V.V. Razevig – Ph.D. (Eng.), Senior Research Scientist, 

Bauman Moscow State Technical University

Е-mail: vrazevig@rslab.ru

S.I. Ivashov – Ph.D. (Eng.), Head of Department,

Bauman Moscow State Technical University

А.S. Bugaev – Dr.Sc. (Phys.-Math.), Professor, Academician of RAS, Head of Department,

Moscow Institute of Physics and Technology (State University)

А.V. Zhuravlev – Ph.D. (Phys.-Math.), Leading Research Scientist,

Bauman Moscow State Technical University

М.А. Chizh – Ph.D. (Phys.-Math.), Junior Research Scientist,

Bauman Moscow State Technical University

During the last years, microwave holography technology has been successfully used for non-destructive testing of dielectric materials used in the aerospace industry, in particular, thermal insulation coatings of fuel tanks. To reduce boiling of cryogenic components of the fuel (liquid hydrogen and oxygen) on the launch pad and prevent icing of the fuel tank, a heat-insulating layer of polyurethane foam 2 to 8 cm thick is applied to its surface. Nondestructive testing of heat-insulating coatings should reveal defects such as delamination on the border with the surface of the tank and air voids in the thickness of polyurethane foam.

This technology allows reconstructing the microwave image of a subsurface object/defect using a electromagnetic field (microwave hologram) scattered by it, recorded either using a set of transmitting and receiving elements, or using a single transceiver, which is scanned along a surface called the measuring plane. In the case of a half-space bounded by a metal surface, which is an ideal reflector for electromagnetic waves, the solution of the inverse scattering problem is complicated by the presence in the microwave hologram the additional contribution from the re-reflections between the object and the metal surface.

In this work, the iterative Gauss–Newton method with Tikhonov regularization is used for reconstruction of the recorded microwave holograms (solving the inverse scattering problem). This method allows reconstructing the contrast function, describing the difference between the electrical properties of the object from the properties of the medium.

To test the method both numerical and physical experiments were carried out. It is shown that to obtain a well-focused microwave image of an object, it is necessary to use the Green function, corresponding to the sounding geometry. For this, the depth of the object should be known with submillimeter accuracy. In the case of unknown depths, it cannot be determined using the well-known best-focusing method, which works well in a homogeneous half-space, due to signal re-reflections between the sheet and the object. Several methods for solving this problem are considered: the use of a wide-band signal, the use of a multi-static configuration of the antenna system, a combination of the first two methods. It is shown that the best results are achieved by a combination of the two methods.

However, since the experiments were carried out using a setup with a single transceiver antenna, but to create a multi-static configuration several antennas are required, the only way for existing equipment is to use a wide-band signal. Due to the wide-band signal, at the reconstruction there will be only few false focusing depths around the true depth, and the last one will be determined using «the best focusing» criterion.

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