Underwater UXO discrimination studies: adapting EMI forward models to marine environments

Abstract

Recently, several sensor technologies, such as magnetometers (total-field and gradiometers) and various types of timedomain and frequency-domain electromagnetic induction (EMI) sensors have been developed and applied successfully to land-based subsurface unexploded ordnance (UXO) detection and mapping. Current researchers of underwater UXO detection commonly apply land-based UXO detection technologies directly to underwater scenarios. Since the electric conductivity of water is much higher than that of soil, an object's EMI response underwater should be different than in a dry environment because inside the conducting water low-frequency electromagnetic signals change both in magnitude and phase, particularly at high frequencies where induction numbers (i.e., wavenumbers) are significantly high. In order to fully explore the capabilities and limitations of land-based EMI sensors for underwater UXO detection and discrimination, in this paper we assess the applicability of current EMI forward models by investigating how the electromagnetic parameters of seawater affect the performance of state-of-the-art EMI sensors. The studies are conducted using the Generalized Standardized Excitation Approach. Objects' locations are inverted for using a reduced version of the HAP technique that combines the magnetic field and its gradient. Particular attention is given to understanding how seawater EM parameters or a multilayer conductive background change objects' EMI responses and affect the UXO discrimination process.