Sound methods of underwater communication

Concert halls, rock bands and stereo equipment may commonly come to mind when acoustics is mentioned, but for Mohsen Badiey, professor of physical ocean science and engineering (POSE) and director of the POSE Program at the College of Marine Studies, the word has an entirely different meaning.

Badiey is investigating the use of acoustic systems, which involve the transmission of sound waves, to communicate with different underwater systems. Those systems include submarines and autonomous underwater vehicles (AUVs), small, submarine-like vessels that can operate without being tethered to the surface or to a ship.

“AUVs are going to revolutionize the way we sample the ocean,” Badiey says. “They are extremely versatile and can go places where ships can’t go—for instance, into a hurricane. In addition, they are cost-effective. You can send in an entire fleet of AUVs to take pictures and collect data without having to tie up a whole ship and the manpower needed to man that ship.”

Currently, the usefulness of AUVs is limited by the ability, or inability, of scientists to communicate effectively with them. Although R2-D2 and C-3PO, the fictional robots in Star Wars, appear to think for themselves, AUVs must respond to commands to complete their tasks at various underwater sites. According to Badiey, the principles behind underwater communication are similar to the way in which cellular phones use electromagnetic waves to allow people to communicate with each other on land.

And, just as cell phone services were unreliable at first, Badiey says, the ability to communicate underwater also must be improved. Electromagnetic waves cannot propagate through water, so sound waves must be used. As a result, scientists at the surface can communicate with underwater systems only by sending data and information through acoustic modems, in the same way that computers communicate with each other with digital modems.

“However, the ocean is a dynamic environment,” Badiey says. “Its physical properties, such as temperature, salinity and current direction and speed, change over time and space. These spatial and temporal changes in the ocean will, in turn, cause the transmitted sound intensity to fluctuate.

“As a result, the transmitted sound signals must be encoded and optimized. In other words, the properties of the sound must be changed to minimize the effect that the ocean has on it and to make the entire process more efficient.”

Badiey is working on a project off the coast of Hawaii to understand how these variations in the ocean environment affect the propagation of sound. The project is supported by the U.S. Navy and involves a number of institutions, including the Applied Physics Laboratory at the University of Washington; the Naval Research Laboratory in Washington, D.C.; the NATO Undersea Research Centre; Science Application International Corp.; Scripps Institution of Oceanography at the University of California San Diego; the University of New Hampshire; and Woods Hole Oceanographic Institution.

This project focuses on the transmission of sound in the shallow, coastal environment. Previously, ocean acoustics focused on finding enemy submarines that were hiding in the deep ocean. However, after the Cold War, the emphasis shifted to finding smaller objects, such as buried mines, in the coastal seabeds and to using sound to study the ocean environment.

“AUVs are not only smaller than submarines, but they also will be operated in waters of various depths depending on the nature of the mission,” Badiey says. “As a result, we need to use sound waves that have a frequency that is comparable to the size of the object that is being investigated.”

According to Badiey, the principles used in ocean acoustics can be compared to echolocation, which is used by mammals such as bats and dolphins. In echolocation, the animal explores its environment by producing sounds and then listening for their echoes.

The methods and techniques used in developing underwater communication systems are evolving. “Every time we go out to conduct a experiment at sea, it is like a brand-new mission,” Badiey says. “It is very exciting and very intense.” For each trip to sea, he says, researchers test a new piece of equipment, which they have designed and built using the knowledge gained in previous work.

This area of ocean science and communication engineering has just begun to capture Badiey’s interest. As an undergraduate, he earned degrees in civil and mechanical engineering, where he realized that mechanical systems were prone to problems with vibrations. His quest to resolve this problem led him into the study of sound, which is a type of mechanical vibration. In graduate school, he became interested in the mechanisms dealing with the interaction of water waves that induce dynamic pressure on the shallow water seafloor. Badiey says that the field of ocean acoustics has changed dramatically over the last 20 years and is still changing.

“Mathematically, the vibrations—whether they are caused by mechanical waves such as water waves and acoustics waves or electromagnetic waves—are all very similar,” he says. “However, the resulting problems can be different when the waves pass through ground, water or even air.”

Closer to home, Badiey has been instrumental in leading a team of marine scientists in equipping the Fourteen Foot Bank Lighthouse in the Delaware Bay with meteorological and oceanographic sensors. These sensors continually record such data as air and water temperature and the speed and direction of winds and currents as part of the Delaware Bay Observing System. This system will be part of a regional network located along the northeast Mid-Atlantic coast and tied in to a national initiative to monitor the coastal ocean. It provides yet another way to sample the physical parameters of the ocean.

Badiey also notes that the College of Marine Studies plans to establish a Coastal Ocean Dynamics Applications Radar (CODAR) system for the Delaware Bay to look at the surface of the bay and the adjacent coastal ocean in real time. CODAR is a system that can measure ocean surface currents remotely from shore and generate a map of surface currents within about 30 miles of the coast.

At the same time, in a different research project, the bay will be modeled for water waves and to determine how different wind fields generate waves. The information garnered in these projects has applications to issues including fisheries management, oil spill response and storm preparedness, and it can improve management of the Delaware Bay and the adjacent coastal region.

The oceanographic community has started establishing more and more of these types of real-time observing systems, which provide information more efficiently and economically than ever before, Badiey says. “There will always be new sensors and new ways of measuring the temporal and spatial variations of the environment around us,” he says. “But, pretty soon, we’ll get to a point that we will have more information than we are able to digest.”

Badiey says he believes that scientists who can analyze the collected data and use their knowledge to improve lives and prevent disasters are going to be increasingly in demand. “I think the interdisciplinary nature of the POSE program prepares our students with the skills needed to tap into this information technology age,” he says.

At the same time, he says, the college provides students with a background in policy issues. “Our students are learning to use technical information to better manage the ecosystem, the coastal waters, the beach erosion,” Badiey says. “All these are decisions that need to be made to protect the environment for future generations.”

—Kari K. Gulbrandsen, EG ’91M