Parametric projectors protecting marine mammals from vessel collisions.

Abstract

Marine mammals are vulnerable to ship collisions. Measurements of controlled ship passages through vertical hydrophone arrays demonstrate a confluence of propagation factors and near surface effects that obscure the sounds of approaching vessels which then pose serious detection challenges for marine mammals. Joe Blue, who first identified these challenges, later conceived of a parametric method to mitigate them. A highly directional, dual‐frequency parametric array has been developed to reduce collision risks by selectively alerting only those animals in the direct path of approaching vessels. The system projector is comprised of multiple elements, band‐centered to transmit a high carrier frequency along with a lower side band signal. A single‐side band modulation and phase‐shift method are employed. The non‐linearity of water then demodulates the mixed high frequency carrier into a lower frequency waveform audible to both manatees and whales. The bow mounted array projects a narrow beam directly ahead of vessels, and “fills in” acoustical shadows to alert marine mammals of approaching danger. Controlled field tests of the manatee device in Florida's NASA wildlife refuge are proving effective. Real‐world deployments on select Navy and DOD vessels are planned this year and sea tests of a larger whale system will start next year. [Funded by U.S. Department of Defense, Navy Legacy Resource Management Program.]