Time-lapse gravity and levelling surveys reveal mass loss and
ongoing subsidence in the urban subrosion prone area of Bad
Frankenhausen/Germany
Abstract. We present results of a sophisticated, high-precision time-lapse gravity survey that was conducted over four years in Bad Frankenhausen (Germany). To our knowledge, this is the first successful attempt to monitor subrosion-induced mass changes in urban areas with repeated gravimetry. The method provides an approach to estimate the mass of dissolved rocks in the subsurface. Subrosion, i.e. leaching and transfer of soluble rocks, occurs worldwide. Especially in urban areas, any resulting ground subsidence can cause severe damage, especially if catastrophic events, i.e. collapse sinkholes occur. Monitoring strategies typically make use of established geodetic methods, such as levelling, and therefore, focus on the associated deformation processes. In this study, we combine levelling and highly precise time-lapse gravity surveys. Our investigation area is the urban area of Bad Frankenhausen in Central Germany, which is prone to subrosion, as many subsidence and sinkhole features on the surface reveal. The city and the surrounding areas are underlain by soluble Permian deposits, which are continuously dissolved by meteoric water and groundwater in a strongly fractured environment. Between 2014 and 2018, a total of 17 high-precision time-lapse gravity and 18 levelling campaigns were carried out in quarter-yearly intervals within a local monitoring network. This network covers historical sinkhole areas, but also areas that are considered to be stable. Our results reveal ongoing subsidence of locally up to 30.4 mm a−1, with distinct spatio-temporal variations. Furthermore, we observe significant time-variable gravity changes in the order of 8 μGal over four years at several measurement points. In the processing workflow, after the application of all required corrections and least squares adjustment to our gravity observations, a significant effect of varying soil water content on the adjusted gravity differences was figured out. Therefore, we place special focus on the correlation of these observations and the correction of the adjusted gravity differences for soil water variations using the global soil water model GLDAS Noah to separate these effects from subrosion-induced gravity changes. Our investigations demonstrate the feasibility of high-precision time-lapse gravity in urban areas for sinkhole investigations. Although the observed rates of gravity changes of 1–2 μGal a−1 are small, we suggest that it is significantly associated with subterranean mass loss due to subrosion processes. We discuss limitations and implications of our approach, as well as give a first quantitative estimation of mass transfer at different depths and for different densities of dissolved rocks.
