Microwave Nondestructive Imaging of Buried Objects Using Improved Scattered-Field Calibration Technique

Abstract

A novel microwave subsurface imaging scheme based on an improved electric field calibration technique is proposed in this paper. The proposed imaging scheme requires RF measurement system comprising of a uniform linear transmitter-receiver antenna array wherein the region under examination is illuminated sequentially to measure the scattering parameters of the test. An appropriate imaging frequency is selected so as to minimize the errors introduced due to coupling among the coresident antennas. The electric field required for the imaging algorithm is obtained from the measured scattering parameters using the proposed calibration methodology in two subsequent steps. The first step of calibration removes the systematic errors inherent in the measured scattering parameters by using known standards such as a metal plate and absorber foam. The second calibration step transforms the calibrated scattering parameters of the test object into electric field values so as to use them in the required inverse reconstruction procedure. The transformation is based on comparison of the measured scattering parameters of a known specimen and test object under the same test environment. The imaging algorithm based on the electric field formulation uses the standard iterative distorted Born procedure. For determining the location of the buried object, a novel data interpretation scheme based on analysis of variance (ANOVA) applied to time domain reflection coefficients obtained from various antenna elements is also proposed here. The applicability including performance evaluation of the proposed imaging scheme is tested by retrieving microwave image of various standard objects buried inside sand.