Case Study of marine UXO and scrap metal discrimination by relative size inferred from electromagnetic polarizability

A multi-component electromagnetic induction sensor for ROV-based investigation and discrimination of marine UXO (SubTEM-ROV) was deployed during a wind farm development project. The system supports multiple modes of operation, using either a five-transmitter setup for wide area surveys or a simplified single transmitter setup, which allows for better maneuverability during dredging operations. The latter mode was used to locate, dredge and visually identify a total of 173 targets. During the dredging, the electromagnetic polarizabilities of these targets were determined by an inversion algorithm, and later compared to values corresponding to the smallest UXO that constitutes a threat to construction operations. The analysis of the recorded data shows that the number of suspected UXO could have been significantly reduced by using even a relatively simple metric which compares polarizability amplitude to decay rate.