Abstract.One of the main features of "post mining" research relates to available methods for monitoring minedegradation processes that could directly threaten surface infrastructures. In this respect, GISOS, a French scientific interest group, is investigating techniques for monitoring the eventual collapse of underground cavities. One of the methods under investigation was monitoring the stability of a salt cavity through recording microseismic-precursor signals that may indicate the onset of rock failure. The data were recorded in a salt mine in Lorraine (France) when monitoring the controlled collapse of 2 000 000 m 3 of rocks surrounding a cavity at 130 m depth. The monitoring in the 30 Hz to 3 kHz frequency range highlights the occurrence of events with high energy during periods of macroscopic movement, once the layers had ruptured; they appear to be the consequence of the post-rupture rock movements related to the intense deformation of the cavity roof. Moreover the analysis shows the presence of some interesting precursory signals before the cavity collapsed. They occurred a few hours before the failure phases, when the rocks were being weakened and damaged. They originated from the damaging and breaking process, when micro-cracks appear and then coalesce.From these results we expect that deeper signal analysis and statistical analysis on the complete event time distribution (several millions of files) will allow us to finalize a complete typology of each signal families and their relations with the evolution steps of the cavity over the five years monitoring.
The aim of the Couy experiment in the 'Gtologie Profonde de la France' programme was to reach the source body of the magnetic anomaly of the 'Bassin de Pans'. The drilling stopped at 3500 m. Magnetic field logging and susceptibility logging were undertaken to obtain as complete a description as possible of the magnetic parameters of rocks crossed by the borehole. Remanent magnetization and susceptibility measurements on the core were also carried out. The entire set of results is presented in this paper for both sedimentary layers and the basement. The values obtained for magnetic field variations and for susceptibility lead to the conclusion that the magnetic source body of the AMBP was not reached.
The monitoring of the stability of old mines constitutes an important research objective for our institution, BRGM. The study reported here shows the contribution of high-frequency (>30 kHz) acoustic emissions to the detection of the damage within a rock mass, during an experiment within a pilot site of an old flooded iron mine. The experiment consisted of recording all the hydroacoustic events in a broad frequency band (between 30 Hz and 180 kHz), during 18 months. The monitoring network has been calibrated by a triggered block fall that made it possible to highlight a relationship between the occurrence of high-frequency/low-frequency hydroacoustic emissions and rock falls. The events recorded have been associated with the micro-failure of the rock mass near the roof, prior to the detachment of the blocks. This monitoring showed important high-frequency hydroacoustic activity, which may be associated with mechanical instabilities generated by the evolution of water pressure during the experiment. In conclusion, the high-frequency hydroacoustic activity appears to be a good indicator of instability and, therefore, this new technique constitutes a promising tool for monitoring abandoned underground cavities.
This work aims at developing the bases necessary for taking into account in an integrated manner the soil and sub-soil issues in urban development. The diversity of actors and of their concerns lead us to develop different tools, with dedicated uses: 1) a multi-actor debate-aid tool based on multi-criteria analysis, that makes more readable the diversity of challenges and viewpoints of actors; 2) a technical aid-tool with a database devoted to stakeholders, that makes an inventory of main issues that may be encountered during typical urban development projects and that enables to agglomerate the information from basic data in order to make it usable in the multi-actor public consultation; 3) a quantitative decision-making aid tool for enlightening the economic value of indispensable investigations. A conceptual framework is proposed, based on the expertise of the project partners in data gathering, in operational urban development or in project management. It is validated and tested on real test cases, corresponding to specific urban projects.
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