Ground motion measurement in the lake Mead area (Nevada, USA), by temporal analysis of multiple interferograms.

Olivier Cavalié(1) , Marie-Pierre Doin(1) , Cécile Lasserre(1) , and Pierre Briole(2)

(1) ENS, 24 rue Lhomond, 75231 Paris Cedex 05, France
(2) IPGP, 4, place jussieu, 75252 Paris cedex 05, France


SAR interferometry has proven to be a reliable method for detecting small displacements due to ground subsidence. In this study, we measure ground motion around the lake Mead (Nevada, USA) using InSAR. This artificial lake has been filled with water in 1935. An earlier study, based on leveling measurements, has shown that the load associated with lake impoundement has induced a subsidence of 17 centimeters. This relaxation process has been argued as analogous to the postglacial rebound, but at a smaller spatial scale and with a much lower viscous relaxation scale. To quantify the deformation and thus constrain the crust and mantle rheological parameters in the lake area, we analyse multiple interferograms (241) based on 43 ERS images acquired between 1992 and 2001. With baselines smaller than 300 m, all inteferograms have a very good coherence due to the desert region. Most of interferograms show strong atmospheric artefacts that are partly due to the variation of water vapor vertical stratification between two satellite passes. Tropospheric delay is computed for each interferogram and then inverted for each date of SAR images before interferograms correction. These corrections are validated using data from global atmospheric models (ERA40). Corrected interferograms are then inverted to solve for time series of the expected deformation in the lake Mead area . The linear inversion treats each pixel independently from its neighbours and use the data redundancy to reduce errors such as local decorrelations or local atmospheric artefacts. We obtain a time series of the expected deformation in the lake Mead area. The analysis of the deformation evolution during 1992-2001 the period shows a very good correlation between the ground motion and the water level changes. Our method allows to obtain a millimetric accuracy of a non linear deformation in time over a large spatial scale. In particulary, we observe a subsidence of up to 1.7 cm between 1995 and 1998 due to a large water level increase, followed by an uplift due to the drop of the water level after 2000. We, also, observe a local anticorrelated movement in a 20 km *10 km area located North of the lake with a significative uplift of 1.4 cm during the period of the lake area subsidence. Finally, west of the lake and independently of its water load variations, uplift is observed in Las Vegas area.



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