High Resolution Displacement Monitoring for Urban Environments in Seattle, Washington using Terrestrial Radar Interferometry

Abstract:

Displacement monitoring using Terrestrial Radar Interferometry (TRI) over an urban environment was conducted to monitor for potential movement of buildings, roadways, and urban infrastructure in Seattle, Washington for a 6 week deployment in March and April of 2015. A Gamma Portable Radar Interferometer was deployed on a the lower roof of the Smith Tower at an elevation of about 100 m, overlooking the historical district of Pioneer Square. Radar monitoring in this context provides wide area coverage, sub millimeter precision, near real time alarming, and reflectorless measurement. Image georectification was established using a previously collected airborne lidar scan which was used to map the radar image onto a 3D 1^st return elevation model of downtown Seattle. Platform stability concerns were monitored using high rate GPS and a 3-axis accelerometer to monitor for building movement or platform instability. Displacements were imaged at 2 minute intervals and stacked into 2 hour averages to aid in noise characterization. Changes in coherence are characterized based on diurnal fluctuations of temperature, cultural noise, and target continuity. These informed atmospheric and image selection filters for optimizing interferogram generation and displacement measurement quality control. An urban monitoring workflow was established using point target interferometric analysis to create a monitoring set of approximately 100,000 stable monitoring points measured at 2 minute to 3 hour intervals over the 6 week deployment. Radar displacement measurements were verified using ongoing survey and GPS monitoring program and with corner reflector tests to verify look angle corrections to settlement motion. Insights from this monitoring program can be used to design TRI monitoring programs for underground tunneling, urban subsidence, and earthquake damage assessment applications.