Measurement strategies for a confined microwave circular scanner

Abstract

International audienceThis article deals with the inverse scattering problem from scattered field data measured inside a closed microwave scanner. This system is presently being developed to demonstrate the potentiality of a non-invasive microwave imaging system for volumetric water content monitoring. The final goal is to retrieve soil moisture information as it is an important parameter for understanding fluid flow modelling, as well as water uptake by plants roots. Based on the actual state of the setup, we are proposing appropriate numerical tools, in particular a finite element formalism combined with a Lagrangian minimization scheme to provide a fast and accurate imaging tool. We will also show how we can improve the reconstruction algorithms by changing in a very simpler manner the measurement configuration, using either off-centred information or impedance boundary matching environment

Quantitative Imaging With Incident Field Modeling From Multistatic Measurements on Line Segments

“…3) even without a classical calibration procedure based on a reference target. This field is then directly used in the imaging procedure to reach the reconstruction results presented in Section V. The elementary fields and are computed with a homemade finite element solver [13]. For this type of configuration, the typical size of the underlying linear system is around 230 000 unknowns, and each field is computed in a few seconds for a given source position.…”

“…The descent direction is derived from the Quasi-Newton Broyden-Fletcher-Goldfarb-Shanno (BFGS) method [14], which requires the cost function gradient . This gradient is computed using an adjoint problem [13] (7)…”

Finite-element contrast source inversion method for microwave imaging

“…For example, in experimental setups used for biomedical imaging (BMI) applications where a lossy matching fluid is used, it is typical to assume a mathematical formulation which incorporates a spatial region of infinite extent and to impose appropriate radiative boundary conditions at infinity. Experimental setups using arbitrarily shaped conductive enclosures have also been investigated where the appropriate mathematical formulation assumes regions of finite extent with perfect electric conductor (PEC) boundary conditions being imposed on the appropriately shaped boundary [10][11][12][13]. In either case, the background medium can be assumed to be homogeneous or inhomogeneous.…”

On the Calibration of a Multistatic Scattering Matrix Measured by a Fixed Circular Array of Antennas

“…Several algorithms have already been implemented to compute the field in this specific configuration, all of them being based on the method of moments with various types of Green functions [12,29,30]. Here, we prefer to use a finite element method (FEM) with conformal meshing adapted to the casing configuration [1,2,13]. It requires typically about 6 seconds of run time on a standard computer, for a firstorder triangular mesh with 45 000 nodes and for 64 emitting antennas.…”

“…This is what has been done in a free space environment [3][4][5], in a liquid environment [6,7] or above a buried target [8,9] for example. Another possibility is to construct it electronically with a fixed array of antennas, each pair of emitter/receiver being selected thanks to hyperfrequency switches or any kind of multiplexer/demultiplexer devices, e.g., [10][11][12][13][14][15]. It is even possible to combine the two approaches as done for example in [16] where an array of modulated receivers is kept fixed and the multiple illuminations are created by rotating the target on itself.…”