Assessing EMI noise due to the marine environment to enhance underwater UXO detection and discrimination

摘要

We assess the noise level caused by marine environments in underwater UXO discrimination studies. Underwater UXO detection and discrimination is subject to additional noise sources, which are not present in land-based scenarios. Particularly, we study the effects of water surface roughness on the diffusion of EMI (electromagnetic induction) fields through the air-water interface and the interaction effects between an underwater conducting object and its surrounding conductive medium. Numerical simulations are conducted using the 3-dimensional setup of the Method of Auxiliary Sources suitable for low-frequency regime. Water surface roughness is modeled as an interference pattern between a finite number of surface waves with varying amplitudes, wavelengths and propagation directions. The results indicate that the perturbations in diffused and scattered EMI fields due to water surface roughness are negligible (although they depend on the shape of water surface) and that these perturbations decay with distance from the interface. Thus, the conducting water body may be assumed to represent a half-space in subsequent calculations for UXO detection. Finally, it is shown that there is some interaction between a conducting object and its surrounding conductive environment for frequencies above 100 kHz. This interaction is attenuated if the object is surrounded by an insulating shell, but is amplified if the shell is conducting. This non-negligible effect needs to be taken into account for the purposes of UXO detection and discrimination.