Detection of dielectric targets buried in a very lossy Earth

In this paper, we propose an efficient method to reconstruct the dielectric objects buried in the lossy Earth using the Born approximation and the Born iterative method. For the detection of buried targets, the diffraction tomographic (DT) algorithm has been an efficient tool. It is fast and stable, and can be used for on-site detection. However, the exact DT algorithm is only valid in the lossless case. Although it was later generalized to the lossy case, the Earth conductivity still cannot be very high. Also, DT algorithms require high-frequency spectrum. Our study shows that DT algorithms provide high-resolution images if the upper frequency is larger than 50 MHz. When the upper frequency is smaller than 30 MHz, DT algorithms become invalid. In this paper, an efficient inversion method is proposed using the least-square minimization and Tikhonov regularization technique, which is valid for the VETEM system. Based on the fast forward solver for buried dielectric objects we developed (Cui et al. 2002), the inversion algorithms are very fast. For a 8 × 8 × 8 pixelled reconstruction domain, only 10 seconds are required for the Born-approximation inversion, and 160 seconds for each Born iterative scheme. Reconstruction examples are given to test the efficiency of the inversion algorithm.