Blending light and sound: mapping underwater from above?

According to a recent study published in the journal, Stanford University engineers have created an aerial method for imaging objects that lie deep underwater, mixing light and sound to penetrate the stubborn interference barrier on the surface of the water. IEEE access.

In essence, we may be approaching the time when flying drones map the entire ocean floor, in high resolution.

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Engineers penetrate the surface, the underwater barrier with a healthy and light blend

The researchers think their new optical-acoustic hybrid system could eventually play a role in conducting drone-based biological marine surveys from the air, as well as performing large-scale aerial searches of sunken ships and planes and mapping the deep black depths of the ocean with opacity and speed similar to those of landscapes above water.

“Airborne and space and laser radar systems, or LIDAR, have been able to map Earth’s landscapes for decades. Radar signals are even capable of penetrating cloud cover and canopy cover,” said Amin Arbabian. study leader and associate professor of electrical engineering at Stanford’s School of Engineering, in a Stanford blog post. “However, seawater is too absorbent for imaging in the water,” he added.

“Our goal is to develop a more robust system that can reproduce images even through murky waters,” added Arbabian.

Loss of energy on the surface, underwater reef

Oceans make up about 70% of the earth’s surface, but only a small fraction of their depths are cataloged with high-resolution images and maps, he reports. TechXplore.

The main barrier to high resolution imaging is physical. For example, sound waves cannot flow from air to water or vice versa without losing most of its energy – more than 99.9%. This happens due to a reflective effect from a different medium.

Systems that use sound waves moving from water to air in the water will again lose this energy twice, losing 99.9999% of its energy.

The conventional seabed imaging method is too slow and expensive

Likewise, electromagnetic radiation – which includes microwaves, radar signals, and conventional light – also loses enormous energy as it passes from one physical medium to another. But most of the energy is absorbed in water, according to Aidan Fitzpatrick, first author of the study and a Stanford graduate student in electrical engineering.

In particular, this air-water absorption also explains why sunlight does not penetrate to the depths of the ocean. It is also the reason why cellular signals from smartphones, which is a form of electromagnetic radiation, cannot send or receive calls underwater.

This means that we cannot map the oceans from air and space in the same way we can map the earth. At the time of writing, most of the underwater mapping efforts have been completed by connecting sonar systems to ships assigned to specific regions of interest. But this method is expensive and slow, and lacks the efficiency needed to map large areas.

Solve an invisible puzzle, the best of both worlds

This is why the Photoacoustic Airborne Sonar System (PASS) was designed to outperform. It combines light and sound to break up the interference typically experienced at the air-water boundaries. The idea came from a different project, which relies on microwaves to perform “contactless” images and sharp characterization of underground plant roots.

Some tools used in PASS were created to work with this revolutionary paradox in mind, with the collaborative help of Stanford’s Professor Butrus Khuri-Yakub of Electrical Engineering.

In short, PASS uses the strengths of light and sound to overcome their relative weaknesses: “If we can use light in air, where light travels well, and sound in water, where sound travels well, we can get the best of both worlds, ”Fitzpatrick said in the Stanford blog post.

The new system compensates for the loss of signal amplitude

To achieve this, the system initially fires a laser from the air, which is absorbed by the water surface. Once this happens, it generates ultrasonic waves, which propagate downward into the depths of the water column until they reflect off an underwater object, before gliding effortlessly across the sunny surface.

Sound waves still lose most of their energy when they hit the air-water barrier, but by generating the sound waves underwater with lasers, the researchers eliminated the secondary energy loss mentioned above.

“We have developed a system that is sensitive enough to compensate for a loss of this magnitude and still allow for signal detection and imaging,” said Arbabian.

We could build a high-resolution map of the entire ocean floor

At the time of this writing, the PASS has only seen laboratory tests with a container the size of a large aquarium. “Current experiments use static water, but we are currently working to tackle water waves,” Fitzpatrick said. “This is a challenging problem but we believe it is feasible.”

Stanford engineers envision the technology could eventually operate on a helicopter or drone, Fitzpatrick said. “We expect the system to be able to fly tens of meters above the water,” Fitzpatrick added. But when it successfully completes tests over real-world oceans, it will only be a matter of time before the abyssal depths of each ocean are available for viewing in high-resolution images.