Near-field imaging of the surface displacement on an infinite ground plane

This paper is concerned with theinverse diffraction problem for an unbounded obstacle which is aground plane with some local disturbance. The data is collected inthe near-field regime with a distance above the surface displacementthat is smaller than the wavelength. In this regime, the evanescentmodes carried by the scattered wave are significant, which makes itdifferent from the far-field measurement. We formulate explicitlythe connection between the evanescent wave modes and the highfrequency components of the surface displacement, and present a newnumerical scheme to reconstruct the surface displacement from theboundary measurements. By extracting the information carried by theevanescent modes effectively, it is shown that the resolution of thereconstructed image is significantly improved in the near field.Numerical examples show that images with a resolution ofλ/10 are obtained.