Gravity field changes during deep exploitation of the coal longwall and their relation to stress distribution and seismic activity

Mining operations cause volumetric deformations within the rock mass and changes in its density, to which the gravimetric method is sensitive. These changes are particularly well seen in periodic measurements of the local gravity field. The paper analyses the relationship between the movement of a longwall in a coal seam and the change in the distribution of the gravity field in time and space observed on the ground surface. Relative gravimetric measurements were carried out in six series between 2018 and 2020; before the start of coal extraction, with the progress of the longwall and after the cessation of mining. Collected data allowed differential maps of changes in gravity to be plotted. Differential anomalies between the subsequent measurement series, and the reference one were then analysed. The distribution and temporal variations of the anomalies suggest a relationship between changes in density distribution of the rock medium in the longwall overburden, and the change in the stress state in the rock mass caused by the passage of the longwall front. An attempt was made to relate the variability of the state of stresses in the longwall overburden to the intensity of seismic tremors with energy > 10-4 J accompanying longwall exploitation.