Relationship between piezometric level and ground deformations measured by means of DInSAR in the Vega Media of the Segura River (Spain)
Roberto Tomás(1) , Yolanda Máquez(1)
, Juan Manuel Lopez-Sanchez(1)
, José Delgado(1)
, Pablo Blanco(2)
, and Jordi Mallorquí(2)
Universidad de Alicante,
Escuela Politécnica Superior, PO. Box 99,
03080 - Alicante,
(2) Universitat Politècnica de Catalunya, c/Jordi Girona, 1-3, Ed. D-3, 08034 - Barcelona, Spain
Differential SAR Interferometry (DInSAR) is a remote sensing method with the well demonstrated ability to monitor geological hazards like earthquakes, landslides and subsidence. Among all these hazards, subsidence involves the settlement of the ground surface affecting wide areas. Frequently, subsidence is induced by excessive overexploitation of aquifers and constitutes a common problem that affects developed societies. The excessive pumping of underground water decreases the piezometric level in the subsoil and, as a consequence, increases the effective stresses with depth causing a consolidation of the soil columns. These consolidations originate a settlement of ground surface that must be withstood by civil structures built on these areas.
In this paper we make use of an advanced DInSAR approach - the Coherent Pixels Technique (CPT)  - to monitor subsidence induced by aquifer overexploitation in the Vega Media of the Segura River (SE Spain) from 1993 to the present. This method assumes that the deformation component of the differential interferometric phase can be broken down into two phase terms, one due to linear deformation and another due to non linear deformation. The retrieval of the linear term includes the estimation of both the DEM error and the mean velocity deformation. These are calculated by adjusting a model function applied only over those pixels of the scene that show good interferometric coherence along time. The non-linear term is estimated by applying spatio-temporal filtering to extract the contribution of atmospheric artefacts and the low and high-resolution components of the non-linear deformation. Atmospheric isolation is possible because of the different behaviour of non-linear movement with respect to atmospheric artefacts in time and space.
28 ERS-1/2 scenes covering a time interval of about 10 years were used to study this phenomenon. The deformation map retrieved with CPT technique shows settlement of up to 80 mm at some points of the studied zone. These values agree with data obtained by means of borehole extensometers, but not with the distribution of damaged buildings, well points and basements because the occurrence of damage also depends on the structural quality of the buildings and their foundations. The most interesting relationship observed is the one existing between piezometric changes, settlement evolution and local geology. Three main patterns of ground surface and piezometric level behaviour have been distinguished for the study zone during this period: 1) areas where deformation occurs while ground conditions remain altered (recent deformable sediments), 2) areas with no deformation (old and non-deformable materials) and 3) areas where ground deformation mimics piezometric level changes (expansive soils). The relationship between deformation pattern and soil has been analysed in this work. Moreover, this technique has also allowed the measuring of ground subsidence for a period (1993-1995) where no instrument information was available.
 O. Mora, J.J. Mallorquí, and A. Broquetas, “Linear and nonlinear terrain deformation maps from a reduced set of interferometric SAR images,” IEEE Trans. Geosci. Remote Sensing, vol. 41, pp. 2243-2253, Oct. 2003.