Understanding the Sound and Motion of the Ocean

 

CTD Image

Conductivity, Temperature, and Depth (CTD)

Seventy-one percent of the Earth’s surface is covered by ocean, and 80% of all life on the planet resides there. It’s no surprise, then, that the deep blue sea also presents some of civilization’s great scientific and engineering challenges. Achieving a better understanding of some of those challenges and finding solutions to them can have a significant impact on the future of life on Earth.

Case in point: Research conducted by Mechanical Engineering Professor Pierre Lermusiaux, the Doherty Associate Professor in Ocean Utilization, could aid in basic interdisciplinary ocean science, naval operations, and the control and location of underwater vehicles. Lermusiaux and a research team of students and group members, in collaboration with Taiwanese and Woods Hole Oceanographic Institution scientists, have been studying and predicting the way ocean variability—the perpetual changing of currents, temperatures, salinity, and the contours of the seafloor—alters the way sound travels through the water.

The results of this research could make it easier for Navy submarines to evade detection or for remotely operated underwater vehicles, like those used to combat the recent oil spill in the Gulf of Mexico, to maneuver more accurately. It could also aid in basic oceanographic and climate research by helping to calibrate systems for using sound waves to measure ocean properties, such as temperature and seafloor topography.

Using both theoretical computer models and on-site experiments close to the islands of Taiwan and Kauai, the team found unexpected changes in the way ocean and sound waves interact when they are emitted near the edge of a continental shelf, where the average slope of the seafloor changes abruptly. For the first time, they were able to make integrated four-dimensional ocean and acoustical predictions of how sound waves would propagate at a given time and location, evaluate the degree of uncertainty in those predictions, then verify them with acoustic measurements.

 

Paving the way for new acoustic applications in underwater operations and communications

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     Bathymetry of the region

The continental shelf area is important because such regions are increasingly being exploited for oil and gas drilling, and are key to the undersea naval operations of many nations. These regions are also significant in assessing the health of the oceans and climate dynamics. “A lot of research interests,” he says, “are now focusing on the complex shallower seas and their interactions with the deep ocean.”

Lermusiaux’s research, which was funded by the Office of Naval Research and published in the IEEE Journal of Oceanic Engineering, predicted and explained how sound waves used for sonar imaging and underwater communications can be affected by the interplay of large-scale currents, eddies, internal tides, and the irregular topography of the seafloor. It also demonstrated where and when each could be a key factor in predicting how sound waves will travel. Some scientists, including researchers at MIT, have suggested that measuring the propagation of sound waves over long distances in the ocean could make it possible to monitor the effects of climate change by allowing them to determine ocean temperatures and circulations over large regions.

Nadia Pinardi, Professor of Oceanography at the University of Bologna, believes that the research “confirms unequivocally that ocean acoustic uncertainties are connected to a detailed knowledge” of features and processes of the ocean at a specific location and time. She adds that these results could pave the way for new acoustic applications in underwater imaging and communications.

Arthur Miller, a research oceanographer and senior lecturer in climate sciences at the Scripps Institute of Oceanography in San Diego, observes that techniques pioneered by Lermusiaux now make it possible to determine which factors are most important, and thus improve the accuracy of predictions used for carrying out underwater measurements—results, he adds, that are critical to practical applications in sonar operations.meche logo

 

Read the full article in IEEE Xplore.

 

Excerpted from an MIT News article by David Chandler.