Imaging Conductive Objects Through Metal Enclosures Using ELF/VLF Magnetic Fields

Imaging through conductive media is a pervasive problem in medical, industrial, and security applications. Several potential modalities such as X-ray, exotic particle beams, and related high resolution techniques have been employed in the past. However, the difficulty of production and safety of these technologies is a concern in practice. Of particular interest in this work are extremely and very low frequency (ELF/VLF, f<; 30 kHz) radio signals. We consider the feasibility of using ELF/VLF signals for detecting and imaging objects that are hidden inside thin-shelled conductive enclosures. It is shown that the hidden perfect electrical conductor (PEC) object partially blocks the incident magnetic field and results in a magnetic-field depletion that can be used to detect the object. Next, using FEKO simulations, a parametric study of through-conductor detection using a magnetic dipole is considered for a cubic aluminum shield. It is shown that signals above 1 kHz can be used to evaluate the outer shield properties, while signals at frequencies below 200 Hz can be used to effectively discern shapes of hidden objects by observing the magnetic distortion within 1 cm of the outer shield. To validate the results of theory and simulations, experiments matching the simulation conditions are conducted with a cubic aluminum container and hidden aluminum block. The experiments clearly demonstrate the presence of the magnetic field depletion as predicted by theory and simulations. The results of this work suggest that near-field ELF/VLF magnetic induction is an effective method for imaging through realistic metallic shields.