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 we deal with a layered medium containing one layer with water filled microheterogeneitis of subseismic scale (less than 0.01 of dominant wavelength). In order to be able to describe these heterogeneities reliably locally refined staggered grid is used and corresponding finitedifference technique developed and implemented. Its stability is studied and intensity of artificial reflections estimated. On this base numerical simulation is performed and structure of scattered waves analyzed in dependence of porosity and geometry of microheterogeneities.