One of the main features of "post mining" research relates to available methods for monitoring mine-degradation 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 (super 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.
Le Ministère de l'Environnement a souhaité que le BRGM apporte son concours à l'Agence de l'Eau Rhin-Meuse, dans le cadre de sa mission de Service public, pour l'identification et la codification des systèmes aquifères du bassin. Le BRGM réalise toujours ce genre d'opération au 1/50 000, pour être cohérent avec l'échelle des investigations géologiques de base (couverture nationale).
The Callovian-Oxfordian (COx) clay formation in the Paris Basin (France) has been the target of many studies investigating the feasibility of deep nuclear waste disposal in a reduced clay formation. To determine the mobility of radionuclides in the host rock formation, modeling of the porewater chemistry, particularly iron solute concentrations, is necessary. Notably, this study aims to understand the supersaturation of Fe(III) oxyhydroxides given by models. Fe(III) oxyhydroxides have been identified magnetically in unpreserved Callovian-Oxfordian samples. In this study, a set of magnetic measurements are used to detect the Fe-bearing magnetic minerals present in the COx clay formation. A core sample from the borehole FOR1118, preserved from air since its collection, is the target of this study. The magnetic measurements performed show that magnetite and goethite are the main magnetic minerals (<0.2%), together with probable greigite, and occur in low concentrations. Goethite occurs as nanoparticles dispersed in the clayey matrix, and not enclosed in other minerals or in organic matter. It is unlikely that the goethite is an alteration by-product, as particular care was undertaken. This finding resolves the discrepancies between observations and previous modeling results.
L'étude IKARE constitue un inventaire sous forme de Base de Données, doublé d'un outil cartographique d'interrogation des renseignements de la Base. L'essentiel des résultats est donc fourni à travers les fichiers et applicatifs informatiques. Ce rapport d'étude n'a pas vocation à être particulièrement développé. Il s'agit seulement de présenter le contexte et la progression de l'étude, la méthodologie employée et les résultats généraux obtenus, ainsi que quelques pistes de réflexions sur la poursuite ultérieure du travail
