InSAR measurement of surface deformations in south Ontario

Abstract

Detection of surface deformation can be measured using InSar, a geodetic technique that calculates the interference pattern which results from a difference in phase between images acquired by synthetic aperture radar (SAR). Ground surface deformation, in particular land subsidence is caused by numerous factors, such as: tectonic motion, sediment compaction, thawing permafrost, increased surface loading, Glacial Isostatic Adjustment (GIA), hydro-chemical erosion of karst, decomposition of organic material in soils, mining, anthropogenic fluid withdrawal and surface water/drainage management. Groundwater applications have commonly been completed in areas of extreme subsidence due to dehydration and collapse of fine grained sediment textures; such as in the central valley of California, Nevada, and the Mexico city area. This study examines whether there is potential application of this technique to measure changes in surface elevation in southern Ontario and whether it can be related to changes in groundwater storage. For the southern Ontario study five datasets were used to assess the ground surface deformation and the hydrogeological/hydrologic conditions within the imagery extent. Datasets included: a set of 40 Radarsat-2 images spanning five years, GPS weekly solutions, Real-time kinematic (RTK) gps data, groundwater levels, terrestrial water storage data derived from GRACE satellites and hydrologic data from the Provincial Groundwater Monitoring Network (PGMN). Differential synthetic aperture radar interferometry (D-InSAR) has sub-centimetre precision and high spatial resolution over a large area. To eliminate some of the noise and to reduce geometrical distortions (multilook) images were averaged to a 50 m resolution. A stable reference site RTK TWOO was used a reference point. The TWOO GPS measurements were then added to the InSAR time series deformation maps. Obtaining a coherent signal was difficult and resulted in clustering of signal return from urbanized areas. The roofs and corners of the buildings in urbanized areas can form permanent scatterers, resulting in a more coherent signal. Across the study area, an annual rate of 1 mm to 10 mm of subsidence is observed. The greatest amount of subsidence (> 8 mm/year) is observed along the shore of Lake Ontario. In the absence of a field campaign to support validation the study has concluded that any viable signal attributed to specific geological - hydrogeological controls is within the signal-noise ratio of the study.