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.
gTOOLS, an open-source MATLAB program for processing high precision, relative gravity data for time-lapse gravity monitoring
