Yes, lidar can penetrate water but can be quite a challenge. Mainly due to various limitations such as refraction and light absorption.
Greenlight (infrared wavelength of 532nm) from Lidar sensors can penetrate water the best and farthest due to its wavelength. It can penetrate ocean water of up to hundreds of meters (up to 300m).
Humans have been trying to learn more about our Mother Earth for centuries now. Even today, we do not know everything about our planet. With the rapid advancements in technology, we are using newer techniques to observe and learn more about the Earth.
Various different methods are used in oceanography, archaeology, surveying, geology, forestry, etc. One such technology is Lidar. Different surfaces can be analyzed with the help of this technology.
Our oceans or water bodies are one of the most rarely explored regions on the planet. This is due to the depth of the water bed, lack of oxygen and visibility below the sea level. However, some depths have been explored with the help of manned and unmanned capsules.
Such oceanic expeditions cannot be conducted easily without months or years of preparation. This is where Lidar can help. With the right equipment, Lidar can help survey the depths of the ocean without having to descend someone into the abyss.
Albeit, in theory, Lidar seems like a perfect technology to wade into deep waters, in practicality, it’s quite complicated and costly.
Most people working with Lidar make use of airborne or terrestrial lidar. But there is also a need for lidar in nearshore and offshore regions. Nearshore regions are relatively shallow and cannot be covered by survey ships, so we need to use bathymetric lidar.
Wavelength
The topographic lidar sensor emits an infrared wavelength of 1064nm in the US and 1550nm in Europe. This manages to penetrate only a few centimeters in water before losing all the power. We need to find a wavelength that can penetrate water and not be absorbed by it.
For pure water, you would need a wavelength of 440nm, but the water in our ocean is not pure. To penetrate this water, you require a wavelength of 532nm, which is the wavelength of the color green.
Increase Power Output
Now that you have found the right wavelength, there are still some more limitations. The deeper your laser penetrates, the more power it loses. And for lidar, the laser has to travel to and fro. Meaning, it may lose twice the power.
So, now you decide to increase the power output. But how much power can you increase without frying the laser medium? You can only obtain so much power from a laser before it starts to pose a threat to human safety. A high power laser beam can blind a human if they look at the laser.
But you can’t just pass lidar through water and expect results. Water has a tendency to bend light, and this phenomenon is known as refraction. It will not only bend the light at an angle but also slow it down.
A Bathymetric Lidar system is quite expensive even after being a decade old technology as it has to overcome various limitations. If the water is not clear enough, the signal may not reach the bottom or back. You may be spending a lot of money and time, sitting and waiting for enough signal to assess the water body.
Conclusion
Lidar is well known for its ability to map out the terrain with aerial and topographic methods but mapping underwater is a different story.
We found out that green light can penetrate water the best and farthest due to its wavelength, penetrating the water of up to hundreds of meters.
We already know that radar is not effective in penetrating water. Sonar is one acoustic alternative to radar, but it is not as efficient as Lidar. NASA proved the potential of LiDAR in oceanography back in 1988 using Airborne Oceanographic LiDAR (AOL).
Due to the properties of visible light frequencies, Lidar signals can be transmitted through the air-water medium and penetrate through the water. This way, we can measure the vertical structure of the ocean without having to risk human lives. Thus, we can set up Lidar systems on water or airborne platforms.