Increasing demand for groundwater for agricultural and industrial needs puts pressure on the water table, especially in arid and semiarid climates. In the Kabudar Ahang plain, in Hamedan province, groundwater overexploitation for agricultural and industrial application has increased significantly over recent decades. The InSAR technique is used to monitor the subsidence induced by groundwater overexploitation in the Kabudar Ahang plain from 2003 to 2010. For this purpose, three different Envisat tracks are used. Interferogram stacking and time-series analysis are performed to study the short-term and long-term behavior of the subsidence. Interferogram time-series analysis from 2003 to 2006 estimates an average subsidence of 19 cm per year with a maximum of 25 cm. The maximum rate of 27 and 28 cm per year is estimated from interferogram stacking in 2004 to 2005 and 2007 to 2010, respectively. The results are further compared with the geological and hydrological information to investigate the relation between the subsidence and groundwater level variations. A high correlation is found in areas affected by subsidence and the rate of water level drop in 10 piezometric wells. The results show that a large part of the plain subsides, mostly in the south and southeast where the aquifer is thicker and in the areas with the highest water table dropdown of about 100 m. With the continued use of groundwater resources, subsidence is another hazard in this plain in addition to the sinkholes.
Land subsidence resulting from groundwater extraction is a widely recurring phenomenon worldwide. To assess land subsidence, traditional methods such as numerical and finite element methods have limitations due to the complex interactions between the different constructor factors of aquifer in each area. We produced a groundwater-induced subsidence map by applying the geological and hydrogeological information of the aquifer system using an artificial neural network (ANN) combined with interferometric synthetic aperture radar (InSAR) and geospatial information system. The main problem with neural networks is providing the ground-truth dataset for training step. Therefore, the subsidence rate used as the network output was estimated using the InSAR time series analysis method. This study indicates the ANN approach is capable of knowing the mechanism of the land subsidence and can be used as a complementary of InSAR method to estimate the land subsidence with effective parameters and accessible data such as groundwater-level data especially in those areas in which measuring the subsidence was not feasible using InSAR. However, the results indicated that average groundwater depth and groundwater level decline were the most effective factors influencing subsidence in the study area using sensitivity analysis.
The potential of synthetic aperture radar (SAR) interferometry was shown to study the compaction of the aquifer system in Darab plain, Iran. In so doing, two different datasets, including Envisat advanced SAR (ASAR) spanning 2010 and Sentinel-1A spanning 2016 to 2017, were applied in small baseline subset time series analysis. To estimate the subsidence in the time period for which there is no SAR data available, i.e., 2010 to 2016, the time series analysis results separately obtained from the two datasets were to be integrated using an appropriate model, which should have been fitted to both sets of results. However, as both deformation time series results were calculated taking into account a distinct temporal reference, fitting the model was not a straightforward task. Accordingly, the main attempt was to find the subsidence value corresponding to the temporal reference of Sentinel-1A time series with respect to that of Envisat ASAR. This shift value was optimally determined using a genetic algorithm so as to minimize the misfit between the model and the deformation time series corresponding to the entire period. The average value of the root mean square error estimated as the misfit between the model and the calculated time series at all pixels is 0.011 m, which is an indication of the high performance of the proposed method for modeling the deformation time series. The integration results were further used to derive the stress–strain relationships to study the storage properties of the aquifer system. The fact that the strain linearly increases along with the decrease in water level in most piezometric wells indicates that the subsidence is highly correlated with groundwater exploitation.
Persistent scatterer interferometry (PSI) techniques using amplitude analysis and considering a temporal deformation model for PS pixel selection are unable to identify PS pixels in rural areas lacking human-made structures. In contrast, high rates of land subsidence lead to significant phase-unwrapping errors in a recently developed PSI algorithm (StaMPS) that applies phase stability and amplitude analysis to select the PS pixels in rural areas. The objective of this paper is to present an enhanced algorithm based on PSI to estimate the deformation rate in rural areas undergoing high and nearly constant rates of deformation. The proposed approach integrates the strengths of all of the existing PSI algorithms in PS pixel selection and phase unwrapping. PS pixels are first selected based on the amplitude information and phase-stability estimation as performed in StaMPS. The phase-unwrapping step, including the deformation rate and phase-ambiguity estimation, is then performed using least-squares ambiguity decorrelation adjustment (LAMBDA). The atmospheric phase screen (APS) and nonlinear deformation contribution to the phase are estimated by applying a high-pass temporal filter to the residuals derived from the LAMBDA method. The final deformation rate and the ambiguity parameter are re-estimated after subtracting the APS and the nonlinear deformation from that of the initial phase. The proposed method is applied to 22 ENVISAT ASAR images of southwestern Tehran basin captured between 2003 and 2008. A quantitative comparison with the results obtained with leveling and GPS measurements demonstrates the significant improvement of the PSI technique.
Precise leveling surveys across southwest of Tehran have revealed a significant subsidence due to the overexploitation of groundwater. In order to monitor the temporal evolution of the deformation, Interferometric SAR time series analysis was applied using ENVISAT ASAR images recorded between 2003 and 2005. Only Interferograms with small temporal baselines are processed to decrease the temporal decorrelation effect caused by the agricultural fields. However, the spatial baselines of the processed interferograms are not as small as in the conventional Small Baseline Subset (SBAS) method. Coherence analysis reveals that the spatial decorrelation is insignificant. However, since the constructed interferograms are affected by topographic artifacts caused by the large spatial baselines, a multi-step procedure was used in order to refine the interferometric phase. Smoothed time series analysis was then carried out to retrieve the atmospheric-error free deformation corresponding to every acquisition time. The mean displacement velocity map extracted from the time series results indicates a maximum subsidence rate of 24 cm/yr. Chronological sequence of the computed deformations for several points located in the subsidence area shows the permanent aquifer system compaction at a long-term constant rate on which the seasonal effects are superimposed. Sustained hydraulic head declines reveal a relatively low correlation with InSAR derived information. Comparison of the subsidence rate to soil type profiles in different parts of the subsidence area was then used to interpret the deformation signal.
The development of the polarimetric synthetic aperture radar (PolSAR) applications has been accelerated by coming of
new generation of SAR polarimetric satellites (TerraSAR-X, COSMO-SkyMed, RADARSAT-2, ALOS, etc.). The aim
of this article is to extract the information content of the polarimetric SAR data. Cross products of four channels "HH,
HV, VH, and VV" could be at least nine features in vector space and by applying the different class separability
criterion, the impacts of each feature, for extracting different patterns, could be tested. We have chosen the large distance
between classes and small distance within-class variances as our criterion to rank the features. Due to high mutual
correlation between some of the features, it is preferable to combine the features which result in the lower number of
features. Also the computational complexity will be decreased when we have lower number of features. Due to these
advantages, our goal would be to decrease the number of features in vector space. To achieve that, a subset of ranked
features consists of two to nine ranked features will be classified and the classification accuracy of different subsets will
be evaluated. It is possible that some of the new features that have been added to the old subsets change the classification
accuracy. Finally different feature subsets which were selected based on the various class-separability approaches will be
compared. The subset that gives the highest overall accuracy would be the best representative of the nine originally
features.
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