Sinkhole hazard assessment in the area of abandoned mining shaft basing on microgravity survey and modelling - Case study from the Upper Silesia Coal Basin in Poland

The article describes the results of a gravimetric survey conducted in the area of an abandoned mining shaft located on the premises of the liquidated “Porąbka-Klimontów” coal mine in Sosnowiec - southern Poland. After cessation of its exploitation in the past, the shaft has probably only been protected by placing a slab on a concrete pit-bank without commencing the proper liquidation. The aim of the survey was to determine whether the shaft had been backfilled and what is the condition of the rock mass where it gets in contact with the shaft barrel. In the interpretation of the Bouguer anomaly map, a forward modelling of gravity effect generated by an empty mineshaft has been used in order to eliminate its influence on the local gravitational field. That approach to survey data analysis allowed to assess not only the type of the shaft's filling but also the changes in the rock mass structure around it. The results allowed planning an adequate type of works aiming to secure the surface from potential sinkhole deformations resulting from the shaft lining collapse or wash out of the soil into the shaft barrel.