A.V. Popov – Dr.Sc.(Phys.-Math.), Main Research Scientist, Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation of RAS (Moscow, Troitsk)
I.V. Prokopovich – Research Scientist, Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation of RAS (Moscow, Troitsk)
D.E. Edemsky – Ph.D.(Eng.), Senior Research Scientist, Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation of RAS (Moscow, Troitsk)
Attempts of subsurface object visualization by means of microwave holography started in the 60-ies of the last century. Those times, measurement and computation possibilities were not enough to perform this task. Recently, a substantial progress has been made in the problem of radio-frequency (RF) wave monitoring of closed rooms and identification of moving objects («through-wall vision»). This work is aimed at another important practical problem – visualization of subsurface environment and embedded contrast objects («in-wall vision»). Here, a number of basic difficulties are encountered: unfavorable relationship between the wavelength, object dimensions, probing range and antenna array aperture; EM wave attenuation in subsurface matter, and strongly interfering interface reflection. These problems are partially mitigated by scanning the material surface with a combined microwave transceiver. However, this method essentially hinders and complicates the process of subsurface RF-vision and limits the range of practical applications.
In this work, we apply another scheme of registration and processing of the subsurface probing data. The object of interest, embedded in a dielectric half-space, is illuminated from outside with harmonic microwave radiation. The amplitude and phase distribution of the scattered signal is captured by a multi-element receiver array separated from the material surface. Imaging of the scattering object is performed via mathematical wave front inversion.
In our previous paper, we described analytical approach and numerical algorithm allowing one to explain quantitatively image formation of a complicated object in microwave holography. It was shown that the main role in the image distortion plays losing a part of the scattered radiation angular spectrum. We derived a quantitative estimate of the holographic radar spectral window and point out a way to increasing spatial resolution of the radar by the method of synthetic aperture. Here, we generalize the inversion procedure to the case of a stratified non-uniform medium. In this case, the integral operator relating the microwave image with the object shape is almost identical to its free-space case counterpart but takes account of the wave reflected from the interface and the refraction in the material medium. Experimental implementation of subsurface imaging and an efficient method of eliminating the interfering reflection from the material boundary are proposed. Experiments have demonstrated a possibility to reconstruct a recognizable microwave image of a buried electrically contrast object.
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