The effects of seawater on the EMI response of UXO

摘要

The electromagnetic induction (EMI) response to objects in the marine environment is significantly altered by conductive seawater. Two effects include skin-depth propagation, and the response associated with perturbation of the currents induced in the surrounding water, (current channeling response - CCR). These effects distort the multi-frequency response of the target from the free-air eddy current response (ECR). Since the relative amplitude of CCR to ECR depends on variable conditions, including sensor-target geometry, seawater salinity, and the target condition (paint or corrosion forces background currents around the target causing negative CCR), accounting for CCR is problematic. Restricting operational parameters to minimize these effects is the most straightforward approach. To quantify the CCR and propagation effects, we developed analytical models, and performed underwater experiments. Algorithms include a two-layered sphere in conductive medium, and a perfectly conducting/insulating spheroid in a conductive medium to quantify the effect of aspect ratio and orientation. As a test on the application of EMI based discrimination, we performed spectral matching on underwater data using a free-air library. Limiting the frequency and target-sensor ranges, ECR dominates CCR, and propagation effects are small, and the algorithm is effective at identifying the target, and identifying clutter based on goodness-of-fit is feasible.