Article By: Kenneth A. Stewart
Researchers from the Naval Postgraduate School (NPS) and The University of California’s Scripps Institution of Oceanography are collaborating to understand the effects of climate change on Arctic ocean acoustic propagation. Their experiment is being carried out in the Beaufort Gyre in an effort dubbed the Canadian Basin Acoustic Propagation Experiment (CANAPE) 2016-2017. The Beaufort Gyre is an ocean current system above the Canadian Arctic located in a region of growing economic and geo-political importance.
The CANAPE 2016-2017 effort is led by NPS Professor John Colosi and Scripps Researcher Peter F. Worcester. Their simultaneous measurements of physical oceanographic processes and acoustic propagation is aimed at providing clarity to the impact of changing Arctic sea ice conditions and ocean thermohaline structure on acoustic propagation – an issue of importance to the submarine and anti-submarine warfare (ASW) community as well as oceanographers and meteorologists studying the region. Some of this analysis is being carried out by NPS Ph.D. student Lt. Cmdr. Dominic DiMaggio who is being co-advised by Colosi and Research Professor of Oceanography, Dr. Wieslaw Maslowski.
This is hardly Colosi and Worcester’s first foray into the sea. They have been working together since Colosi did his postdoctoral studies under Worcester in the 90s on similar and related experiments from California to Hawaii to the Philippine Sea.
“My equipment consists of largely oceanographic instrumentation for this experiment while the Scripps team has the acoustical instrumentation. We bring both data sets together to try to understand how the ocean environment is affecting the acoustic fields,” explained Colosi.
|NPS Oceanographers Chris Miller, left, and Marla Stone, right, prepare to deploy research instrumentation from the U.S. Coast Guard Cutter Healy (WAGB-20) in August 2016. The effort is part of a comprehensive study, entitled the Canadian Basin Acoustic Propagation Experiment (CANAPE), to understand the effects of climate change on Arctic ocean acoustic propagation.|
For Colosi, understanding the acoustics is important to developing the ability to discern the difference between isotropic, uniform, and anisotropic sea conditions, which are characterized by variations that he calls acoustic “twinkles.” Those twinkles in the acoustic field are critically important to members of the ASW community. They can reveal everything from changing oceanographic conditions to the presence of an enemy submarine.
“To understand the anisotropy of ‘twinkling’ you need several measurements. You need to be able to see the large-scale thermal structure, but you also need to look at small-scale waves that perturb sound speed in random ways. It’s kind of like the problem of a telescope looking through the atmosphere,” explained Colosi. “The telescope is trying to look at a star, but the atmosphere has all sorts of turbulence and irregularities in it. We try to build our telescopes on the tops of mountains to avoid these effects, but there is still atmosphere in the way so we build special mirrors and carry out sophisticated data processing to adjust for those disturbances.
“[Similarly] the ocean is not just a quiescent fluid with only large-scale variation in temperature. It has small random fluctuations and turbulence of its own … We try to measure and observe those fluctuations, and use them to predict fluctuations in the acoustic field,” Colosi continued.
NPS Faculty Associate Chris Miller and Oceanographer Marla Stone recently embarked aboard the heavy icebreaker U.S. Coast Guard Cutter Healy (WAGB-20) en route to the Beaufort Gyre to deploy seven, deep-water moorings loaded with audio hydrophones that they hope will gather the data Colosi and Worcester are so anxious to analyze.
“They will be at sea for a month deploying moorings that will transmit and receive audio as well as deploying sensors that will measure salinity, temperature and currents,” explained Colosi. “The acoustic instrumentation is very unique. It is designed to handle the high pressure of the deep ocean and is very reliable. It can function for an entire year relying solely on battery power to send and receive acoustic signals.”
If succesful, measurements from the moorings deployed by Miller and Stone will be used to create a 3-D temperature map of the Arctic Sea in a process known as ocean acoustic tomography, which Colosi likens to an “ultrasound of the ocean.”
“We measure the time it takes a signal to go from Point A to Point B. If the water is warm, sound travels fast and its travel time will go down. If the water cools down, the corresponding sound speed is less and it takes longer to receive a signal,” explained Colosi. “If you have enough signals crossing paths, you can actually build a three-dimensional image of the warm and cold regions of the sea.
“Our instruments allow us to look at the ASW acoustical anisotropy detection problem - how well can you detect in any given direction where a source [submarine] is,” Colosi continued. “Imagine a naval scenario in which you are trying to track a quiet submarine through the region. Often times, you won’t be able to detect it because it is below the noise level of the surrounding ocean. Our measurements will help us to find optimal hydrophone arrays and deployment locations to best determine the submarine’s location.”
Posted October 26, 2016