On the use of the ISBAS Acronym in InSAR Aapplications. Comment on Vajedian, S.; Motagh, M.; Nilfouroushan, F. StaMPS Improvement for Deformation Analysis in Mountainous Regions: Implications for the Damavand Volcano and Mosha Fault in Alborz. Remote Sens. 2015, 7, 8323–8347

Vajedian et al. [1] present an improved method for the derivation of deformation parameters using satellite Interferometric Synthetic Aperture Radar (InSAR) data. The method is a modification of the Small Baseline Subset (SBAS) method as implemented in the StaMPS (Stanford Method for Persistent Scatterers) software. The modification includes many steps including the filtering of the differential interferograms, integration with GPS data and advanced phase unwrapping “to overcome a lot of short- and long-wavelength artifacts that are clearly visible in StaMPS results” (cf. [1], p. 8331). The authors refer to this new approach as the Improved SBAS, or ISBAS, method. [...

Utilisation of InSAR for Monitoring of Subsidence over Mining Caving Zones

The utilisation of InSAR techniques for the monitoring of subsidence over mining areas, employing open pit and underground mining methods, has a large potential due to inaccessibility and safety issues associated with the usage of classical surveying techniques. InSAR can also be very competitive concerning the cost of provided results. However, there are a few issues that may significantly limit InSAR applicability for subsidence monitoring in mining areas. The highly dynamic character of subsidence induced by mining, especially employing caving as a mining system, may lead to ambiguity issues. This could happen when the vertical movement between the neighbouring cells (pixels) of the SAR image is greater than quarter of the wavelength of a radar signal over the period between image acquisitions. The altered terrain topography, involving steep slopes and deep pits, may also lead to radar signal layover issues for specific satellite and pit slope geometry.In this paper the authors analyse the above-mentioned issues and present how the InSAR technology was applied as a help to monitor large scale and highly dynamic subsidence for a real case study in Western Australia. It was recognised that the analysis of ground deformation dynamics, based on topographical surveys, may provide a basis for the resolution of ambiguity issues existing in InSAR processing. Also, the new technique involving generation of a detailed DEM based on the current topographical surveys and pixel-by-pixel analysis were applied in order to identify a precise extent of layover areas

Small Baseline Subset (SBAS) pixel density vs. geology and land use in semi-arid regions in Syria

36 ENVISAT ASAR images acquired in 2002 to 2010 along descending passes with nominal revisiting time of 35 days were processed over the whole region of Homs, western Syria, by implementing the low-pass Small Baseline Subset (SBAS) technique. More than 280,000 coherent pixels with ~100m ground resolution were obtained. We analysed pixel spatial distribution in respect of local geology and land use, to assess to what extent these factors can influence the performance of an interferometric deformation analysis in a semi-arid environment. Filtering out the amount of pixels associated with the urban fabric of Homs and surrounding villages, it is apparent that limestone and marl units are less prone to generate coherent pixels if compared with the basalt units in the north-western sector of the processed region. The latter resulted in pixel density of ~50-60 pixels/km2, which is comparable with that found over urban settlements and man-made structures

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

REDUCING THE DEM ERROR EFFECT IN DIFFERENTIAL INTERFEROMETRY

ABSTRACT/RESUME Differential interferometric synthetic aperture radar (DInSAR) products are subject to a number of errors, some intrinsic in the phase measurements and others from third-party data such as orbits and digital elevation models (DEMs). The effect of these on product quality depends on the magnitude of the errors and the type of method employed to generate the differential phase. This paper assesses the effect of such errors on 2-Pass and 3-Pass methods. It is shown that the accuracy of these methods may be improved upon by phase screening and filtering, assuming optimal combinations of phase coherence and DEM accuracy. If a standard 90m SRTM DEM is available, it is shown that it is possible to generate results for an ERS-type configuration that are superior to the application of a 2-Pass method alone using the same DEM. It is also shown that, for spaceborne data, the parallel ray or far-field approximation may not be appropriate. The process is demonstrated using ENVISAT data of the Bam earthquake in Iran. INTRODUCTION Since it was first demonstrated in 1989 by Gabriel et al. [1] differential interferometric synthetic aperture radar (DInSAR) has been successfully applied to the study of land deformation, earthquakes, volcanoes, ice sheets, landslides and terrain subsidence Current methods of spaceborne DInSAR follow a very similar pattern: two interferograms are formed, one containing deformation information and one without, and their difference, following compensation for the differing geometries, gives the differential phase characteristic of the change in radar signal path length caused by the deformation. The interferogram that does not contain the deformation information is commonly simulated using a Digital Elevation Model (DEM) -this approach is termed 2-Pass as it simply requires a single pair of images with at least one being acquired after the deformation event. When a suitable DEM is not available, an interferogram formed using another one or two images (3-and 4-Pass respectively) may be used to form the reference interferogram. A review of these methods may be found in Massonnet and Feigl [8]. In this paper we will refer only to 3-Pass and not 4-Pass for convenience, although the analysis applies to both methods. A 3-Pass method has the advantage that it does not require a DEM, an asset in regions for which a DEM is not forthcoming or of unspecified quality or prohibitive cost. However, when a suitable DEM is available, 2-Pass is generally superior. This is primarily because, as identified by Massonnet and Rabaute [9], the 2-Pass method only contains atmospheric and baseline anomalies from a single InSAR pair; a 3-Pass method clearly contains more. Any DEM, however, will also contain errors and this will affect the final quality of any 2-Pass result. The error is essentially random, depending on its method of production, and its effect on the quality of the differential interferogram depends on the size of the orbital baseline used, as in Massonnet and Feigl [8]. Thus, when a DEM is available there appear to be two choices for DInSAR: use a 3-Pass method with no DEM error but with additional atmospheric and baseline errors, or use a 2-Pass method with a DEM error but with reduced atmospheric and baseline effects. In practice, the choice is clear -choose a 2-Pass with a baseline appropriate to the DEM such that the effect is minimized. This stark choice overlooks a remaining option: to use the DEM within a 3-Pass scenario. Coarse DEMs are already used in conventional interferometric synthetic aperture radar (InSAR) to 'seed' a DEM generation process and so a similar approach may help to subtract the anomalous atmospheric and baseline effects found in a 3-Pass and give a better overall result than 2-Pass alone. This paper will explore the use of a DEM in monitoring land movements and identify scenarios for which a 3-Pass solution is superior to a 2-Pass solution. DINSAR ERRORS If we consider the quality of a single synthetic aperture radar (SAR) interferogram, it is clear that there are many potential sources of error. In addition to atmospheric errors, there are phase errors due to signal-to-noise ratios, number of looks, pixel misregistration and baseline decorrelation, as presented by Li and Goldstei

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

Delineating ground deformation over the Tengiz oil field, Kazakhstan, using the Intermittent SBAS (ISBAS) DInSAR algorithm

Changes in subsurface pore pressures and stresses due to the extraction of hydrocarbons often cause deformation over oil and gas fields. This can have significant consequences, including ground subsidence, induced seismicity and well failures. Geodynamic monitoring is an important requirement in recognising potential threats in sufficient time for remedial measures to be implemented. Differential interferometric synthetic aperture radar (DInSAR) is increasingly utilised for monitoring ground deformation over oil and gas reservoirs, achieving greater spatial coverage than traditional field-based surveying techniques. However, ground deformation over oil and gas fields can extend regionally into the surrounding rural landscape, where many conventional DInSAR techniques are of limited use due to the dynamic nature of the land cover. The Intermittent Small Baseline Subset (ISBAS) method is an advanced DInSAR technique, which considers the intermittent nature of coherence over dynamic land cover types to obtain markedly more ground motion measurements in non-urban regions. In this study, the ISBAS technique is used to delineate deformation over the super-giant Tengiz oil field in rural Kazakhstan. Analysis of ENVISAT data for 2004–2009 reveals a well-defined bowl subsiding with a maximum rate of −15.7 mm/year, corroborated by independent DInSAR studies and traditional levelling data. Subsequent application of ISBAS to Sentinel-1 data reveals significant evolution of deformation over the field in 2016–2017, with subsidence increasing dramatically to a maximum of -79.3 mm/year. The increased density of measurements obtained using the ISBAS technique enables accurate and comprehensive delineation and characterisation of ground deformation in this rural landscape, without the need for corner reflectors. This enhanced information could ultimately aid reservoir characterisation and management, and improve understanding of the risk posed by ground subsidence and fault reactivation

Assessing the feasibility of a National InSAR Ground Deformation Map of Great Britain with Sentinel-1

This work assesses the feasibility of national ground deformation monitoring of Great Britain using synthetic aperture radar (SAR) imagery acquired by Copernicus’ Sentinel-1 constellation and interferometric SAR (InSAR) analyses. As of December 2016, the assessment reveals that, since May 2015, more than 250 interferometric wide (IW) swath products have been acquired on average every month by the constellation at regular revisit cycles for the entirety of Great Britain. A simulation of radar distortions (layover, foreshortening, and shadow) confirms that topographic constraints have a limited effect on SAR visibility of the landmass and, despite the predominance of rural land cover types, there is potential for over 22,000,000 intermittent small baseline subset (ISBAS) monitoring targets for each acquisition geometry (ascending and descending) using a set of IW image frames covering the entire landmass. Finally, InSAR results derived through ISBAS processing of the Doncaster area with an increasing amount of Sentinel-1 IW scenes reveal a consistent decrease of standard deviation of InSAR velocities from 6 mm/year to ≤2 mm/year. Such results can be integrated with geological and geohazard susceptibility data and provide key information to inform the government, other institutions and the public on the stability of the landmas