In this age of satellites and search engines, you would be justified in thinking that every corner of the globe has now been mapped out—and this is largely true for the Earth’s terrain. However, the deep seas continue to elude us—only about a fifth of Earth’s ocean floor has been mapped to modern scientific standards. In other words, after years of concentrated global efforts, we still have an area twice the size of Mars left to scan.
The conventional way of mapping underwater terrain uses sonar technology. Sound waves are sent into the deep blue, and how long it takes for them to bounce back to their source tells us how far the bottom is and what it looks like. Sound waves travel well underwater, as opposed to electromagnetic radiation, which is why sonar is preferred to radar and optical imaging. Unfortunately, these same sound waves do not transmit very well through the air-water interface, which means that the sonar system must be mounted below the water’s surface. To scan an area of interest, a ship must sail across the area with a sonar system mounted underneath it, and sailing has never been a very speedy, cost-efficient mode of travel. Scientists estimate that it would take a single ship 350 years to map the remaining parts of the ocean that are deeper than 200 meters.
New research from Stanford University aims to quicken the pace of ocean mapping, however, by allowing the oceans to be scanned from the skies. The new technology presented recently in IEEE Access looks to use both electromagnetic radiation and sound waves, utilizing the best of both worlds with the aim of being useable from aboard aircraft. The key to this new technology is the photoacoustic effect: When a laser comes in contact with water, a sound is generated within the water. This is because the laser heats the water up ever so slightly, causing the water to expand. This minute expansion of the water generates a pressure wave—an inaudible sound that travels through the water. Utilizing the photoacoustic effect therefore allows a sound wave to be generated inside the water from outside of it, defeating the air-water interface on the way in. On the way out, high-sensitivity sensors are used to pick up the acoustic waves from above the water.
This groundbreaking system has shown promising results in the lab; the engineers at Stanford have already succeeded at locating underwater targets in a fish tank using an entirely airborne system—a feat that they say has never been documented before. With the success of their proof-of-concept system, chief investigator Amin Arbabian is certain that the system can be scaled up to real-world applications.
“This proof of concept is to show that you can see through the air-water interface,” he said. “That’s the hardest part of this problem.”
Arbabian and his team are next looking to tackle larger objects submerged in a swimming pool, as well as looking for ways to minimize interference caused by wind waves.
Why does it matter? Research like Arbabian’s can be tremendously beneficial to our everyday lives. Knowing the shape of the seabed would make it much easier to find sunken wrecks in the ocean, directly affecting lives. This knowledge would also provide a better understanding of ocean circulation patterns and marine ecosystems, enabling more accurate weather forecasts and even improved prediction of natural disasters. It would allow for more efficient wave-energy conversion, which is a sustainable source of energy, and would help us maintain a prosperous aquatic food supply. The list goes on and on.