Surface subsidence induced by underground mining activity may cause danger to buildings and infrastructure. The issue of the surface subsidence modelling is known for many years. The Knothe's theory is the most often used method of surface deformation modelling. Surface subsidence predictions accuracy depends mainly on the correct identification of model parameters and geometrical conditions of the strata occurrence. The paper presents a methodology for modelling of the seam occurrence in the computer system called MODEZ. That program is dedicated to surface subsidence prognosis. Shown system enables to take into account not only the variability of the seam deposit parameters but also the exploitations parameters. As a result it gives considerable improvement of the accuracy of the surface subsidence prognosis.