Acoustic characterisation of unexploded ordnance disposal in the North Sea using high order detonations.

Abstract

Results are presented of acoustic measurements made during the disposal of 54 items of unexploded ordnance (UXO) in the North Sea during the pre-construction phase of two offshore windfarms. The disposals were conducted using high-order controlled detonation of donor charges placed on the seabed adjacent to the UXOs. The total charge masses ranged from 2.5kg to 295kg TNT equivalent, and acoustic measurements were made at ranges of 1.5km to 58km from the UXO. High-order detonations can present a risk of injury or death to marine mammals and other fauna from the high sound levels produced, and these results represent the largest data set of acoustic measurements ever assembled for publication. Acoustic measurements were also made on small scare charges, used as mitigation. The sound pressure pulses are presented with their spectra, and the levels of peak sound pressure and sound exposure are presented as a function of range from the source. Measured levels are compared to data from a shallow-water propagation model, and to widely-adopted exposure level thresholds used for marine mammals, illustrating the potential for injury at distances of several kilometres.