Computation of the eddy-current modes of three-dimensional conducting bodies

摘要

Low-frequency electromagnetic induction (EMI) sensors are commonly used in subsurface detection applications because of their efficacy at detecting even small fragments of metal when they are buried near the surface. This efficacy can become a shortcoming when the detector is expected to locate specific classes of targets that are buried among metallic clutter. For these applications, broadband EMI sensors have shown considerable promise at being able to detect, classify and locate targets such as land mines, and discriminate between them and the clutter with low false-alarm rates. In such cases, where differentiating targets from clutter is a significant obstacle, detection strategies based on the discrete spectrum of relaxation frequencies (DSRF) have been shown to be highly effective. For such purposes, a dictionary of DSRF of targets of interest must be computed a priori. Several classes of targets such as sphere and rings have DSRF that can be derived analytically, however, in general, the DSRF must be computed numerically. Previously, numerical strategies have been presented for thin conducting shells and rotationaly symmetric targets. In this paper, we will present a strategy to compute the DSRF of arbitrary conducting targets using a null space free Jacobi Davidson iteration (NFJD).