Detection of cavity migration and sinkhole risk using radar interferometric time series

Upward migration of underground cavities can pose a major hazard for people and infrastructure. Either via sudden collapse sinkholes, or by eroding the support of building foundations, a migrating cavity can cause the collapse of buildings, water defense systems, or transport infrastructure. The main problem for risk assessment is the lack of a priori knowledge on the location of a potentially hazardous cavity. Here we demonstrate the feasibility of satellite radar interferometry to detect a migrating cavity under the city of Heerlen, the Netherlands, leading to foundation instability and the near-collapse of a part of a shopping mall in December 2011. We exploit the data archives of four imaging radar satellites, between 1992 and 2011, to investigate the dynamics of the area and detect shear strain within the structure of the building. Time series analysis shows localized differential vertical deformation rates of ~ 3 mm/yr during 18 years, followed by a dramatic increase of up to ~ 15 mm/yr in the last few years. These results imply that the driving mechanism of the 2011 near-collapse event had a very long lead time and was likely due to a long-lasting gradual process, such as the upward migration of a cavity.