Assessment of urban slope instability in Dunedin, New Zealand, using orbital differential synthetic aperture radar interferometry

Abstract

A number of landslides in Dunedin have affected residential properties, roads and infrastructure. Many of these slides have had, or currently have, surface deformation monitoring networks, with repeat surveys over various periods to measure horizontal and vertical deformation. However, repeated measurements of millimetre- and centimetre-scale slope movement are costly to obtain, and are time-consuming to acquire a high spatial resolution if using traditional field-based techniques for resurveying, such as levelling and differential GPS (global positioning system) measurements.

In this study, we investigate the viability of orbital differential synthetic aperture radar interferometry (DInSAR) as a potential solution to this problem. DInSAR is a technique developed to measure centimetre-scale surface movement from pairs of remotely sensed synthetic aperture radar (SAR) images collected from orbiting satellites. DInSAR has already proven successful for monitoring slope instability in the French and Austrian Alps.

In this report we collate the survey data made available to us by the Dunedin City Council for comparison, and present comprehensive documentation of landslide motion in the Dunedin urban area. We then provide a detailed review of orbital differential SAR interferometry, and investigate its applicability for measuring slope instability in Dunedin.

In general, we found that the technique worked well over the urban, built-up areas of Dunedin. However, there were relatively few, or only very small, independently documented landslide movements within the intervals of the DInSAR image acquisitions, and ERS DInSAR did not yield the results as we had hoped for. On the other hand, we were able to make several interesting observations, and these results will be invaluable to other groups considering similar studies to this in similar environments and scenarios. Our study is unique, because thus far, as only successful studies applying ERS SAR to landslides have been published, and this study injects some realism into the difficulties of using the technique in areas with moist, variable climates.