Risk prevention related to complex extended cavities raises often strong economic, social and/or technical issues for local authorities and decision-makers. Remote monitoring offers a median way of vulnerability mitigation. However relevant monitoring of those geohazards calls for innovative volumetric methods to meet reasonable cost/benefit ratios, as microseismic and aerial acoustic monitoring techniques. Aerial acoustic technique has been developed and tested by INERIS in several underground contexts. This paper presents feedback from both experimental and operational case studies, presents results and recommendation for proper field application.
Natural cavities or abandoned underground excavations are to be found in various contexts and numerous parts of the world. In France, shallow natural or anthropogenic cavities count rises up to several hundreds of thousands all over the country. It includes complex cavities as old abandoned multilevel underground quarries and extended shallow mines or naturally interlaced karsts. Although major accidents and disasters are rare, cavity danger remains a national threat: subsidence, large-scale ground collapse or sinkholes phenomena threaten public safety, properties and infrastructures. Risk assessment and prevention of these geohazards are then a major concern to both national and local authorities. To reduce the risk, definitive remediation may be achieved by mitigating the hazard or reducing the vulnerability of the stakes. However, these two main options may reveal much difficulties to risk managers, raising severe financial, social and/or technical issues, with both decision-making and mitigation solution processes calling for distant deadlines. It is then common to set up a waiting but watchful strategy usually based on regular visual inspections whenever it is possible. Such an alternative monitoring strategy, with nearly no investment cost, aims to take proper and timely action to reduce risk related to public safety, but without any impact on the hazard itself or on the vulnerability of materials.