Evaluation of subsidence from DinSAR techniques using Envisat-ASAR data at Toluca Valley Basin, Mexico.

Transcription

1 Evaluation of subsidence from DinSAR techniques using Envisat-ASAR data at Toluca Valley Basin, Mexico. Norma Angélica Dávila Hernández 1 Delfino Madrigal Uribe 1 Xanat Antonio Némiga 1 1 Autonomous University of Mexico State (UAEM) Faculty of Geography. Cerro de Coatepec, Ciudad Universitaria s/n, Toluca, Estado de Mexico CP nadavilah@uaemex.mx Abstract. The Aquifer of Toluca Valley (ATV) belongs to the upstream basin of Lerma River in Central Mexico. It extends over 2, sq km, and is considered the second most overexploited aquifer in Mexico. Its overexploitation began since 1942, providing drinking water for Toluca and Mexico City inhabitants. This has produced the compaction of geologic strata (sequences of volcanoclastic and lacustrine deposits), thus giving rise to crackings and spatial patterns of fractures associated to land subsidence. The increasing drilling of pumping wells around urban and industrial areas has also caused the gradual extinction of springs, wetlands and groundwater recharge areas. In this paper we studied land subsidence at ATV, based on PSI time series technique (for the period ) considering the aquifer over-exploitation as a factor of land subsidence. Persistent Scatterer Interferometry (PSI) is one the most used DinSAR methods to model land subsidence. It is based on coherent pixels called persistent scatterers through time intervals and over large areas of repeat-pass SAR dataset. Preliminary results show a subsidence rate of 60 mm/year, with maximum displacements by land subsidence over the city center and farming areas in northern Valley. The statistical validation showed a direct correlation between PS candidates (maximum subsidence) and water level drop at this aquifer, which reached a peak declination in Keywords: Persistent Scatterer Interferometry, radar images, pumping wells, aquifer. 1. Introduction The Aquifer of Toluca Valley (ATV) belongs to the upstream basin of Lerma River in Central Mexico (Figure 1). It extends over 2, sq km, and is considered the second most overexploited aquifer in Mexico (CONAGUA, 2009). Its overexploitation began since 1942, providing drinking water for Toluca and Mexico City inhabitants. Nowadays, by means of pumping wells, this is the main source of water for 30% of Mexico Valley and 100% of Toluca Valley inhabitants. Near to 256 pumping wells operate in the ATV, at depth range from 20 to 300 meters, generating groundwater overexploitation. This in turn, has produced the compaction of geologic strata (sequences of volcanoclastic and lacustrine deposits), thus giving rise to crackings and spatial patterns of fractures associated to land subsidence. The increasing drilling of pumping wells around urban and industrial areas has also caused the gradual extinction of springs, wetlands and groundwater recharge areas (Garfias et al., 2007). The use of remote sensing techniques such as Differential Interferometry (DinSAR) has become a powerful tool for the spatial evaluation of land subsidence in urban and agricultural areas, due to its efficiency to obtain high-accuracy measurements of large areas (Raucoles et al., 2003; Croseto et al., 2005, Herrera et al., 2009; Lopez et al., 2009; Lopez et al., 2009; Akcin et al., 2010; Honh Hong et al., 2010 ; Osmanoglu et al., 2011; Cigna et al., 2011 ; Rodriguez et al., 2012; Chen et al., 2012; Calderhead et al., 2012). Persistent Scatterer Interferometry (PS1) is one the most used DinSAR methods to model land subsidence, which is based on coherent pixels called persistent

2 scatterers through time intervals and over large areas of repeat-pass SAR dataset (Ferreti et al., 2000; Mora et al., 2003; Werner et al., 2003). In this paper we studied land subsidence at ATV based on a PS time series technique of years 2003 to 2010, considering the aquifer overexploitation, due to urban growth, as determining factor of land subsidence. Figure 1. Aquifer of Toluca Valley (AVT) with pumping wells mostly located in Toluca City. 2. Methodology Preliminary studies on land subsidence around urban areas at Toluca Valley have used conventional DinSAR techniques (see Calderhead et al., 2010 and Calderhead et al., 2012). We used PSI instead for being a more accurate method, due to selection of coherent pixels called persistent scatterers through time intervals and over large areas in repeat-pass SAR (Ferreti et al., 2000; Colesanti et al., 2003; Crosseto et al., 2005). To do so, 30 Envisat-ASAR SLC (Single Light Complex) data for the period were acquired. The conditional rule minimum baselines was established, selecting only 20 SLC data for PSI analysis; in addition coherence values upper to 0.75 were accepted. From baseline process, the time acquisition 2003/03/26 was selected as reference data (Figure 2). Then we obtained a deformation map, which shows the displacement velocity in mm/year. The PSI validation consisted of the spatial correlation between near group of PS pixels with high displacements velocity (deformation velocity) and the lowering of water level from the nearest piezometric data. Finally, field evidences of land subsidence were recorded using a differential Geographic Positioning System (dgps).

3 Figure 2. Minimum baselines for SLC Envisat-ASAR candidate pair selection for PSI analysis. 3. Results and Discussion Initially 20 differential interferograms were obtained for the period under study ( ), but the spatial continuity of fringes cycle (-π to π) was developed only on the urban area because the coherence values were null around the mountainous area. According to Figure 3, an evident increase in terms of differential phase (ground deformation) was observed with respect to the following time acquisition data: 2006/05/24, 2008/04/23 and 2009/11/04. Subsequently, a deformation map was calculated with PS method for the above mentioned. Figure 4 shows a map deformation composite by PS candidates with capability to detect displacements in millimeters and therefore to know the deformation (displacement) velocity within a given period of time. Thus, PSI showed a subsidence rate of 60 mm/year, with maximum displacement velocities over the region of industrial parks, recent civil infrastructure and farming areas. However, field evidence did not provide physical evidences related to differential movements over the farming areas. 3.1 Piezometric validation As validation, records of the nearest piezometers to PS pixels with high displacements velocity are next discussed. Figure 5 shows the historical evolution for the piezometer PL-201(sound 12 m depth) located at the historical city center of Toluca and Figure 6 shows the piezometer PL-218 (sound 12 m depth) located at north of Valley. In both cases, the groundwater level begins to have a decrement of near 2 m around the first period of Meanwhile, selected PS have the same tendency: before that date the subsidence is about and after it presents an exponential growth (values up 175 mm). It is important to note that the subsidence rates from PS have a direct relation with the decrease of groundwater level; however this water level could vary depending of the period of aquifer recharge and the compaction of geological units.

4 Figure 3. Differential interferograms (DinSAR) for , using 2003/03/26 as reference Figure 4. Deformation map (in mm/year) derived from Persistent Scatterer method. Note: The Pink box remarks the group of PS pixels with high displacements velocity.

5 Figure 5. Correlation between piezometer 201 and PS pixels with high displacements velocity. Figure 6. Correlation between piezometer 218 and PS pixels with high displacements velocity. 5. Conclusions We applied the Persistent Scatterer Interferometry (PSI) method to diagnose the process of subsidence at Toluca Valley using 20 Envisat-ASAR from the period Preliminary results show a subsidence rate of 60 mm/year, with maximum displacements associated to land subsidence in the historic city center and farming areas in northern Valley. The statistical validation showed a direct correlation between PS candidates (maximum subsidence) and water level drop at this aquifer, which reached a peak declination in Therefore, PSI was a useful Interferometry Differential method (DinSAR) by means of pin point accuracy to evidence the historical evolution of land deformation in urban areas due to aquifer overexploitation.

6 6. Bibliography Calderhead, A; Martel, A; Alasset, P.J; Rivera, A; Garfias, J. Land subsidence induced by groundwater pumping, monitored by D-InSAR and field data in the Toluca Valley, Mexico. Canadian Journal of Remote Sensing, v. 36, n. 1, p Calderhead, A; Therrien, R; Rivera, A; Martel; Garfias, J. Simulating pumping-induced regional land subsidence with the use of InSAR and field data in the Toluca Valley, Mexico. Advances in Water Resources, v. 34, n. 1, p Werner, C; Wegmüller. U; Strozzi, T; Wiesmannn, A. Interferometric point target analysis for deformation mapping. In: Geosciences and remote sensing symposium, Toulouse, France, Jul 2003, pp 1 3 (CDROM). Colesanti, C; Ferretti, A; Novali, F; Prati, C; Rocca, F. SAR monitoring of progressive and seasonal ground deformation using the Permanent Scatterers Technique. IEEE Transactions on Geoscience and Remote Sensing 41(7): Acknowledgements We gratefully acknowledge ESA (European Space Agency) for providing ENVISAT-ASAR data by means of Proposal-Project PI This study is part of the project CONACYT