NDTnet 1998 Aug, Vol.3 No.8

Abstract

The penetration of microwaves in various materials gives active microwave imaging a large potential for subsurface radar, medical imaging, civil engineering etc. However, its practical applications are so far very limited because existing inversion algorithms suffer from serious problems when either discontinuous profiles of high contrast are to be reconstructed, or the medium is lossy, or the frequency band of operation is essentially restricted from both sides,i.e. low and high frequency data are not available.

In this work, a new one-dimensional imaging approach overcoming all these difficulties is proposed. The method represents a further development of the frequency domain reconstruction algorithm described in (1) and includes the following:

step 1	solution of a forward problem for some initial profile (constant permittivity distribution equal to I can be adopted) using well-known formulas for the calculation of reflection coefficient from a multilayered medium. The problem should be formulated in terms of an optical path length x rather than in geometrical depth. Calculation of a discrepancy between the calculated and given (measured or simulated) reflection coefficient data r(k) in the frequency band of operation (k denotes the free space wave number)
step 2	determination of a depth, at which the derivative of reaches its maximum. Installation at this point of a step-like correction to the dielectric permittivity profile minimizing the maximal error between given and calculated reflection coefficient data
step 3	return to the step I as long as the error calculated in the previous step is larger than an acceptable error. Otherwise, the iterations are stopped and the optical path length which has been so far an argument is transformed into the geometrical distance.

A similar procedure can be formulated to obtain both the dielectric permittivity and conductivity profiles. However, their simultaneous determination requires the knowledge of a total thickness of inhomogeneous slab and the background permittivity.
Numerical simulations demonstrated fast convergence of the solution and good robustness of the approach for complicated multilayered dielectric profiles. Therefore, it can be used for the data processing in the frequency-swept imaging radar at microwaves and millimeter waves.Reference:
1. Mikhnev, VA., Vainikainen, P. Profile inversion of stratified dielectric media using the two-step reconstruction. Proc. of the 27th European Microwave Conference, Jerusalem, 8-12 September 1997

Abstract Source:
Book of Abstracts, 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN: 87-986898-0-00

Full-Text Source:
Proceedings of the 7th European Conference on Non-Destructive Testing, 26-29 May 1998, ISBN:

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