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. We present results ofsophisticated, high-precision time-lapse gravity monitoring that wasconducted over 4 years in Bad Frankenhausen (Germany). To our knowledge, thisis the first successful attempt to monitor subrosion-induced mass changes inurban areas with repeated gravimetry. The method provides an approach toestimate the mass of dissolved rocks in the subsurface. Subrosion, i.e. leaching and transfer of soluble rocks, occurs worldwide.Mainly in urban areas, any resulting ground subsidence can cause severedamage, 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 deformationprocesses. In this study, we combine levelling and highly precise time-lapse gravityobservations. Our investigation area is the urban area of Bad Frankenhausenin central Germany, which is prone to subrosion, as manysubsidence and sinkhole features on the surface reveal. The city and thesurrounding areas are underlain by soluble Permian deposits, which arecontinuously dissolved by meteoric water and groundwater in a stronglyfractured environment. Between 2014 and 2018, a total of 17 high-precisiontime-lapse gravimetry and 18 levelling campaigns were carried out inquarterly intervals within a local monitoring network. This network covershistorical sinkhole areas but also areas that are considered to be stable.Our results reveal ongoing subsidence of up to 30.4  mm a−1 locally, with distinct spatiotemporal variations. Furthermore, we observe asignificant time-variable gravity decrease on the order of 8  µGal over 4 years at several measurement points. In the processing workflow, after the application of all required correctionsand least squares adjustment to our gravity observations, a significanteffect of varying soil water content on the adjusted gravity differences wasfigured out. Therefore, we place special focus on the correlation of theseobservations and the correction of the adjusted gravity differences for soilwater variations using the Global Land Data Assimilation System (GLDAS) Noah model to separatethese effects from subrosion-induced gravity changes. Our investigations demonstrate the feasibility of high-precision time-lapsegravity monitoring in urban areas for sinkhole investigations. Although theobserved rates of gravity decrease of1–2  µGal a−1 are small, we suggest that it is significantlyassociated with subterranean mass loss due to subrosion processes. We discusslimitations and implications of our approach, as well as give a firstquantitative estimation of mass transfer at different depths and fordifferent densities of dissolved rocks.

Time-lapse gravity and levelling surveys reveal mass loss and ongoing subsidence in the urban subrosion-prone area of Bad Frankenhausen, Germany