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Acoustics

Subaquatic sounds

Hydrophones reveal a world of sound from the bottom of the sea

From a depth of 20 meters, hydrophone equipment records the sounds of boats and fish at Laje de Santos

Lúcio Júnior/POLI-USPFrom a depth of 20 meters, hydrophone equipment records the sounds of boats and fish at Laje de SantosLúcio Júnior/POLI-USP

The Laje de Santos State Marine Park lies 42 miles off the coast of the state of São Paulo. Fishing is prohibited at the park, which serves as a spawning reserve for aquatic organisms, but guided diving is allowed on certain days, during scheduled hours. The occasional laxity of these rules, however, has allowed Linilson Padovese and his team opportunities to utilize the area to test standalone subaquatic acoustic monitoring equipment. Designed for installation on the sea floor, the equipment was developed at the Dynamics and Instrumentation Laboratory (LADIN) of the University of São Paulo’s Polytechnic School (Poli/USP). “We can measure, for example, sounds from motorboats between 8:30 and 11:00 p.m.,” explains Padovese. “The fishermen arrive, turn off their motors, stay two or three hours, and then leave.”

The equipment includes a hydrophone (a kind of microphone that reads underwater sound waves) and a set of electronic recording devices and batteries. “The test conducted at Laje dos Santos [ecological park] was one of our first experiments with the device,” says Padovese, explaining that things got their start when he decided to look into the measurement of undersea acoustic signals, a field of research that was just beginning in Brazil. “The problem is that there are no hydrophone or hydro-acoustic equipment factories in Brazil; and abroad, this equipment costs between $5,000 and $30,000, depending on how it is configured and used.” Another obstacle is that the most sophisticated hydrophones are used by the military in their vessels and submarines. To be sold on the private market, these devices require special authorization from the government where the factory is located.

Obstacles such as these led Padovese to develop his own technology. “We developed a new, very energy-efficient electronic reader that records signals on SD memory cards—just like the ones used in cameras—installed in a sealed cylinder and powered by alkaline batteries,” he explains. “We built some models that we distributed to partner research groups, and we are experimentally monitoring Lage dos Santos and Alcatrazes, an archipelago off the northern coast of the state of São Paulo, adjacent to the Tupinambás Federal Ecological Station, where fishing and boating are also prohibited.”

Each model consists of four 128 GB SD cards and batteries capable of up to five months of continuous monitoring. “The equipment can be programmed for continuous or scheduled readings,” says Padovese. Using this approach, the device can remain submerged for up to one year. The device’s sealing system was tested at a depth of up to 300 meters. However, divers at Laje de Santos and Alcatrazes installed and removed the equipment at 20 meters.

Inspections proceed more efficiently when speedboats from the Forest Foundation, the State Department of the Environment, responsible for administration of the Laje de Santos Park or the Environmental Military Police have knowledge of when violations in marine areas most likely will occur. “We’ve also looked into a system of real-time underwater acoustic monitoring, with the hydrophone connected to a transmitter that sends radio signals from Laje to foundation headquarters in São Vicente,” says Padovese.

According to José Edmilson Mello Júnior, administrator of the Laje de Santos Park, the hydrophone has proven to be important for environmental oversight and protection. “The area is an integrated conservation and protection site, and if the inspectors stop a boat—even if it is just passing through with fishing gear—the crew can be fined and have their equipment seized,” explains Mello Júnior. According to Padovese, a number of other types of acoustic events can also be recorded and studied—some at distances of several kilometers. Because sound travels through water at five times its normal speed, it can be detected at much greater distances than in the air. Whale calls and the movement of schools of fish can be identified. In general, data are analyzed via graphs called spectrograms, which show variations of acoustic frequency over time.

070-071_Hidrofone_242Data processing
A hydrophone is able to record large volumes of data. To process all of this information, the Poli group developed software that can and recognize signal patterns. For example, the researchers are able to As far as schools of fish, any specific association between acoustic signals and species would require a multidisciplinary effort by researchers in the fields of biology, oceanography, or marine sciences.

For Mello Júnior, the foremost need is an assessment of the animals, such as whales and dolphins that inhabit the Laje. “Three sounds can disrupt the lives of these mammals in the Santos Basin: oil drilling in the pre-salt areas, underwater pipelines [that take treated wastewater out to sea], and the anchoring of ships at the Port of Santos.” Padovese has formed a partnership, distributing hydrophones to researchers studying humpback whales (Megaptera novaeangliae) in the municipality of Uruçuca, near Ilhéus, in the state of Bahia.  “We visually monitor the whales between July and November from a hill at Serra Grande, 90 meters above sea level,” says Professor Júlio Baumgarten of the State University of Santa Cruz (UESC) in Ilhéus. “The equipment’s acoustic recordings were important because they complemented the visual record, especially when whales are calving or swimming with their calves.” Twice in one year, three devices were placed on the sea floor that could record sounds within a range of 200 square kilometers. “With these recordings,” explains Baumgarten, “we can track the whales’ activity, even at night.”

Deep-sea standalone equipment is now ready for service. The devices, similar to those used worldwide, received FAPESP funding. According to Padovese, the model that was built by his team cost between $2,000 and $4,000. The technology developed is being implemented and upgraded further through a partnership with the Naval Research Institute (IPqM) in Rio de Janeiro.

“The demand has greatly increased for studies measuring the impact of undersea acoustic sound from industrial operations like the expansion of ports, rivers and hydroelectric projects on aquatic animals.”  Besides continuing to add information to the signal-processing software, Padovese motivates his students to form their own business. “The idea,” he explains, “is to provide services in undersea acoustics in the areas of infrastructure and scientific research, but there are also opportunities for partnerships with electronics equipment businesses that have shown an interest in marketing these devices. Possibilities such as these are still being assessed,” concludes Padovese.

Projects
1. Underwater acoustic observatory for marine park monitoring (nº 2012/04785-0); Grant Mechanism Post-doctoral research grant; Principal Investigator Linilson Rodrigues Padovese (USP); Investment R$ 238,194.70 and US$ 24,207.17.
2. Multi-sensor platform for underwater sensor networks (nº 2012/23016-7); Grant Mechanism Post-doctoral research grant; Investigator Manuel Alfredo Caldas Morgan (USP); Principal Investigator Linilson Rodrigues Padovese (USP); Investment R$ 166,859.21.

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