  In seismic monitoring, one is usually interested in the response of a
changing target zone, embedded in a static inhomogeneous medium. We introduce
an efficient method which predicts reflection responses at the earth's surface
for different target-zone scenarios, from a single reflection response at the
surface and a model of the changing target zone. The proposed process consists
of two main steps. In the first step, the response of the original target zone
is removed from the reflection response, using the Marchenko method. In the
second step, the modelled response of a new target zone is inserted between the
overburden and underburden responses. The method fully accounts for all orders
of multiple scattering and, in the elastodynamic case, for wave conversion. For
monitoring purposes, only the second step needs to be repeated for each
target-zone model. Since the target zone covers only a small part of the entire
medium, the proposed method is much more efficient than repeated modelling of
the entire reflection response.
