In this study, the shape and size of a combustion cavity with a fracture zone in the gasified coal seam was determined with the use of control boreholes and a ground-penetrating radar (BGPR) test. The underground coal gasification (UCG) field-scale experiment was performed in Carboniferous strata in coal seam 501 at a depth of approx. 460 m in the Wieczorek hard coal mine in the Upper Silesian Coal Basin, Poland. After the termination of the UCG reactor, five coring boreholes were drilled to identify the geometry of the resulting combustion cavity and the impact of the UCG process on the surrounding rock mass. Borehole ground-penetrating radar measurements were performed using a 100 MHz antenna in three boreholes with a length of about 40–50 m. This enabled the identification of the boundaries of the combustion cavity and the fracture zone in the coal seam. The fracture zones of rock layers and lithological borders near the control borehole were also depicted. As a result, the cavity was estimated to have a length of around 32 m, a width of around 7 m and a height of around 5 m. The analyses performed with the control boreholes and the BGPR provided sufficient information to determine the geometry of the combustion cavity and the fracture zone.

In this study, the shape and size of a combustion cavity with a fracture zone in the gasified coal seam was determined with use control boreholes and a ground-penetrating radar (BGPR) test. The underground coal gasification (UCG) field-scale experiment was performed in Carboniferous strata in coal seam 501 at a depth of approx. 460 m in the Wieczorek hard-coal mine in the Upper Silesian Coal Basin, Poland. After the termination of the UCG reactor, five coring boreholes were drilled to identify the geometry of the resulting combustion cavity and the impact of the UCG process on the surrounding rock mass. Borehole ground-penetrating radar measurements were performed using a 100 MHz antenna in three boreholes with a length of about 40-50 m. This enabled the identification of the boundaries of the combustion cavity and the fracture zone in the coal seam. The fracture zones of rock layers and lithological borders near the control borehole were also depicted. As a result, the cavity was estimated to have a length of around 32 m, a width of around 7 m and a height of around 5 m. The analyses performed with the control boreholes and the BGPR provided sufficient information to determine the geometry of the combustion cavity and the fracture zone.

P-wave velocity in coal seams increases with depth and reflects the in situ stress state in the rock mass. Anomalous velocity can indicate changes in the stress state resulting from various mining and geological disturbances. This information could contribute to the more efficient mining of coal at greater depths, particularly in seismically prone areas. The empirical relationship between P-wave velocity in coal seams and depth, developed by Dubiński in 1989, enables the determination of the magnitude of the velocity anomaly. However, this relationship was determined based on measurements taken to a depth of approximately 900 m. Currently, mining in the south-western Upper Silesian Coal Basin extends to greater depths, reaching around 1300 m. This study aims to update the empirical relationship for calculating reference P-wave velocities in coal seams by including new data. The archival 252 measurements were combined with 74 new velocity data from greater depths up to 1281 m. Regression analysis revealed that the updated power model offers a more reliable description of velocity changes in coal seams with increasing depth. This updated model can be used to identify anomalous stress zones and implement special protective measures in endangered mine workings. Our findings may contribute to reducing the risk of dynamic phenomena and enable more efficient exploitation of deep-seated coal seams.