Spatio-Temporal Changes in the Gravity And Post-Mining Seismic Activity in Abandoned Deep Coal Mine Kazimierz-Juliusz During Flooding

The flooding of an underground hard coal mine, resulting in the filling of post-mining voids (workings and goafs) and the repeated saturation of pore and interstitial spaces of the adjacent rock with water, change the mass and its distribution in the near-surface part of the earth's crust. This process is slow and lasts many years. As a result, the local gravity field undergoes dynamic changes. It depends on the surface morphology and the density of geological strata, which is related to their water content. Changes in the local gravity field can be monitored by periodic gravity surveys performed in the area of the flooded mine using portable spring gravimeters. This paper presents the results of gravity monitoring surveys conducted in the area of the abandoned coal mine Kazimierz-Juliusz (K-J) in Poland, which is in the process of flooding. Gravity surveys were conducted at yearly intervals spanning 2021–2023. We used a high-precision CG-6 gravity meter manufactured by Scintrex LTD, allowing for relative gravity measurement with an accuracy of order 0.005 mGal. In the collected data set, the rates of annual gravity change ranged from -0.04 mGal to 0.04 mGal. The K-J mine was closed in 2016, and since then, the rock mass in the zone between the deepest workings (about 750 m) and the surface has been undergoing repeated water saturation. In 2021–2023, the water table in carboniferous rock mass was located at an estimated depth of 380 m and moved up to a depth of 330 m. An average yearly increase of the water table level oscillated around 18 m per year. During this period, over 50 seismic tremors of local magnitude ranging from 0.6–2 and ground deformations on the surface were recorded in the K-J mine's area. Surface deformations recorded in this area by GNSS surveys had a form of subsidence and uplifts. The annual rates of surface movements ranged between –11 cm (subsiding regions) and +3 cm (uplifting regions). The main objective of this paper is to identify the relationship between seismic phenomena and observed variations in the local gravity field distribution, factoring in the mining activity performed and changes in terrain relief. However, the data presented in the article indicate that in the coal mines, in the process of flooding, there are threats to the surface similar to those during the mining period (surface deformations and soil vibrations). In view of the collected data, their intensity is lower than of those observed during the mine's operation. As the gravity field is the primary factor shaping the dynamics of a geological medium, the research results indicate that gravity monitoring is essential in studies of safety in post-mining lands.