108 / 2021-09-30 03:13:57

Impact of past mining on public safety – Gardanne abandoned coalmine (France)

flooded abandoned mine,post-mining risk management,post-mining seismicity,seismic hazard,microseismic monitoring,fault reactivation

In former underground mine districts, prior mining activity can cause irreversible effects on the rock equilibrium. The abandoned mining sites management therefore often involves assessment and mitigation of the risk of ground instabilities, which can remain long after the mine closure and can impact the public safety. The ground stability can be affected by different forms of mine collapses causing ground failures, such as sinkholes or subsidence. As the failure initiation in rocks and expected ground instabilities can be accompanied by precursory seismic events, microseismic monitoring plays an important role in post-mining risk mitigation.  Additional concerns for public safety can be raised from triggered rupture on pre-existing faults resulting from pore pressure increase following the mine flooding. The seismic hazard related to the ground shaking due to fluid-induced seismicity on fault however still remains poorly known in flooded post-mining districts, due to limited amount of research studies available.



We present the case of the former coal mining site in the Gardanne basin in Southeastern France, where industrial exploitation lasted from 18^th century until closure in 2003. The depth of the mine varied between few meters (shallow mine) to more than 1000m. Following the closure, mine was flooded and a permanent microseismic monitoring network was established in zones with high-risk of ground instabilities to detect early signs of damage in the mining works. With the advance of the flooding front from deeper mine to shallower part, a significant microseismicity has started appearing periodically, in an area outside of monitoring zones. The strongest events with a local magnitude close to 2 are occasionally felt by the population. This led to new concerns regarding the seismic hazard and risk. As the first seismic data analysis contradicted an origin of the seismicity linked to underground collapses (seismic sources mostly located below the mining works, presence of multiplet- type events), the questions were raised concerning seismicity origin. Is it related to ongoing deformation in the mining works and overburden, or the reactivation of deeper faults below the mine? Two hypothesis carry different consequences in terms of seismic hazard, related to maximum expected magnitude and persistence of seismic activity.

Most recent study provided us with the detailed seismic catalog for the period 2014-2017, based on data from a scientific microseismic network installed in area of enhanced seismic activity. This allowed a more detailed overview of the seismic activity and its potential relationship with variations in the water level in the mine. The results indicate clearly that the origin of the seismicity is linked to the reactivation of deeper faults located under the mining structures.

Despite the limitations of available data, the main question concerned possibility for eventually having events of M>3. Based on current knowledge, this does not appear to be the case, as seismic activity is present in the form of earthquake swarms, which means that stress relief (caused by increased pore pressure) rather occurs along several small structures.



References

[1] Matrullo, E., Contrucci, I., Dominique, P., Bernard, P., Aochi, H. & Bennani, M., 2015, Induced microseismicity by flooding of the abandoned Gardanne coal field (Provence, France): analysis and interpretation, Rapport INERIS DRS-15-151231-10877A; Rapport BRGM/RP-65968-FR.

[2] Dominique, P., 2016, Bassin houiller de Provence (13). Bilan de l’activité sismique 2015. - Secteur de Fuveau ouest - Rapport BRGM/RP-66203-FR

[3] Kinscher, J., 2017, Rapport INERIS DRS-17-164272-00556A. Origine de la sismicité après mine du bassin houiller de Gardanne Programme EAT-DRS06 - Rapport de fin d’opération 4 « sismicité induite par les activités industrielles et extractives ».

[4] Kinscher, J., Gombert, P., Contrucci, I. & Thoraval, A., 2021,. Rapport Ineris-201216-2215842. Origine de l’essaim sismique de Fuveau et son lien avec l’ennoyage des anciens travaux miniers du bassin de Gardanne : apport des analyses sismique, hydrogéologique et géomécanique

[5] Namjesnik, D., Kinscher, J., Gunzburger, Y., Poiata, N., Dominique, P., Bernard, P., & Contrucci, I., 2021, Automatic Detection and Location of Microseismic Events from Sparse Network and Its Application to Post-mining Monitoring. Pure and Applied Geophysics, 178(8), 2969-2997

[6] Namjesnik, D., et al. 2019. Seismic multiplets induced by flooding of an abandoned coal mine and their implication for seismic hazard assessment. Geophysical Research Abstracts. Vol. 21.

[7] Namjesnik, D. 2021. Origin of the seismicity related to a flooded abandoned coal mining district at Gardanne, Provence, France. PhD thesis, Ineris, University of Lorraine, France