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InSAR time series analysis with water vapour correction model for mapping postseismic deformation after the 2003 Bam, Iran Earthquake

Zhenhong Li(1), Eric Fielding(2) and Paul Cross(1)

(1) University College London, Gower Street, London WC1E 6BT, United Kingdom
(2) JPL/Caltech, Pasadena, California, United States

Abstract

A major source of error for repeat-pass InSAR is the phase delay in radio signal propagation through the atmosphere and the part due to tropospheric water vapour only could cause errors as large as 10-20 cm in deformation retrievals. Reduction of the spatial variation of InSAR path delays using space-based water vapour measurements (e.g. the NASA Moderate Resolution Imaging Spectroradiometer (MODIS) and the ESA MEdium Resolution Imaging Spectrometer (MERIS)) have been successfully demonstrated [Li et al., 2005; 2006]. Although MERIS and MODIS near IR water vapour products are sensitive to the presence of clouds, we found that the Middle East, North Africa, South Africa, Australia, Chile, Antarctica, Southern California and North Mexico show cloud-free frequencies as high as 60% or even more.

The small baseline subset algorithm (SBAS) is a robust time series analysis approach, which mainly uses interferograms with small baselines to minimise the effects of baseline decorrelation and inaccuracies in topographic data used. Unlike persistent scatterer InSAR, phase is unwrapped in 2D image space first for SBAS (instead of in time first or in 3D), making it easy not only to implement in standard interferometric processing, but also to integrate with existing water vapour correction models. The SBAS algorithm is perfect for processing data over arid and urban regions where the coherence is generally high even for long periods with the existing C-band radar archives of ERS, Envisat and Radarsat 1. Therefore, it is clear that the integration of SBAS and MERIS/MODIS water vapour correction models shows promise to map small deformation signals over certain regions (such as the aforementioned desert areas): (1) it has the ability to map surface deformation as it evolves in time; and (2) it is able to better separate deformation signals from water vapour effects.

The postseismic surface deformation after the 2003 Bam, Iran earthquake is one order of magnitude smaller than the coseismic deformation, so the signal in individual postseismic interferograms is affected strongly by atmospheric variations. For the first time, we have been exploring the use of InSAR time series with water vapour correction model (InSAR TS + PWV) to examine the postseismic time history in the three years since the earthquake. Because MERIS near IR water vapour data is not available for 2 dates (out of 27 that used in this study) due to the presence of clouds, an Atmospheric Phase Screen (APS) model has been developed for the InSAR TS + PWV to estimate atmospheric effects for a given date using water-vapour-corrected interferograms; Cross validation reveals that the estimated APS agreed to MERIS derived path delays in line of sight with a small standard deviation (0.3~0.5 cm) and a high correlation coefficient (0.84~0.98).

Preliminary InSAR time series analysis (without water vapour correction) shows small spatial-scale ground deformation that decayed with a time constant of about 1/2 year after the earthquake but any larger spatial-scale deformation was masked by the water vapour variations [Fielding et al., 2006]. It is demonstrated that a better time series of postseismic deformation is achievable after reduction of water vapour effects by c. 50% using the InSAR TS + PWV approach with coincident MERIS near IR water vapour data. In addition, special attention is paid to short-wavelength atmospheric ripples observed in several interferograms in this presentation.

References: Fielding, E. J., P. Lundgren, Z. Li, G. Funning, and R. B├╝rgmann (2006), Post-seismic deformation after the 2003 Bam, Iran earthquake from time series analysis of Envisat InSAR, presented at the 2006 SSA Annual Meeting, 18-22 April 2006, San Francisco, California, 18 - 22 April 2006. Li, Z., E. J. Fielding, P. Cross, and J.-P. Muller (2006), Interferometric synthetic aperture radar atmospheric correction: MEdium Resolution Imaging Spectrometer and Advanced Synthetic Aperture Radar integration, Geophysical Research Letters, 33, L06816, doi:06810.01029/02005GL025299. Li, Z., J.-P. Muller, P. Cross, and E. J. Fielding (2005), Interferometric synthetic aperture radar (InSAR) atmospheric correction: GPS, Moderate Resolution Imaging Spectroradiometer (MODIS), and InSAR integration, Journal of Geophysical Research, 110, B03410, doi:03410.01029/02004JB003446.

 

Workshop presentation

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry