Singly scattered waves form the basis of many imaging techniques that are used, for example, in radar and in seismic exploration. If one knows the arrival time of a singly scattered wave, along with its velocity and direction of propagation, one can determine the location of the scatterer. For multiply scattered waves it is more difficult to determine the locations of the scatterers because the waves propagate over many possible scattering trajectories involving a number of scatterers. Thus, multiply scattered waves are not so useful for imaging.
Multiply scattered waves are very useful, however, for detecting temporal changes in the medium through which they propagate. The changes may arise from the motion of the scatterers, changes in their scattering properties, or changes in the properties of the matrix in which the scatterers are embedded. The singly scattered wave in figure 1(a) traverses the medium at most twice. By contrast, the...
