Kinscher, Jannes (2015) The analysis and interpretation of microseismicity induced by a collapsing solution mining cavity: A contribution for progress in hazard assessment of underground cavities. PhD thesis, UNIVERSITE DE LORRAINE.
Full text not available from this repository.Abstract
Ground failures, caving processes and collapses of large natural or man-made underground cavities can produce significant socio-economic damages and represent a serious risk envisaged by the stakeholders and municipalities. In order to improve our understanding of the mechanisms governing such a geohazard and to test the potential of geophysical methods to prevent them, the development and collapse of a salt solution mining cavity was monitored. This experiment was conducted by the research “Groupement d’Int´erˆet Scientifique sur l’impact et la S´ecurit´e des Ouvrages Souterrains” (GISOS), at the SOLVAY exploitation site of Cerville-Buissoncourt (Lorraine basin, France). The potential to create large- scale, shallow cavity structure at the study site is suggested from the presence of a stiff Dolomite layer, acting as a beam that keeps the overburden stable, and thus retards its collapse. During the experiment, an important catalogue (∼ 50, 000 event files) was recorded by a local high frequency microseismic network, installed by INERIS, which provides a unique opportunity to study in detail the microseismic response of a caving and collapsing underground cavity. However, ∼ 80% of the data comprised unusual swarming sequences with complex clusters of superimposed microseismic events. These waveforms could not be processed through automatic standard, body wave phase based detection, location and source analysis routines. Further problems in signal processing arise from the limited network station coverage, short hypocentral recording distances and propagation and near-field effects. This thesis presents a detailed analysis and interpretation of this unique microseismic data base, using and adapting innovative methods proposed from tectonic, volcanic and mining seismic swarm studies. In the first part of this thesis, a probabilistic, automatic detection location design has been developed, able to resolve the spatio-temporal characteristics of microseismic swarms in a first order. In this approach, mainly signal polarization attributes were used to identify P wave phases. Event locations were then obtained by using the information of the corresponding polarization angles, as well as signal amplitudes, enabled by a detailed study of the attenuation characteristics, describing the amplitude decay with distance. Moreover, seismic source mechanisms and standard source parameters were investigated by comparing observed and synthetic wave forms, amplitude spectra and peak-to-peak amplitude ratios, assuming different standard source models. Within this source study, it was possible to assess the common source mechanisms of many swarming events at once, using the observation from only one single three component station. The second part of this thesis summarizes and discusses the results obtained by the developed methods, as well as by detailed statistical analysis of the spatio-temporal and energetic microseismic attributes. The principal results are summarized as follows: (i) microseismic events are comparable to small natural tectonic earthquakes, demonstrate a self- similar scaling behavior and are associated with moment magnitudes MW ranging from around −3 to 1; (ii) the entirety of microseismic sources represents dominant shear faulting (standard doublecouple shear model) at the cavity roof, in the Dolomite layer and in the overburden. The presence of tensile faulting could not be observed from the dataset, but cannot be completely excluded; (iii) this observation contradicts geomechanical models predicting a dominant mode of traction failure in the center of the Dolomite layer; (iv) source mechanisms for most microseismic events are remarkable stable and demonstrate a predominant thrust faulting regime with faults similarly oriented NW-SE, dipping around 35◦ − 55◦, what might be related to the presence of systematically arranged pre-existing fractures; (v) similar to tectonic earthquake swarms, the origin of microseismic swarming is suggested in an incapacity to sustain larger strains and to release larger stresses, what seems to be related to the mechanical constitution of the rock strata overlying the cavity (i.e. low strength materials); (vi) caving and collapsing periods at the cavity roof are associated with systematic, selfreinforcing dynamics and have a distinct microseismic response, clearly observable from statistical analysis; The performed analysis and interpretation of the microseismicity at Cerville-Buissoncourt has shown that microseismic monitoring is a useful tool to constrain the mechanical and dynamical characteristics of an evolving and collapsing hazardous underground cavity. The here documented observations and trends provide new important constraints for the design of adequate geomechanical models that will improve hazard assessment at underground openings. In future research, further investigation will assess the relationship between microseismic emissions and observed in-situ deformation including observations from seismic broadband data.
| Item Type: | Thesis (PhD) |
|---|---|
| Subjects: | Methodology > Method and procesing > Collective properties of seismicity Region > France > Lorraine salt basin Inducing technology > Underground mining |
| Project: | EPOS-IP > GISOS-SOLVEY: underground solution mining |