On the quantitative determination of coal seam thickness by means of in-seam seismic surveys

By means of in-seam seismic (ISS) surveys carried out at Karvina coal mine of OKD, Czech Republic, in two neighbouring panels it is shown how the distribution of coal seam thickness can be investigated if the coal seam consists of almost pure coal. The coal seam under investigation was considerably affected by erosion, which resulted in thickness changes amounting from ∼30 cm up to ∼4.40 m. First, by applying ISS-tomographic inversion to the group travel times for a constant frequency value, the distribution of the related group velocity was determined, which showed the extension of the erosions within the survey areas. By correlating the group velocity distribution with known values of the coal seam thickness along a gate, a relation in terms of a polynomial approximation between these quantities could be derived. This relation is specific for a chosen frequency value and resolves in general only some of the thickness range. For the application presented in this paper, a value of 200 Hz was chosen for the constant frequency value by which a range of coal seam thicknesses from about 100 cm to about 300 cm was resolved. This was the main objective of the surveys. For the investigation of a greater variation of coal seam thickness (or even the complete thickness distribution), the described procedure has to be repeated for different constant frequency values. The final thickness map then has to be combined from the individual results for the different constant frequency values. The thickness range that was investigated by the surveys was in good agreement with the thickness encountered by mining the panel.