New numerical method for large-scale computer simulation of seismic waves in multiscale heterogeneous media is developed. As multiscale hereafter we mean media with heterogeneities of extremely different sizes. In the paper some specific multiscale medium is dealt with - karst intrusions within thin layered package. Karsts are produced as a result of interaction of water (surface or ground) with underlying bedrock, often limestone or marble. This causes very complicated, extremely irregular structure of karst intrusions - as a rule they form conjunction of local heterogeneities of different scales and very complicated shape. In order to be able to perform reliable finite-difference simulation of seismic wave's propagation through these structures the finite-difference techniques based on grids with local spatial refinement is developed and implemented.