InSAR Methods Research Articles

Monitoring of Guadalentín valley (southern Spain) through a fast SAR Interferometry method

Abstract Ground subsidence is a critical problem in populated areas, which requires attention and routine monitoring. Specifically, subsidence related to the exploitation of aquifer systems is of particular interest in terms of risk for the possible damage to buildings and availability of water resources. We propose to realise a survey of the aquifer systems near the city of Lorca, southern Spain, using Synthetic Aperture Radar Interferometry (InSAR) with a light method necessitating a small number of interferograms rather than analysing hundreds of radar images. We computed nine interferograms with good coherence over the years 2004 and 2005. We analysed them by a two-step filtering process in order to characterise the atmospheric contribution and the ground deformation. We are then able to identify two deforming areas and quantify subsidence rates at 0.20 ± 0.02 mm/day south of Lorca and southeast to the city of Alhama de Murcia with the same efficiency as the studies using sophisticated InSAR methods. Using continuous records of water levels in boreholes, we deduced a mean storage coefficient ( S k ) at 4.1 × 10 − 3 which is very close to the previous published values, and coincides in this case to an inelastic deformation mode of all the system. With this value, we determine the groundwater level variations from the measurements of the ground subsidence.

SAR interferometry at Venus for topography and change detection

Since the Magellan radar mapping of Venus in the early 1990’s, techniques of synthetic aperture radar interferometry (InSAR) have become the standard approach to mapping topography and topographic change on Earth. Here we investigate a hypothetical radar mission to Venus that exploits these new methods. We focus on a single spacecraft repeat-pass InSAR mission and investigate the radar and mission parameters that would provide both high spatial resolution topography as well as the ability to detect subtle variations in the surface. Our preferred scenario is a longer-wavelength radar (S or L-band) placed in a near-circular orbit at 600km altitude. Using longer wavelengths minimizes the required radar bandwidth and thus the amount of data that will be transmitted back to earth; it relaxes orbital control and knowledge requirements. During the first mapping cycle a global topography map would be assembled from interferograms taken from adjacent orbits. This approach is viable due to the slow rotation rate of Venus, causing the interferometric baseline between adjacent orbits to vary from only 11km at the equator to zero at the inclination latitude. To overcome baseline decorrelation at lower latitudes, the center frequency of a repeated pass will be adjusted relative to the center frequency of its reference pass. During subsequent mapping cycles, small baseline SAR acquisitions will be used to search for surface decorrelation due to lava flows. While InSAR methods are used routinely on Earth, their application to Venus could be complicated by phase distortions caused by the thick Venus atmosphere.

Time Evolution of Deformation Using Time Series of Differential Interferograms: Application to La Palma Island (Canary Islands)

Differential interferometry is a very powerful tool for detecting changes in the Earth’s crust where coherence conditions are good, but is difficult to employ in some volcanic areas due to dense vegetation. We apply two differential InSAR methods using the time series associated with the interferograms to perform a phase analysis on a data set for La Palma island (Canary Islands) from the ERS-1 and ERS-2 European Space Agency (ESA) satellites for the time period 1992 to 2000. Both methods involve choosing a master image from the database and creating a series of interferograms with respect to this image. The “Coherent Pixel Time Series” (CPTS) technique chooses pixels with good average coherence, aligns the unwrapped interferograms with a stable area and then performs an inversion to calculate the linear velocity to quantify the deformation. The Coherent Target Modeling (CTM) method calculates the temporal coherence of each pixel to identify stable targets and then determines the best velocity for each pixel by using a linear fit that maximizes the temporal coherence. Using these two methods we have been able to detect deformation on La Palma Island that has been previously undetectable by conventional InSAR methods. There is a roughly circular region on the Southern part of the island that is actively deforming at ∽ -4 to -8 mm/yr. This region is located near the Teneguia valcano, the host of the last known eruption on La Palma in 1971. A thorough investigation of the possible sources for this deformation revealed that it was most likely created by a subsurface thermal source.