Sinkholes are circular to elliptical depressions or collapse structures at the Earth's surface, caused by subrosion processes, i.e., underground leaching of soluble rocks such as rock salt, anhydrite, or limestone. Diameters of sinkholes may range from a few metres to several hundred metres. Controlling factors are the local geological structure and the specific generation process as summarised in .
Especially in urban areas sinkholes are of societal relevance where they pose a severe hazard for infrastructure and life. The joint research project SIMULTAN (Sinkhole instability: integrated multi-scale monitoring and analysis) aims to develop and apply a system for early detection of sinkhole instabilities, unrests and collapses to gain deeper insight into these subsurface processes and related surface deformations. Concepts for monitoring, multi-scale description, and prediction of sinkhole evolution especially in urban areas are studied and applied since they are not yet fully developed. SIMULTAN focuses on aspects such as the understanding of interactions between surface and subsurface, the separation of different superimposing processes, as well as the prediction of future sinkhole development and the assessment of damage potential. In detail, the integrated approach is based on four scientific questions and methods:
- 1.Application of dedicated, high-resolution methods (e.g. seismics, geoelectrics, downhole logging, seismology, and direct-push methods) and inversion techniques to characterise structures, physical properties, and seismicity at different depth levels and scales.
- 2.Combination of geodetic (GNSS, levelling) and geophysical (relative and absolute gravimetry) measurements at identical and adjacent points to monitor deformation and mass changes on different spatio-temporal scales.
- 3.Interdisciplinary studies, time-dependent analyses, and joint interpretation of multiple data streams to derive process models, partly by improved modelling techniques.
- 4.Set up of an information platform to interpret measurements, to visualise information, and to evaluate case studies.
SIMULTAN focuses on aspects such as the separation of different superimposed processes and the understanding of interactions between surface and subsurface, as well as the prediction of future sinkhole development and the assessment of damage potential. The research is performed on different scales with respect to time, lateral extent, and depth  and is designed to complement investigations and results from similar and related research projects [2, 6, 15, 18, 19, 20].
Target areas of SIMULTAN are located in Northern Germany and Thuringia, where the development of sinkholes is linked to salt structures . In Central and Southern Germany the soluble rocks are mostly carbonates (Fig. 1, ). Hexagons in Fig. 1 represent evaporitic sinkhole formations that are targeted by SIMULTAN. The first area, Hamburg-Flottbek, is densely populated and slowly subsiding locally due to the leaching of the Othmarschen-Langenfelde salt diapir . The second investigation area is Bad Frankenhausen, where sinkhole diameters are some tens of metres and which is discussed in more detail in this paper.
Several sinkholes have developed along the Kyffhäuser Southern Margin Fault and these processes are still ongoing. In Hamburg-Flottbek as well as Bad Frankenhausen the spatio-temporal evolution of surface deformation is still unknown in detail and an integrated interpretation of the data is missing. The geodetic work package in SIMULTAN combines geodetic and gravimetric surface monitoring techniques in close cooperation between the Leibniz Institute for Applied Geophysics (LIAG) and the Institut für Erdmessung (Leibniz Universität Hannover, LUH). This comprises absolute and relative gravimetry, levelling, and GNSS (GPS and GLONASS) campaigns.
The paper is organised as follows: section two presents the concept of the network and the monitoring strategy. Sections three and four address the geophysical and geodetic field measurements in detail. We will show that space geodetic and terrestrial geophysical and geodetic techniques are capable to detect even small displacements during the ongoing deformation monitoring in Bad Frankenhausen. Similar combinations of methods are already established in Hamburg Flottbek.
2 Concept and monitoring strategy
Surface deformation often consists of superimposed signals from several sources at different depths. The combination of classical geodetic and geophysical techniques, e.g., borehole extensometers, levelling instruments, gravimeters, and GNSS provides complementary information and allows to separate different sources [5, 14, 16].
In SIMULTAN, geophysical parameters of critical geological zones are resolved by monitoring vertical and horizontal surface displacement caused by mass relocations in the subsurface with different techniques at identical points. Precise GNSS and levelling campaigns are combined with gravimeter measurements at identical or adjacent points to study long periodic effects caused by subrosion processes that might be superimposed by seasonal effects. The term integrated describes a system characterisation in order to achieve an improved comprehensive solution from the joint interpretation of different kinds of results. Thus, identical or adjacent points are established to perform co-located geo-monitoring with different sensors. Whenever possible the same marker is used to setup the instruments at the physically identical place; otherwise the instrumental reference points are linked by local ties. However, due to the urban conditions, sometimes not all of the used geodetic and geophysical techniques are combined at one identical measurement point. The derived key parameters are expected to become important results to assist and improve the modelling of rock-soil interaction in sinkhole formations and the subsurface cavity and collapse evolution [7, 9, 10, 11]. These studies are carried out by other groups in this joint research project. For numerical modelling, synthetic and most realistic models will be used both, the latter derived from geophysical and geological site surveys. Furthermore, the integration step comprises the forward-modelling and backward-evaluation between realistic datasets and related realistic model-prediction of the areas of interest.
2.1 Measurement points
The combination of various geodetic methods requires an observation network that is accessible for each of the used methods. The planning, selection, and installation of measurement points were carefully carried out. In Bad Frankenhausen they are located in active subsidence areas, e.g., around the leaning church tower, historical sinkholes, and in assumed stable areas to obtain clear evidence of gravity changes and vertical movements due to subsidence (cf. white bordered areas in Fig. 3). The gravimeter platforms GRAV2, GRAV11, and GRAV12, installed in January 2014, are concrete pillars, cast to reduce the influence of micro-seismic noise. They have a diameter of 0.30 m, a depth of 0.80 m, and a flat surface with north arrow (Fig. 2(b) and Fig. 5(d)). All other points are marked by 0.25 m long stainless steel tubes with a white cap and a fillister head.
2.2 Monitoring network
The monitoring network in Bad Frankenhausen consists of about 120 points for precision levelling. Thirteen of these points define a local gravimetric network (Fig. 3) with two more distal gravity points outside the city serving as reference points (AP2 and AP4; Fig. 3). Precise levelling and gravimetric campaigns are conducted quarterly since March 2014 to gain time-lapse data sets. An absolute gravimeter point, as part of the control network, was established in 2015 to monitor the long-term stability of the gravity reference annually and thus, to identify regional gravity changes related to subrosion over several years or even decades.
The GNSS part of this multi-sensor monitoring network consists of six identical points (simultaneously used for gravimetry and levelling, Fig. 3) and two distal points (GGP1, SL03), where only GNSS is observed. Station SL03 serves as a local reference for the GNSS network. GNSS campaigns started in September 2015 and will be repeated every six months, closely aligned to the gravimeter and levelling campaigns. Additionally, the Geological Survey of Thuringia operates a borehole extensometer at GRAV7.
3 Levelling and gravimetry campaigns
The integrated study in SIMULTAN comprises monitoring at co-located points in quarterly (gravimetry, levelling) and semi-annually (GNSS) field campaigns, the combination and integration of the results of the different monitoring techniques, and their interpretation as an integrated solution.
The levelling network of about 120 points was established to observe subsidence in the northern area of the city and to provide evidence for measuring campaigns using GNSS. Furthermore, levelling supports the processing and interpretation of the gravity data by providing heights and height changes for different processing steps, e.g., further gravity anomaly calculation.
Levelling campaigns are performed with Leica Geosystems digital levels DNA03 and bar code invar staffs (manufacturer accuracy information:
Results of the absolute gravimetry campaign with the gravity meter FG5X-220 on the Rathaus point (cellar vault, Fig. 5(a)).
|Point Rathaus||Measurement run (orientation)||Date in 2015||Drops|
|Setup 1||20150622a (N)||June, 22.–23.||988||−2.678||9811717.488|
|Setup 2||20150623a (W)||June, 23.–24.||798||−2.678||9811717.458|
A brief overview on relative and absolute gravity is given by Timmen . During every relative gravimetry campaign four different gravimeters of the types Scintrex CG3, CG5, and LaCoste & Romberg (LCR) are used to optimise the network surveys. In total, 15 gravity points (13 local and 2 distant reference points) are observed using the step method for drift control of the spring gravimeters .
Data processing includes the elimination of outliers, jumps, and other failures in the data sets as well as the reduction for earth tides, different instrument heights and air mass redistributions. Moreover, calibration and the sealing of the sensors against instrumental air pressure effects are checked regularly. Inside the local observation area, the maximum gravity difference is
The absolute gravimeter point was established in Bad Frankenhausen in June 2015 to determine and control the absolute gravity level (datum) for the relative gravimetry campaigns. The absolute gravity point is located in the basement of the town hall of Bad Frankenhausen. Measurements were performed by LUH using the Hannover absolute gravimeter FG5X-220 (Fig. 5(a)). This gravimeter participated in the latest international comparisons (Walferdange, Luxembourg, Nov. 2013 and Belval, Luxembourg, Nov. 2015) and agrees within a few
The local tie between the basement-located absolute gravity point and the levelling mark (cf. Fig. 5(c)) in front of the town hall (simultaneously the starting point of relative gravimetry) was determined using the CG3M-4492. The determined
4 GNSS campaigns
For consistent monitoring, equal or very similar GNSS receivers (Leica GRX1200+GNSS, GRX1200GG Pro and GX1230GG) with GPS and GLONASS capability are used. Local reference stations and challenging stations are equipped with 3D choke ring antennas (Leica AR25 Rev. 3) to mitigate most of expectable multipath. All other stations are equipped with rover antennas for economic reasons. The used GNSS antennas were absolutely calibrated at the LUH facility using the robot based approach [21, 29]. Additionally to the horizontal components, the height component is of particular interest. Therefore, the special tripod adaptor FG-ANA100B (ref. Fig. 6) is used in SIMULTAN campaigns. This tripod, in combination with the near-field calibrated carrier phase center corrections (PCCs), is used for high-precision GNSS levelling surveys . The adaptor contains a 0.5 m scale, directly connected to the antenna reference point (ARP), enabling a precise and contactless determination of the antenna height during the GNSS sessions.
4.1 Session setup
Each GNSS campaign consists of three sessions of four hours. Data is recorded with one second interval (internal purposes) but processed with ten seconds to adjust the final GNSS network.
To meet the requirements of a stable and reliable local reference station, we decided to use SL03 (indicated by a red star in Fig. 3), a concrete pillar with well-defined centring that is also part of another monitoring network. Along with the point GRAV11, the local reference is observed continuously during all 6 sessions. A star-like network is formed for each campaign with baselines starting at SL03. GNSS stations with square symbol in Fig. 3 are occupied during three sessions.
4.2 Campaign preparation
Concerning GNSS, there are two challenges in SIMULTAN. First, the horizontal and vertical components should be accurate in order to determine even small deformation. Second, urban environments are challenging for a stable acquisition of GNSS signals due to signal obstructions, multipath, and diffractions. Detailed station analysis verified moderate obstructions in the field of view for the receiving antennas at all selected GNSS points. Additional studies are focused on adaptive and dynamic elevation masks that will improve the signal availability and stabilise the ambiguity resolution.
A zero baseline (ZB) test at the laboratory network of LUH examines the expectable performance of the used GNSS equipment and delivers quality parameters for:
- 1.Achievable carrier-to-noise ratio (C/N0) for receiver-antenna combination under ordinary campaign settings; cf. Fig. 7,  and determination of nominal C/N0 curves.
- 2.Analysis of double differences (DD) between the individual receivers.
The processing of the GNSS network is performed using Bernese 5.2  and ESA (European Space Agency) products, e.g., clock, orbits, earth rotation parameters and differential code biases.
Advanced station analysis (study of dynamic and adaptive elevation masks, DD analysis on the observed-minus-computed level (OMC)), is performed using the GNSS MATLAB Toolbox, that is developed at LUH .
4.3 Network solution
The network processing was evaluated using both a GPS only and a GPS/GLONASS combined solution to study the impact on the network performance.
Final results of the first GNSS campaign are summarised in Fig. 8 and prove the valuable improvement for the combined solution. Especially the height component is much more reliable but also the repeatability for the horizontal component is improved. Gaining an optimal estimate for the height component is a challenge due to several urban reflectors and obstructions. The advantage of GLONASS is the higher inclination of the orbital planes. Subsequently, especially the northern hole (a characteristic at mid-latitudes) can be reduced. Furthermore, the double amount of observations stabilises the result.
First studies concerning the usage of adaptive and dynamic elevations masks show that their application stabilises the position repeatability, so that satellite arcs with interrupted and disturbed observations are identified and reduced to a minimum. The quality of the obtained data is improved, since deficiencies of satellite visibility are strongly related to azimuth and elevation of the incoming satellite signal.
Comparison of relative GNSS and levelling height differences.
|Number||Name||NHN92 Height [m]||GNSS Height [m]||Δ Levelling [m]||Δ GNSS [m]||Δ GNSS − Δ Levelling [mm]|
Table 2 evaluates the GNSS and levelling heights against each other with the NHN (Normal Heights; German Height System 1992) from the latest levelling campaign, carried out in September 2015 (hence, temporally close to the GNSS campaign). The point GRAV10 is chosen as reference, since this point represents the medium repeatability for all GNSS-heights (cf. Fig. 8). The last column of Table 2 validates differences of relative heights derived from GNSS and levelling. Results are comparable to each other at the
5 Summary and outlook
First results from the geodetic campaigns within SIMULTAN to monitor sinkholes in Bad Frankenhausen are shown. The established network consists of 120 levelling points. Fifteen of these points are used in a gravity network, whereof six points are additionally occupied by GNSS. The first multi-sensor monitoring campaigns were performed. Quarterly levelling confirms an annually subsidence rate of 4–5 mm in the main subsidence areas that reaches a local maximum of 10 mm. The quarterly observed gravity network covers a gravity range of
Semi-annually GNSS campaigns were evaluated and are ongoing. In contrast to a single GPS processing, the GNSS solution provides improved estimates for the horizontal and height component of 2–3 mm for optimal points and up to 5 mm in the horizontal components and 5–6 mm in the height components for challenging stations.
Urban sites are a challenge for all kind of applied measurement techniques, but we show that reliable solutions are feasible. Additional campaigns will be conducted to achieve and improve our understanding of land subsidence. Moreover we intend to evaluate the potential of InSAR to complement the monitoring concept.
The authors thank the TLVerm Thuringia, Glückauf Vermessung GmbH Sondershausen, and the city of Bad Frankenhausen for their kind cooperation. Furthermore, we thank the LGLN (Lower Saxony) for providing additional FG ANA100B GNSS height adaptors and corresponding accessories. The European Space Agency (ESA) is thanked for providing freely GNSS products.
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