Lafortune, S, Albinet, R, Lefebvre, O & Delaunay, T 2026, 'Reassessment of long-term monitoring of underground heating: 
a 25-year case study at a former lignite mine', in AB Fourie, G Boggs, J Heyes & M Tibbett (eds), Mine Closure 2026: Proceedings of the 19th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, pp. 1-15, https://doi.org/10.36487/ACG_repo/2615_86
(https://papers.acg.uwa.edu.au/p/2615_86_Lafortune/)
Abstract: The former Bois d’Asson underground lignite mine, located in southeastern France, has been monitored since 1999 following the detection of a surface manifestation of underground heating characterised by the emission of fumes. Mitigation works were implemented in 2000 to confine the emission point and reduce air circulation feeding the heating source. Despite these measures, the underground combustion was not fully extinguished and a long-term monitoring has operated since then.
This paper presents a reassessment of the relevance and performance of the monitoring system based on more than 20 years of temperature measurements, gas composition analyses and flow rate observations. High-frequency monitoring data acquired between 2017 and 2023 are analysed using descriptive statistics and time-series decomposition to separate long-term trends from seasonal and short-term variability. The results indicate that the underground heating source remains active but has stabilised in recent years, with a gradual reduction in intensity observed since the mid-2000s.
The study highlights the influence of soil temperature on fumarole temperature measurements and demonstrates the limitations of absolute temperature thresholds established early in the monitoring history. A revised criterion based on the temperature difference between fumarole and soil measurements is proposed to improve robustness against weather-driven variability. Gas analyses confirm persistent combustion signatures while underlining the importance of representative sampling protocols.
The Bois d’Asson case provides a long-term reference for mine closure practitioners dealing with persistent low-intensity heating in abandoned coal mines and demonstrates how long-term monitoring objectives and thresholds should evolve once an underground combustion process has stabilised. It also provides a transferable framework for proportionate post-closure surveillance at abandoned coal mines.

Keywords: gas emissions, lignite, long-term monitoring, mitigation, spontaneous combustion, telemonitoring, underground heating