Radar Interferometry For Monitoring Land Subsidence due to Over-Pumping Groundwater in Crete, Greece

Abstract

The level of water in several wells at the Messara valley, Crete, Greece has dropped 40 meters over the last 20 years. Anecdotal reports describe cracks in the concrete foundations of some residential structures around the valley. These also suggest that ground subsidence may exceed one centimeter, at least, over a period of recharging and withdrawal ground water. Conventional differential SAR interferometry using ERS-SAR and ALOS-PALSAR images, as well as the stacking technique have been applied to monitor this ground deformation. The used images covered the period from 1992 to 2000 and 2007-2009, respectively. A total of 29 ERS-1&2 SAR and 7 ALOS images have been used for forming interferograms. Image pairs with Doppler difference less than 0.20 pulse repetition frequency, and perpendicular baseline smaller than 100m have been used for processing. A Digital Elevation Model with 20-m pixel size and ±7 m height accuracy has also been incorporated in interferometric processing. Atmospheric artifacts have been compensated by using image stacking. The valley to be monitored is densely cultivated and irrigated. Thus, loss of coherence in images has been observed in the C-Band and could not be overcome. Consequently, interferometric results with the ERS-SAR images have been limited. On the other hand, processing of L-band data has brought up a ground deformation that amounted to a subsidence of at least 4 cm/yr. The correlation of the observed ground deformation with respect to water pumping and other geological parameters has also been investigated.