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Light-driven Sonar Could Survey the Oceans From the Air

Drones or airplanes might be able to quickly map seafloors and inspect shipwrecks using the photoacoustic method

3 min read
An artist rendition of the photoacoustic airborne sonar system operating from a drone to sense and image underwater objects.
An artist rendition of the photoacoustic airborne sonar system operating from a drone to sense and image underwater objects.
Illustration: Kindea Labs

Sonar, which measures the time it takes for sound waves to bounce off objects and travel back to a receiver, is the best way to visualize underwater terrain or inspect marine-based structures. Sonar systems, though, have to be deployed on ships or buoys, making them slow and limiting the area they can cover.

However, engineers at Stanford University have developed a new hybrid technique combining light and sound. Aircraft, they suggest, could use this combined laser/sonar technology to sweep the ocean surface for high-resolution images of submerged objects. The proof-of-concept airborne sonar system, presented recently in the journal IEEE Access, could make it easier and faster to find sunken wrecks, investigate marine habitats, and spot enemy submarines.

“Our system could be on a drone, airplane or helicopter," says Amin Arbabian, an electrical engineering professor at Stanford University. “It could be deployed rapidly…and cover larger areas."

Airborne radar and lidar are used to map the Earth's surface at high resolution. Both can penetrate clouds and forest cover, making them especially useful in the air and on the ground. But peering into water from the air is a different challenge. Sound, radio, and light waves all quickly lose their energy when traveling from air into water and back. This attenuation is even worse in turbid water, Arbabian says.

So he and his students combined the two modalities—laser and sonar. Their system relies on the well-known photoacoustic effect, which turns pulses of light into sound. “When you shine a pulse of light on an object it heats up and expands and that leads to a sound wave because it moves molecules of air around the object," he says.

The group's new photoacoustic sonar system begins by shooting laser pulses at the water surface. Water absorbs most of the energy, creating ultrasound waves that move through it much like conventional sonar. These waves bounce off objects, and some of the reflected waves go back out from the water into the air.

At this point, the acoustic echoes lose a tremendous amount of energy as they cross that water-air barrier and then travel through the air. Here is where another critical part of the team's design comes in.

experiment setupImage: Aidan Fitzpatrick

To detect the weak acoustic waves in air, the team uses an ultra-sensitive microelectromechanical device with the mouthful name of an air-coupled capacitive micromachined ultrasonic transducer (CMUT). These devices are simple capacitors with a thin plate that vibrates when hit by ultrasound waves, causing a detectable change in capacitance. They are known to be efficient at detecting sound waves in air, and Arbabian has been investigating the use of CMUT sensors for remote ultrasound imaging. Special software processes the detected ultrasound signals to reconstruct a high-resolution 3D image of the underwater object.

An animation showing the 3D image of the submerged object recreated using reflected ultrasound waves. An animation showing the 3D image of the submerged object recreated using reflected ultrasound waves.Gif: Aidan Fitzpatrick

The researchers tested the system by imaging metal bars of different heights and diameters placed in a large 25cm-deep fish tank filled with clear water. The CMUT detector was 10cm above the water surface.

The system should work in murky water, Arbabian says, although they haven't tested that yet. Next up, they plan to image objects placed in a swimming pool, for which they will have to use more powerful laser sources that work for deeper water. They also want to improve the system so it works with waves, which distort signals and make the detection and image reconstruction much harder. “This proof of concept is to show that you can see through the air-water interface" Arbabian says. “That's the hardest part of this problem. Once we can prove it works it can scale up to greater depths and larger objects."

This article appears in the February 2021 print issue as “Laser Sonar Scans Ocean Depths."

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Europe Expands Virtual Borders To Thwart Migrants

Our investigation reveals that Europe is turning to remote sensing to detect seafaring migrants so African countries can pull them back

14 min read
A photo of a number of people sitting in a inflatable boat on the water with a patrol ship in the background.

Migrants in a dinghy accompanied by a Frontex vessel at the village of Skala Sikaminias, on the Greek island of Lesbos, after crossing the Aegean sea from Turkey, on 28 February 2020.

ASSOCIATED PRESS

It was after midnight in the Maltese search-and-rescue zone of the Mediterranean when a rubber boat originating from Libya carrying dozens of migrants encountered a hulking cargo ship from Madeira and a European military aircraft. The ship’s captain stopped the engines, and the aircraft flashed its lights at the rubber boat. But neither the ship nor the aircraft came to the rescue. Instead, Maltese authorities told the ship’s captain to wait for vessels from Malta to pick up the migrants. By the time those boats arrived, three migrants had drowned trying to swim to the idle ship.

The private, Malta-based vessels picked up the survivors, steamed about 237 kilometers south, and handed over the migrants to authorities in Libya, which was and is in the midst of a civil war, rather than return to Malta, 160 km away. Five more migrants died on the southward journey. By delivering the migrants there, the masters of the Maltese vessels, and perhaps the European rescue authorities involved, may have violated the international law of the sea, which requires ship masters to return people they rescue to a safe port. Instead, migrants returned to Libya over the last decade have reported enslavement, physical abuse, extortion, and murders while they try to cross the Mediterranean.

If it were legal to deliver rescued migrants to Libya, it would be as cheap as sending rescue boats a few extra kilometers south instead of east. But over the last few years, Europe’s maritime military patrols have conducted fewer and fewer sea rescue operations, while adding crewed and uncrewed aerial patrols and investing in remote-sensing technology to create expanded virtual borders to stop migrants before they get near a physical border.

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