 2000s. Subsidence velocities from the Sentinel-1 analysis are, in places, in excess of -24 cm/year along the satellite line-of-sight, equivalent to over ~-40 cm/year vertical rates. This paper demonstrates the potential of Sentinel-1 IW TOPS imagery to support wide-area DInSAR surveys over what is a very large and diverse area in terms of land cover and topography.

Evaluation of InSAR and TomoSAR for Monitoring Deformations Caused by Mining in a Mountainous Area with High Resolution Satellite-Based SAR

Interferometric Synthetic Aperture Radar (InSAR) and Differential Interferometric Synthetic Aperture Radar (DInSAR) have shown numerous applications for subsidence monitoring. In the past 10 years, the Persistent Scatterer InSAR (PSI) and Small BAseline Subset (SBAS) approaches were developed to overcome the problem of decorrelation and atmospheric effects, which are common in interferograms. However, DInSAR or PSI applications in rural areas, especially in mountainous regions, can be extremely challenging. In this study we have employed a combined technique, i.e., SBAS-DInSAR, to a mountainous area that is severely affected by mining activities. In addition, L-band (ALOS) and C-band (ENVISAT) data sets, 21 TerraSAR-X images provided by German Aerospace Center (DLR) with a high resolution have been used. In order to evaluate the ability of TerraSAR-X for mining monitoring, we present a case study of TerraSAR-X SAR images for Subsidence Hazard Boundary (SHB) extraction. The resulting data analysis gives an initial evaluation of InSAR applications within a mountainous region where fast movements and big phase gradients are common. Moreover, the experiment of four-dimension (4-D) Tomography SAR (TomoSAR) for structure monitoring inside the mining area indicates a potential near all-wave monitoring, which is an extension of conventional InSAR.

Land motion in the urban area of Nottingham observed by ENVISAT-1

Persistent scatterer interferometry (PSI) analysis of land motion in the City of Nottingham has been undertaken using advanced synthetic aperture radar (ASAR) data from the ENVISAT-1 satellite covering the period between November 2002 and February 2009, and some interesting areas of motion are evident from the results. The aims of this article are twofold: first to relate these areas to other sources of information such as geology, GPS, and precise levelling, and second to discuss a novel method of atmospheric correction for PSI using precise point positioning (PPP) derived GPS zenith wet delay (ZWD) estimates. PSI validation using the various sources of data is discussed with contextual information regarding historical activity. This article shows that land motion in an urban environment can be widespread and anisotropic, with many different causes, with the consequences that sparse networks of ground-based instruments may be incapable of monitoring land motion in such a complex environment. A ZWD correction method is also presented and results are shown, which improve upon those without the correction. The ZWD estimates are produced using an undifferenced global navigation satellite system (GNSS) processing technique known as PPP. This, coupled with the chosen PSI method, provides a low noise solution when compared to similar experiments involving the integration of GNSS and interferometric synthetic aperture radar (InSAR) data. The results show that there is good temporal and spatial correlation between the PPP ZWD estimates and the PSI-derived atmospheric phase contributions. Noticeable improvements in the PSI solution when comparing before and after processing runs to precise levelling observations are shown.

Orthorectification and interpretation of differential InSAR data over mountainous areas: a case study of the May 2008 Wenchuan Earthquake

This paper illustrates the capabilities of the Advanced Land Observing Satellite (ALOS) Phased Array Type L-band Synthetic Aperture Radar (PALSAR) instrument in the identification of macro- and meso-scale land deformation caused by the Wenchuan Earthquake of 12 May 2008. The specific location analysed is an area near the south west termination of the rupture zone where fault slip and surface deformations were relatively low and topography is relatively high. A full geometric processing of the interferometric synthetic aperature radar (InSAR) data is performed, including the removal of the stereoscopic effect, and the terrain effect fully compensated by identification and removal of layover and shadow areas. This allows for the full characterization of the main earthquake deformation.

Intermittent small baseline subset (ISBAS) InSAR analysis to monitor landslides in Costa Della Gaveta, Southern Italy

This work presents a Differential SAR Interferometry (DInSAR) analysis of slow-moving landslides in Costa della Gaveta (southern Italy) exploiting the improved spatial density of radar targets provided by the novel Intermittent SBAS (ISBAS) algorithm. Several landslides occurred in this area over the past decade, producing a ground displacement of several centimeters and causing unsafe road and rail traffic conditions. In the 2.4 km2 study area, ISBAS analysis of TerraSAR-X data acquired in 2010-2011 has shown the presence of sixteen phenomena with a southeastern main direction of motion. The DInSAR results agree with both the magnitude and the deformation mechanisms that were mapped during field observations and reported in the geotechnical literature.

Intermittent Small Baseline Subset (ISBAS) monitoring of land covers unfavourable for conventional C-band InSAR: proof-of-concept for peatland environments in North Wales, UK

This paper provides a proof-of-concept for the use of the new Intermittent Small Baseline Subset (ISBAS) approach to study ground elevation changes in areas of peat and organic soils in north Wales, which are generally, unfavourable for conventional C-band interferometric applications. A stack of 53 ERS-1/2 C-band SAR scenes acquired between 1993 and 2000 in descending mode was processed with both the standard low-pass SBAS method and ISBAS. The latter revealed exceptional improvements in the coverage of ground motion solutions with respect to the standard approach. The number of identified coherent and intermittently coherent pixels increased by a factor of 26 with respect to the SBAS solution, and extended the coverage of results across unfavourable land covers, particularly for coniferous woodland, bog, acid grassland and heather. The greatest increase was achieved over coniferous woodland, which showed ISBAS/SBAS pixel density ratios above 300. Despite the intermittent nature of the ISBAS solutions, ISBAS provided velocity standard errors generally below 1-1.5 mm/yr, thus preserving good quality of the estimated ground motion rates.

Evaluation of PS-DInSAR technology for subsidence monitoring caused by repeated mining in mountainous area

Abstract The high resolution TerraSAR-X dataset was employed with DInSAR and persistent scatterer interferometry (PSI) technique for subsidence monitoring in a mountainous area. For DInSAR technique, the generally used SRTM and relief-DEM, which was derived from aerial topographic map, were used to evaluate the influence of external DEM. The results show that SRTM could not fully compensate the complex topography of the research area. The corner reflectors installed during the acquisition of SAR dataset were used to estimate the accuracy of geocoding. The terrain corrected geocoding results based on relief-DEM were much better than using SRTM, with the root mean square error (RMSE) being 6.35 m in X direction and 11.65 m in Y direction (both in UTM projection), around one pixel of the multilooked intensity image to be geocoded. For PSI technique, the results from time-series analysis of multi-baseline differential interferograms were integrated to restrict only persistent scatterer candidates near the boundary of subsiding area for regression analysis. The results demonstrate that PSI can refine the boundary of subsidence, which could then be used to derive some angular parameters to help people to learn the law of subsidence caused by repeated excavation in this area.

Intermittent SBAS (ISBAS) InSAR with COSMO-SkyMed X-band high resolution SAR data for landslide inventory mapping in Piana degli Albanesi (Italy)

In the context of recent advances in InSAR processing techniques to retrieve higher persistent scatterer and coherent target densities over unfavourable land cover classes, this study tests the Intermittent Small Baseline Subset (ISBAS) approach to update the landslide inventory around the town of Piana degli Albanesi (Italy), an area where only 2% of the land appears suitable to generate radar scatterers based on a pre-survey feasibility assessment. ISBAS processing of 38 ascending mode and 36 descending mode COSMO-SkyMed StripMap HIMAGE SAR scenes at 3m resolution allows identification of ~726,000 and ~893,000 coherent and intermittently coherent pixels for the ascending and descending data stacks respectively. Observed improvements in the number of ISBAS solutions for the ascending mode are greater than 40 times compared to the conventional SBAS approach, not only for urban and rocky terrains, but also rural and vegetated land covers. Line of sight ground motion rates range between -6.4 and +5.5 mm/yr in 2008-2011, although the majority of the processed area shows general stability, with average rates of -0.6 mm/yr in the ascending and -0.1 mm/yr in the descending mode results. Interpretation of the ISBAS deformation rates, integrated with targeted field surveys and aerial photo-interpretation, provides a new and more complete picture of landslide distribution, state of activity and intensity in the test area, and allows depiction of very slow and extremely slow landslide processes even in areas difficult to access, with unprecedented coverage of results.