Sensitivity of electromagnetic waves to a heterogeneous bianisotropic structure

We study how the perturbations of a generally heterogeneous bianisotropic structure manifest themselves in the wave field, and which perturbations can be detected within a limited aperture and a limited frequency band. A short-duration broad-band incident wave field with a smooth frequency spectrum is considered. Infinitesimally small perturbations of the constitutive tensor are decomposed into Gabor functions. The wave field scattered by the perturbations is then composed of waves scattered by the individual Gabor functions. The scattered waves are estimated using the first-order Born approximation with the paraxial ray approximation. For each incident wave, each Gabor function acts like a 3-D Bragg grating and generates at the most 3 scattered Gaussian packets propagating in specific directions. For a particular source, each Gaussian packet scattered by a Gabor function centred at a given spatial location is sensitive to just a single linear combination of the elements of the constitutive tensor corresponding to the Gabor function. This information about the Gabor function is lost if the scattered Gaussian packet does not fall into the aperture covered by the receivers and into the legible frequency band.