1. A method of imaging using wave propagation data, comprising carrying out wave migration to model wave propagation layerwise, said migration comprising: using a Krylov space expansion of an exponent of a square root operator to approximate a predetermined wave equation, and using said approximation to model propagation of a wave between a first depth level and a second depth level of a geological structure, and using said migration, together with said wave propagation data, to map positions of features through which said wave propagation has occurred.