Showing papers by "Howard A. Zebker published in 2003"

Inverse modeling of interbed storage parameters using land subsidence observations, Antelope Valley, California

Abstract: [1] We use land-subsidence observations from repeatedly surveyed benchmarks and interferometric synthetic aperture radar (InSAR) in Antelope Valley, California, to estimate spatially varying compaction time constants, τ, and inelastic specific skeletal storage coefficients, S*kv, in a previously calibrated regional groundwater flow and subsidence model The observed subsidence patterns reflect both the spatial distribution of head declines and the spatially variable inelastic skeletal storage coefficient Using the nonlinear parameter estimation program UCODE we estimate compaction time constants between 38 and 285 years The S*kv values are estimated by linear estimation and range from 0 to almost 009 We find that subsidence observations over long time periods are necessary to constrain estimates of the large compaction time constants in Antelope Valley The InSAR data used in this study cover only a three-year period, limiting their usefulness in constraining these time constants This problem will be alleviated as more SAR data become available in the future or where time constants are small By incorporating the resulting parameter estimates in the previously calibrated regional model of groundwater flow and land subsidence we can significantly improve the agreement between simulated and observed land subsidence both in terms of magnitude and spatial extent The sum of weighted squared subsidence residuals, a common measure of model fit, was reduced by 73% with respect to the original model However, the ability of the model to adequately reproduce the subsidence observed over only a few years is impaired by the fact that the simulated hydraulic heads over small time periods are often not representative of the actual aquifer hydraulic heads Errors in the simulated hydraulic aquifer heads constitute the primary limitation of the approach presented here

Prospecting for horizontal surface displacements in Antelope Valley, California, using satellite radar interferometry

Abstract: [1] We compare interferometric synthetic aperture radar displacement maps for Antelope Valley, California, derived for two different satellite orbit geometries to test the common assumption that surface displacements above deforming aquifer systems are purely vertical. This assumption greatly simplifies models of aquifer system deformation related to changes in groundwater flow and storage, permitting the use of one-dimensional codes for characterizing the compaction occurring in the flow model. The analysis is complicated by interferometric phase noise and atmospheric signal contributions. We use a variance-weighted least squares approach to optimally use the information contained in several interferograms. Our analysis of the displacement maps does not identify significant displacement differences for the two acquisition geometries, implying that the motions are indeed nearly vertical. Uncertainties in the interferometric phase measurement and atmospheric delay signals preclude any conclusion regarding the existence of horizontal displacements smaller than ∼2 cm. The absence of spatial correlation between the observable differences and the known subsidence field indicates that horizontal displacements are indeed negligible for inelastic aquifer system compaction in Antelope Valley.

Radar stereo- and interferometry-derived digital elevation models: comparison and combination using Radarsat and ERS-2 imagery

Abstract: In this experiment, we derive and compare radar stereo and interferometric digital elevation models (DEMs) of a study site in Djibouti, East Africa A Radarsat stereo pair, as well as Radarsat and ERS-2 interferometric data, comprise the test images The primary objective of the study was to analyse and compare the results obtained by the two techniques and explore possible synergisms between them We find that in regions of high coherence, the DEMs produced by interferometry are of much better quality than the stereo result However, the corresponding error histograms also show some pronounced errors due to decorrelation and phase-unwrapping problems on forested mountain slopes On the other hand, the more robust stereo reconstruction, with an error standard deviation of 45 m, is able to capture the general terrain shape, although finer surface details are lost In the second part of our experiment, we demonstrate that merging the stereoscopic and interferometric DEMs by applying a user-defined weighting

An L-band SAR for repeat pass deformation measurements on a UAV platform

Abstract: We are proposing to develop a miniaturized polarimetric L-band synthetic aperture radar (SAR) for repeat-pass differential interferometric measurements of deformation for rapidly deforming surfaces of geophysical interest such as volcanoes or earthquakes that is to be flown on a unmanned aerial vehicle (UAV) or minimally piloted vehicle (MPV). Upon surveying the capabilities and availabilities of such aircraft, the Proteus aircraft and the ALTAIR UAV appear to meet our criteria in terms of payload capabilities, flying altitude, and endurance. To support the repeat pass deformation capability it is necessary to control flight track capability of the aircraft to be within a specified 10 m tube with a goal of 1 m. This requires real-time GPS control of the autopilot to achieve these objectives that has not been demonstrated on these aircraft. Based on the Proteus and ALTAIR's altitude of 13.7 km (45,000 ft), we are designing a fully polarimetric L-band radar with 80 MHz bandwidth and a 16 km range swath. The radar will have an active electronic beam steering antenna to achieve a Doppler centroid stability that is necessary for repeat-pass interferometry. This paper presents some of the trade studies for the platform, instrument and the expected science.

Inverse modeling of interbed storage parameters using land subsidence observations, Antelope Valley, California

Abstract: [1] We use land-subsidence observations from repeatedly surveyed benchmarks and interferometric synthetic aperture radar (InSAR) in Antelope Valley, California, to estimate spatially varying compaction time constants, τ, and inelastic specific skeletal storage coefficients, S*kv, in a previously calibrated regional groundwater flow and subsidence model. The observed subsidence patterns reflect both the spatial distribution of head declines and the spatially variable inelastic skeletal storage coefficient. Using the nonlinear parameter estimation program UCODE we estimate compaction time constants between 3.8 and 285 years. The S*kv values are estimated by linear estimation and range from 0 to almost 0.09. We find that subsidence observations over long time periods are necessary to constrain estimates of the large compaction time constants in Antelope Valley. The InSAR data used in this study cover only a three-year period, limiting their usefulness in constraining these time constants. This problem will be alleviated as more SAR data become available in the future or where time constants are small. By incorporating the resulting parameter estimates in the previously calibrated regional model of groundwater flow and land subsidence we can significantly improve the agreement between simulated and observed land subsidence both in terms of magnitude and spatial extent. The sum of weighted squared subsidence residuals, a common measure of model fit, was reduced by 73% with respect to the original model. However, the ability of the model to adequately reproduce the subsidence observed over only a few years is impaired by the fact that the simulated hydraulic heads over small time periods are often not representative of the actual aquifer hydraulic heads. Errors in the simulated hydraulic aquifer heads constitute the primary limitation of the approach presented here.