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

Abstract

Abstract. We present results of sophisticated, high-precision time-lapse gravity monitoring that was conducted over 4 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. Mainly 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 observations. 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 gravimetry and 18 levelling campaigns were carried out in quarterly 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 up to 30.4 mm a−1 locally, with distinct spatiotemporal variations. Furthermore, we observe a significant 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 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 Land Data Assimilation System (GLDAS) Noah model to separate these effects from subrosion-induced gravity changes. Our investigations demonstrate the feasibility of high-precision time-lapse gravity monitoring in urban areas for sinkhole investigations. Although the observed rates of gravity decrease 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.

Geophysical analysis of an area affected by subsurface dissolution – case study of an inland salt marsh in northern Thuringia, Germany

“…Both reflection seismic and electromagnetic methods do not deliver information about local mass movements or cavities induced by dissolution and subsurface erosion. For this gravimetry is the preferred method for near-surface investigations in order to depict density contrasts and local gravity anomalies (e.g., Butler, 1984;Kersten et al, 2017;Kobe et al, 2019). In our study, we show that structures and zones affected by dissolution, as identified by the reflection seismic and electromagnetic methods, correlate with local minima of the Bouguer anomaly due to reduced densities.…”

“…This was accomplished using ERT and TEM, as well as seismic facies analysis of P and SH waves. The top of the soluble Permian rocks was detected at 250 to 300 m depth, so near-surface dissolution, as is the case for the town of Bad Frankenhausen north of the KSMF (Wadas et al, 2016;Kobe et al, 2019), is not possible. The unsaturated groundwater needs to reach greater depths in order to leach the evaporites such as salt and anhydrite.…”

“…Altogether, the tectonic and depositional history indicates that dissolution processes were probably triggered by tectonic movements and fault development, and therefore they were probably most active during and after the tectonic phases of the Tertiary but are still ongoing, as evidenced by continuous saltwater rise at the Esperstedter Ried (TMLNU, 2008) as well as recent subsidence and sinkhole development (Jankowski, 1964;Wadas et al, 2016;Kobe et al, 2019).…”

Sinkhole subsidence monitoring combining terrestrial laser scanner and high‐precision levelling

“…High-precision levelling is a conventional method that, thanks to the development of new digital levels in the 1990s (Algarni & Ali, 1998), allows the measurement of vertical deformation with very high accuracy and outstanding precision (submillimetre); thus it has successfully been applied to the monitoring of active sinkholes (Desir et al, 2018;Kersten et al, 2017;Kobe et al, 2019). Nonetheless, the displacement data gathered by this approach is spatially restricted to the measuring benchmarks installed at suitable locations.…”

Gravity as a tool to improve the hydrologic mass budget in karstic areas

“…Recently, the advent of new, high-performance instruments such as superconducting gravimeters (SGs) has raised new interest in this method for monitoring water mass movements (Van Camp et al, 2017). Gravity measurements have been successfully applied to study the groundwater flow in other karstic environments (Fores et al, 2017;Jacob et al, 2011;Meurers et al, 2021;Mouyen et al, 2019;Van Camp et al, 2006;Watlet et al, 2020) or to monitor the subsidence and underground mass redistribution in sinkhole-prone areas (Kobe et al, 2019). Other studies utilized gravimetry to characterize water flow in porous media (Güntner et al, 2017;Weise and Jahr, 2017) or to monitor fluids relevant for geothermal exploitation (Portier et al, 2018).…”