The primary objectives of this project were to 1) extend electromagnetic induction (EMI) forward models in an effort to understand how marine environments affect the performance of EMI sensors, 2) study EMI phenomena from highly conducting and permeable metallic objects in underwater environments, and 3) investigate the impact of the electromagnetic parameters of water (e.g., its conductivity or conductivity gradient) on the ability to discriminate unexploded ordnance (UXO) from non-UXO items.
The Method of Auxiliary Sources (MAS) was adapted for scenarios involving metallic objects placed in heterogeneous conducting media such as seawater. The contribution of the electric field was computed accurately using a surface impedance boundary condition that relates the tangential components of the electric and magnetic fields at frequencies such that the skin depth is small. Numerical experiments were conducted for both homogeneous and heterogeneous UXO-like objects subjected to frequency- or time-domain illumination. The near and far EMI fields and induced eddy-current distributions were calculated to illustrate the underlying physics of EMI scattering phenomena in aqueous environments. The coupling effects between an object and its surrounding conductive medium were analyzed and demonstrated at high frequencies (or early times for time-domain sensors). This project investigated the performance in underwater environments of current frequency- and time-domain state-of-the-art EMI sensors. The EMI scattering was analyzed due to highly conducting and permeable heterogeneous objects placed underwater and interrogated by the electromagnetic fields of the EM-63 and TEMTADS sensors.
This project found that marine environments have negligible effects on the performance of next-generation EMI sensors, which operate from 100 µs (10 kHz) to 25 ms (40 Hz). Similar effects were observed for inversion schemes like the generalized standardized excitation approach, the orthonormalized volume magnetic source model, joint diagonalization data preprocessing, etc. It was shown that currently available EMI sensors and signal-processing approaches could be used to detect and discriminate submerged metallic targets. This project studied ultra-wideband EMI field scattering from heterogeneous conducting and permeable rough surfaces; these investigations were carried out using the MAS supplemented by a surface impedance boundary condition. This project modeled current EMI sensors and used the models to illuminate objects placed in free space and conducting host media at various locations and orientations. This project’s studies demonstrated that rough conducting surfaces have a negligible effect on the EMI responses of highly conducting and permeable metallic objects at low frequencies (<15 kHz); at high frequencies, however, the effect is not negligible. The Debye dielectric relaxation model was employed for salt water and was implemented within the MAS-based numerical code to investigate how dielectric relaxation Closed-form solutions were developed for spherical and spheroidal geometries.
All of this project’s studies show that marine environments have a negligible effect on the EMI response of an object that can be detected by current EMI sensors and EMI response vary as functions of temperature and salinity.