Assessing sinkhole activity in the Ebro Valley mantled evaporite karst using advanced DInSAR
Introduction
Subsidence sinkholes are closed depressions commonly generated by subsurface dissolution of soluble rocks and the downward displacement of the overlying materials and the ground surface (Waltham et al., 2005). Three main subsidence mechanisms may operate independently or jointly: sagging, collapse, and suffusion (Gutiérrez et al., 2008a, Cooper and Gutiérrez, 2013). The settlement of the land surface associated with sinkholes, regardless of the subsidence mechanism, may cause severe damage in any human structure borne by the undermined sediments. For the latest reviews on sinkhole related damage, see the introduction section in Guerrero et al. (2008), Table 1 in Gutiérrez et al. (2009) and supplementary material in Galve et al. (2012). Collapse sinkholes related to the failure of cavity roofs may occur catastrophically and cause personal losses. These rapid sinkholes may directly engulf buildings or people (e.g. De Bruyn and Bell, 2001) and lead to fatal accidents when they form in transportation routes. Previous investigations document slow ground subsidence (creep) preceding sudden collapse, suggesting that the occurrence of these highly dangerous phenomena might be anticipated through the detection of precursory displacements (e.g. Ferretti et al., 2004, Nof et al., 2013).
The avoidance of the existing sinkholes and the areas more prone to new sinkhole occurrences is usually the most cost-effective mitigation option (Cooper and Calow, 1998, Gutiérrez, 2010). This preventive strategy is commonly based on the production of cartographic sinkhole inventories and susceptibility and hazard models derived from them (e.g. Galve et al., 2009a, Galve et al., 2011). However, sinkhole mapping is frequently a difficult task due to factors like the obliteration of the geomorphic expression of the depressions by anthropogenic and natural processes. Moreover, there is frequently a high uncertainty regarding the activity of the existing subsidence sinkholes. It is desirable to discriminate which are the active sinkholes and obtain information on their kinematics: (1) deformation regime (continuous, episodic or a combination of both); (2) subsidence rate; and (3) spatial variability of the deformation pattern, including the precise limits of the area affected by ground motion. The assessment of the activity of sinkholes is usually based on qualitative data, like the freshness vs. degradation state of the depressions, the presence of recent surface deformation features or damaged structures. The application of traditional geodetic methods over large areas is not practical due to the dispersed and localized nature of the karst subsidence process.
Since the early 2000s (e.g. Baer et al., 2002), Differential Interferometric Synthetic Aperture Radar (DInSAR) techniques have opened promising and innovative prospects in sinkhole investigation. The use of SAR interferometry for the study of dissolution-induced subsidence was first applied along the Israeli Dead Sea shores (Derauw and Moxhet, 1996a, Derauw and Moxhet, 1996b, Cornet et al., 1997, Derauw, 1999, Baer et al., 2002). Subsequently, DInSAR studies have also been developed in karst areas of Germany, Italy, Jordan, Spain and USA. These works document: (1) gradual settlement in areas affected by sinkholes (Abelson et al., 2003, Al-Fares, 2005, Castañeda et al., 2009, Castañeda et al., 2011, Rucker et al., 2013); (2) displacements preceding the catastrophic collapse of cavities (Ferretti et al., 2000, Ferretti et al., 2004) or human structures underlain by karstified sediments (Closson et al., 2005, Closson et al., 2010); and (3) the relationships between karst subsidence and its triggering factors, such as earthquake shaking (Closson et al., 2010). Gutiérrez et al. (2011) presented a review of the abovementioned works and a research based on the combination of DInSAR displacement maps with conventional geomorphological methods, Ground Penetrating Radar (GPR) and trenching in the Ebro Valley (NE Spain). Dahm et al. (2010) combined geophysical data sets with InSAR-derived subsidence rates obtained by Schäffer (2009) in Hamburg, Germany. Paine et al. (2012) applied ALOS interferograms to guide site-specific gravimetric investigations over large sinkholes (90 to 200 m in diameter) related to deep-seated dissolution of salt in Texas, USA. The subsidence areas and rates (up to 30 mm/yr) generally matched previous data obtained by geodetic surveys. Since 2007, the availability of high resolution data and the improvement of DInSAR techniques and computing processes make advanced-DInSAR (PS, SBAS) a promising technique even in areas that may pose temporal decorrelation limitations. This paper evaluates the capability of SAR data with different wavelengths and advanced DInSAR techniques to overcome some of the constraints identified in previous works related to sinkhole activity: 1) the general limited coherence in agricultural areas and specially the frequent atmospheric noise in the study area (Castañeda et al., 2011), 2) insufficient spatial resolution to detect most of the active karst features (Castañeda et al., 2009); and 3) the restricted measurable deformation range that determines the identification of the active sinkhole according to their subsidence rates. Three DInSAR velocity maps are compared with a comprehensive sinkhole inventory in a sector of the Ebro Valley to analyze (1) the improvements and limitations of the more recently developed DInSAR products; (2) the contribution of the new data in active sinkhole detection and characterization; and (3) the implications for sinkhole risk management. Additionally, areas affected by subsidence identified using DInSAR that were not previously inventoried were surveyed to semi-quantitatively assess sinkhole-related damage on human structures.
Section snippets
Geological setting
The investigation is focused on the mantled evaporite karst of Zaragoza metropolitan area, located in the middle reach of the Ebro Valley, NE Spain (Fig. 1a and b). The bedrock in this sector of the Ebro Cenozoic Basin consists of sub-horizontally lying evaporites of the Zaragoza Gypsum Formation (Quirantes, 1978). Secondary gypsum is the dominant rock type in outcrop and highly soluble halite and glauberite units up to 70 m and 30 m thick, respectively, occur a few tens of meters beneath the
DInSAR processing
Differential Synthetic Aperture Radar Interferometry (DInSAR) is a microwave remote sensing technique that enables measuring surface displacement with sub-centimeter accuracy by subtracting the topographic phase from SAR interferograms (e.g. Massonnet and Feigl, 1998) by assuming that the scattering characteristics of the ground surface (ground nature and geometry) remain undisturbed (Rosen et al., 2000). Advanced Time-Series DInSAR techniques use two main approaches to measure ground
Assessment of DInSAR velocity maps for sinkhole detection
The main results of the comparative analysis between the three DInSAR velocity maps (Fig. 4b, c and e) and the available sinkhole map (Fig. 4a) are featured below. The results are presented according to a multi-scale approach. A general analysis covering the entire study area (Section 4.1) is followed by a site- and sinkhole-specific analysis (Section 4.2). See Fig. 4a for location of analyzed sites and sinkholes.
Detection of active sinkholes using DInSAR techniques
The previously produced and analyzed SBAS-map (Castañeda et al., 2009), with a pixel size of ~ 90 m, is compared with the two new velocity maps (C-SPN- and L-SPN-maps) with higher spatial resolution (Table S1, see online Supplementary material). Both C- and L-SPN-maps have increased the density of measurement points by 23 and 6 times, respectively (Table S2, see online Supplementary material), resulting in the recognition of a higher number of active sinkholes and detection of smaller subsidence
Acknowledgments
We wish to thank Prof. Adrian Harvey, Prof. Jo De Waele and five anonymous reviewers for their helpful comments. This research has been funded by the Spanish national projects CGL2010-16775, AGL2012-40100 and CGL2013-40867-P (Ministerio de Ciencia e Innovación and FEDER), the Regional projects 2012/GA-LC-021 and 2012/GA LC 036 (DGA-La Caixa) and the European Interreg IV B SUDOE project DO-SMS-SOE1/P2/F157. Jorge Pedro Galve has been contracted under the DGA-La Caixa project and the contract of
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