Shimon Wdowinski
Land Subsidence
Space-borne synthetic aperture radar interferometry (InSAR) is a powerful new geodetic technique providing accurate measurements (cm level) of surface changes over wide areas (typically 100x100 km2). For more than a decade, the method has been widely used to monitor earthquake-induced and volcanic-induced deformation in tectonically active areas. In recent years, InSAR has also been used for mapping and monitoring land-surface in various environments, due to withdrawal of ground water or other fluids (oil, natural gas), soil consolidation, and sediment compaction in flood plains. Land subsidence occurring in urban areas can be monitored by highly accurate Permanent Scatterers InSAR (PSInSAR) technique, which deals better with atmospheric noise and calculates the displacement time series of selected reliable scatteres. We used both standard InSAR and PSInSAR analyses to study land subsidence in New Orleans, Mexico City, the Venice Lagoon, the Dead Sea and the city of Mokpo (South Korea). Our studies show significant subsidence patterns in all locations with variable rate reaching as high as 30 cm/yr.
New Orleans
It has long been recognized that New Orleans is subsiding and is therefore susceptible to catastrophic flooding. In a Nature study of Dixon et al. (2006), we present a new subsidence map for the city, generated from space-based synthetic-aperture radar measurements, which reveals that parts of New Orleans underwent rapid subsidence in the three years before Hurricane Katrina struck in August 2005. One such area is next to the Mississippi River–Gulf Outlet (MRGO) canal, where levees failed during the peak storm surge: the map indicates that this weakness could be explained by subsidence of a meter or more since their construction.
Figure 1. Velocity map for permanent scatterers in New Orleans and vicinity. Values are range change in the direction of radar illumination. Negative values indicate motion away from the satellite, consistent with subsidence. Int. Airpt. is location of International Airport, MRGO is Mississippi River-Gulf Outlet Canal. Inset (location marked by white rectangle) shows close up of PS velocity map for parts of eastern St Bernard's parish and western part of Lake Borgne. Note high rates of surface motion (~subsidence) on levee bounding MRGO Canal. Large sections of the MRGO levee were breached during and shortly after Hurricane Katrina in August 29, 2005.
More details at Dixon et al. (2006)
Mexico City
The eastern portion of Mexico City is undergoing rapid subsidence due to extraction of ground water in excess of natural recharge and consequent compaction of clay-rich lacustrine sediment. In a recent study by Osmano
Figure 2: PSI results showing subsidence through part of Mexico City. The data were acquired by the Envisat ASAR system between January 2004 and July 2006. Maximum rate (300 mm/yr) occurs in eastern Mexico City and the gradient of subsidence increases towards the remnant of Lake Texcoco (dark rectangle). Master PSI point (white triangle) is located 454.5 meters away from UCHI GPS station, on a remnant volcanic hill.
More details at Osmanoglu et al. (2010)
Mokpo City, Korea
Mokpo city locating at the southwestern coast of the Korean Peninsula is one of the largest reclaimed costal lands, suffering a significant ground subsidence due to soil consolidation. In this study, we aim to estimate the subsidence rate over Mokpo city (8 x 8 km) based on InSAR and permanent scatterers InSAR (PSInSAR) with 26 JERS-1 SAR dataset from 1992-1998 and 6 ENVISAT ASAR data from2004-2005. Mean subsidence velocity, which was clearly related to a reclaimed land, was computed from JERS-1 PSInSAR analysis. The results indicated continuous and significant subsidence in three sites (Dongmyung, Hadang and Wonsan), where the subsidence velocity reached over 5-7 cm/yr in the area of maximum subsidence. The subsidence field measured from JERS-1 data (1992-1998) was validated by comparing ENVISAT ASAR measurements during the period of 2004-2005. Using linear model for PSInSAR analysis, which was adopted to predict the forthcoming subsiding, the subsidence velocity had decreased in Wonsan and Hadang, while it had been steady or increased in Dongmyung. The result shows that PSInSAR-based subsidence map is very useful for long-term monitoring of soil consolidation and defining risk zones in coastal reclaimed regions.
Figure 3. Subsidence maps derived from JERS-1 SAR dataset. Color scale represents vertical displacement of the surface. Red color shows fast subsiding areas, and blue shows stable areas. Background image is the multi-image reflectivity map generated from mean of 26 SAR images.
In a recent GRL paper by Kim et al (2010), we present a space-borne SAR interferometric technique for measuring and predicting ground subsidence associated with soil consolidation. Instead of a conventional constant velocity model, a hyperbolic model is introduced for persistent scatterer SAR interferometry (PSI) processing. Twenty three JERS-1 SAR acquired between 1992 and 1998 were used to measure land subsidence in Mokpo city, Korea which had been primarily built on land reclaimed from the sea. Two subsidence field maps were derived and compared: a constant velocity model and a hyperbolic model. Nonlinear components depending on the stage of soil consolidation are well represented by the hyperbolic model. The maximum subsidence velocity reaches over 6 cm/yr, while the maximum acceleration is about −0.3 to −0.4 cm/ year2. The predicted subsidence rate with the new model was validated by using later ENVISAT SAR data for 2004–2005. Prediction accuracy with the non-linear model is improved significantly, indicating the importance of a physically-based deformation model
More details at Kim et al (2008) and Kim et al. (2010)
