When the laser is absorbed, it generates ultrasound waves that propagate down through the water column and reflect off underwater objects before racing back toward the surface. To do this, the system first fires a laser from the air that gets absorbed at the water surface.

“If we can use light in the air, where light travels well, and sound in the water, where sound travels well, we can get the best of both worlds,” Fitzpatrick said. Some of PASS’s instruments were initially designed for that purpose in collaboration with the lab of Stanford electrical engineering professor Butrus Khuri-Yakub.Īt its heart, PASS plays to the individual strengths of light and sound. The idea for it stemmed from another project that used microwaves to perform “non-contact” imaging and characterization of underground plant roots. (Image credit: Aidan Fitzpatrick) An invisible jigsaw puzzleĮnter the Photoacoustic Airborne Sonar System (PASS), which combines light and sound to break through the air-water interface. A Stanford “S” submerged beneath the water (middle) is reconstructed in 3D using reflected ultrasound waves (right). The experimental Photoacoustic Airborne Sonar System setup in the lab (left). But this technique is slow and costly, and inefficient for covering large areas. To date, most underwater mapping has been achieved by attaching sonar systems to ships that trawl a given region of interest. The upshot of all of this is that oceans can’t be mapped from the air and from space in the same way that the land can. Incidentally, this absorption is also the reason why sunlight can’t penetrate to the ocean depth and why your smartphone – which relies on cellular signals, a form of electromagnetic radiation – can’t receive calls underwater. “Light also loses some energy from reflection, but the bulk of the energy loss is due to absorption by the water,” explained study first author Aidan Fitzpatrick, a Stanford graduate student in electrical engineering.

Similarly, electromagnetic radiation – an umbrella term that includes light, microwave and radar signals – also loses energy when passing from one physical medium into another, although the mechanism is different than for sound. A system that tries to see underwater using soundwaves traveling from air into water and back into air is subjected to this energy loss twice – resulting in a 99.9999 percent energy reduction. The main barrier has to do with physics: Sound waves, for example, cannot pass from air into water or vice versa without losing most – more than 99.9 percent – of their energy through reflection against the other medium. Oceans cover about 70 percent of the Earth’s surface, yet only a small fraction of their depths have been subjected to high-resolution imaging and mapping. “Our goal is to develop a more robust system which can image even through murky water.” Energy loss However, seawater is much too absorptive for imaging into the water,” said study leader Amin Arbabian, an associate professor of electrical engineering in Stanford’s School of Engineering. Radar signals are even able to penetrate cloud coverage and canopy coverage. “Airborne and spaceborne radar and laser-based, or LIDAR, systems have been able to map Earth’s landscapes for decades. Their “Photoacoustic Airborne Sonar System” is detailed in a recent study published in the journal IEEE Access. The researchers envision their hybrid optical-acoustic system one day being used to conduct drone-based biological marine surveys from the air, carry out large-scale aerial searches of sunken ships and planes, and map the ocean depths with a similar speed and level of detail as Earth’s landscapes. An airborne sonar system for underwater remote sensing and imaging.

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