Imaging under irregular surface using microwave holography

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In many cases reconstruction of microwave images of subsurface targets is performed by a back propagation method with rather simple model of the medium, such as the model of a homogeneous half-space with plane interface, or the model of a layered medium. If the medium has non-planar interface which does not allow moving the radar sensor directly over it, a flat sheet of radio transparent material or two-coordinate electromechanical scanner can be used for scanning and data acquisition. This allows moving the radar sensor in close proximity to the surface and obtaining radar samples belonging to a plane. In these conditions, the reconstruction of the microwave image is complicated by reflections from the irregular surface which usually dominate the reflections from the subsurface target. Surface reflections can be suppressed on the reconstructed image if the depth of the target is greater than the radar system depth resolution, which is defined by the bandwidth of the sounding signal. If the buried target is closer to the interface and the bandwidth is not wide enough, using a simple model for the medium doesn't allow filtering the surface reflections. In this case their influence can be suppressed by using the reconstruction method that takes into account information about the surface geometry. In this paper the possibility of using an RGB-D video sensor for registering the depth map of the investigated surface is considered. The microwave image is reconstructed by a modified back propagation method taking into account the registered depth map of the surface. The efficiency of this approach is illustrated by the experiments with subsurface holographic radar. The microwave images reconstructed with the new approach are compared with the results obtained by traditional back propagation methods in which the medium is considered homogeneous with planar interface.