Mexico City subsidence: Spatio-temporal analysis using interferometry
Lopez-Quiroz Penelope(1), Doin Marie Pierre(1), Tupin Florence(2), Briole Pierre(1) and Nicolas Jean Marie(2)
(1) Ecole Normale Superieure, 24 rue Lhomond, 75231 Paris Cedex 05, France
(2) ENST, 46 rue Barrault, 75634 Paris Cedex 13, France
In Mexico, one of the most populated cities in the world, the excessive water consumption leads to a serious problem of subsidence with rates as high as 40 cm per year in some parts of the city. This is one of the highest subsidence rates ever registered in a city of the world.
The appearance of Mexico city has been changing through time. Tenochtitlan, the ancient Mexican capital, was built over an island located in the middle of a lake filling the southern part of the Mexico Valley, an endoreic basin surrounded by mountains. After the Spanish conquest, flooding problems forced conquerors to dry the nearby lakes by drainage. Nowadays, the Mexico Valley lakes have almost completely disappeared; their surfaces have been progressively replaced by buildings of the current Mexican capital.
The simplified hydrogeologic structure of Mexico Valley includes a superficial 50 to 300 m thick lacustrine aquitard overlying a thicker aquifer made of alluvial deposits (sand, gravel, pyroclastic material). The aquitard layer plays a crucial role in the subsidence process due to the extremely high compressibility of its clay deposits separated by a less compressible sand layer where the biggest buildings of the city are anchored. Underground water originating from the aquifer contributes to 70 % of the water supply of Mexico city. The aquifer over-exploitation leads to a large scale 30m depression of its piezometric level, inducing water downwards flow in the clays, yielding compaction and subsidence (Rivera, 1990). In order to quantitatively link subsidence to water pumping, the Mexico city subsidence needs to be mapped and analyzed through space and time.
Radar interferometry techniques (InSAR, Interferometric Synthetic Aperture Radar), have been successfully applied to map wide-scale subsidence caused by water pumping (e. g., Amelung et al., 2000). We present a work based on interferometry that allow to measure the subsidence spatial pattern and temporal evolution. Using a 33 ENVISAT images data base from 2003 to 2006, we compute 49 differential interferograms with a perpendicular baseline smaller than 500 m and a temporal baseline smaller than 9 months, linking all images by at least 1 interferogram. Interferograms are corrected from a residual orbital effect and from a layered atmospheric phase screen effect responsible for a phase elevation relationship. The analyse of the few well unwrapped 1-2 months interferograms shows that the subsidence patterns are very stable from one time period to another. However a very accurate quantification of subsidence through space and time requires the use of all images and thus interferograms with time span larger than 2 months. This also allows to mitigate the impact of atmospheric artifacts. Because of the extremely high subsidence rates, interferograms performed with a time span larger than 2 months contain very high fringe rate gradients. The high number of fringes and the loss of coherence seriously complicates the unwrapping step. Unwrapping of each individual interferogram is then performed iteratively, using as a priori information the subsidence spatial pattern derived from a stack of the best 1-2 months well unwrapped interferograms. This method was successfull for unwrapping all 49 interferograms over most of Mexico city urban area. Inverting the interferograms into time series then proves that the subsidence process is extremely linear through time for the observed time span (2003-2006).
Keywords: ESA European
Space Agency - Agence spatiale europeenne,
observation de la terre, earth observation,
satellite remote sensing,
teledetection, geophysique, altimetrie, radar,
chimique atmospherique, geophysics, altimetry, radar,