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WP1: Key properties of critical zones in sinkholes                         

Here, the scientific aims of this topic are to understand: what is the structural character of the subsidence areas and their surrounding? How can we survey disaggregation zones and describe them geophysically? and, how strong is their variability in space and time?

For this work package, three main research topics are defined: 1) identify and predict seismic attributes for subrosion from surface measurements, 2) define physical properties in situ and test their geothecnical application potential, and 3) render specific and sensitive parameters for long-term monitoring concept.

In order to investigate these topics, three main methods suggested in this work are: 1) combined surface (S-waves and P-waves) and borehole (VSP/MSP) seismic survey, 2) borehole and lab measurements of physical and structural parameters, and 3) tomographic and cross-hole studies (Fig. 1).

 

Fig1. Schematic illustration of required surveying and monitoring methods to reveal key structures at surface and in the subsurface. Yellow circles show the focus methods in this work package (modified after Krawczyk & Dahm, 2011).

Fig 2. Interpretation of migration and depth converted seismic section in Bad Frankenhausen. The results are: interpreted section (left), processed section (middle) and processed section with velocity model (right). 

WP2: Seismic monitoring and characterisation

This WP is trying to investigate, 1) what are the seismogenic characteristics of the subsidence areas? 2) how is it possible to identify weak seismogenic signals in the noisy urban environment?, 3) how does the seismogenic signature compare with gravity and deformation and the prognoses from the numerical modelling?, and finally, 3) how can the knowledge of the seismogenic behaviour with possible precursor events be used to improve existing decision protocols?

The main research topics are: 1) detection and localization of microseismic events to characterize the seismogenic behaviour of the sinkhole structures, 2) evaluate ambient seismic noise data, and 3) comparison of seismogenic behaviour of the sinkhole structures with observed deformational character and spatio-temporal evolution as expected from the distinct element (DE) modelling.

Fig1. Schematic illustration of required surveying and monitoring methods to reveal key structures at surface and in the subsurface. Yellow circles show the focus methods in this work package (modified after Krawczyk & Dahm, 2011).

Fig2. event detection and localization in Schmalkalden. Up; onset time detemination, bottom: localization by grid search

WP3: Surface deformation and mass dislocation

This WP is trying to understand: 1) how large are deformation rates and gravity changes at sinkholes in two study area in Hamburg and Thuringia? 2) How do sinkhole-induced deformations and mass movements vary in space and time? And, 3) how can this information constrain geophysical modelling and contribute to a more comprehensive process understanding of sinkhole evolution?

This topic aims at at developing a novel combination of geodetic and geophysical methods, comprising GNSS, leveling & time-lapse gravimetry, borehole extensometers, and InSAR to monitor surface deformation and mass dislocation on different spatio-temporal scales caused by the development of sinkholes; interpretation will be based on geophysical models. Finally the existing time-series data (InSAR, GPS, levelling) will be integrated to study long-periodic effects.

Fig1. Schematic illustration of required surveying and monitoring methods to reveal key structures at surface and in the subsurface. Yellow circles show the focus methods in this work package (modified after Krawczyk & Dahm, 2011).

Fig2. Statistical compilation of the station determination with the Hannover FG5X-220 absolute gravimeter  at the “Rathaus” in Bad Frankenhausen. June 2015

WP4: Rock-soil-water interaction

The research topics of this WP are: 1) develop a concept for sinkhole assessment integrating 3-D electric and electromagnetic tomography as well as in situ probing (direct push (DP), petrophysics), 2) investigation of parameters relevant to speleogenesis, and 3) develop a monitoring concept by optimizing field instrumentation and interpretation to detect changes of geophysical, geotechnical and hydrogeological parameters as an indicator for sinkhole processes. The research teams here, are aiming to:

(1) systematically combine 3-D geophysical imaging technique with geomechanical, physical, and sedimentological in situ probing for high-resolution characterization of relevant parameters for trigger processes and mechanisms of vertical sinkhole propagation (2) to develop a strategy for monitoring, and (3) to combine all field data and the numerical process model from other WP in SIMULTAN to find the optimized survey design for the monitoring system.

Fig1. Schematic illustration of required surveying and monitoring methods to reveal key structures at surface and in the subsurface. Yellow circles show the focus methods in this work package (modified after Krawczyk & Dahm, 2011).

Fig 2. Hydraulic probing and direct push measurements in Münsterdorf.

WP5: Sub-surface cavity and collapse evolution

This work package aims to provide an integrative numerical modelling approach that investigates the various physical, chemical and temporal aspects of the process chain of sinkhole formation into two sub-topics. The first topic mainly deals with the speleogenetic evolution of the karst rock and the development of sub-surface cavities and critical zones by dissolution of the host rock, which concerned with the early stages of the sinkhole process chain. The second topic focuses more on the mechanical destabilisation and subsidence of the overburden above such critical zones, which can eventually lead to a collapse sinkhole at the surface. This division reflects the fact that sinkhole formation occurs at the end of a chain of events (initial void evolution, void growth, breakdown of overburden, final collapse) that operate under different governing laws and on varying length/time scales.

The innovation and application of this work package lies in:
(1) the use of the KARSTAQUIFER package to simulate processes leading to the initiation and evolution of subsurface cavities prone to later collapse at the focus-areas of SIMULTAN
(2) the use of 3D-Distinct Element Method models to better understand geodetic and geophysical data for the onset and evolution of sinkhole collapse at these field locations,
(3) the simulation of hydraulic and groundwater effects induced by human activity on both dissolution and mechanical behaviour, leading to enhanced hazard forecasting potential.

Fig1. Schematic illustration of required surveying and monitoring methods to reveal key structures at surface and in the subsurface. Yellow circles show the focus methods in this work package (modified after Krawczyk & Dahm, 2011).

Fig 2. 3D evolution of solution sinkhole. Vertical displacement is shown of the top layer and a view inside the model box is given, providing important information of rock movement before or during the collapse of the overburden. Dimensions refer to a case study in France (Cerville).

WP6: Protocols and decision process

In this WP, all geological surveys will contribute with their site specific expertise, background information, data, and monitoring results of hydrological and geochemical analyses of aquifers.They will provide the link between science and decision makers, as well as the communication with the public (e.g., information sheet).The surveys will contribute to the training of affected population and crowd-sourcing via questionnaires in case of sinkhole unrest. The outcomes of the early recognition system are integrated into the development of hazard management plans, thereby defining guidelines and responsibilities.

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