  We explore how ambient-noise interferometry can serve to track variations in
the distribution of scatterers within a medium. Experiments are conducted in
the presence and absence of a single scatterer that can occupy different
positions within the propagation medium. We calculate the similarity between
the Green's functions obtained in these cases (i.e., with versus without the
scatterer) as a measure of the temporal variations in the Green's function,
which shows exponential decay with respect to time. We also develop a formula
that relates this decay to the scattering strength of the scatterer. The
formulae are experimentally validated using experiments in a strongly
reverberating duralumin plate. This proposed method for finding the scattering
strength of the scatterer is based on information carried by the coda of the
multiple echo arrivals. We confirm the reliability of this approach through a
conventional method for the measurement of the scattering strength that is
based on direct arrivals.
