Greetings.

Does lidar surveying works to map sea bottom ? NOT from flying above water surface, but hovering under the surface ? ( as towed side scan sonar, as long as light propagation allows on reasonable transparency )

No

LIDAR uses IR light at around 1 micron, and the attenuation of such light is very strong. Much of the light would be absorbed or scattered before it traveled a meter

https://www.researchgate.net/figure/Penetration-depth-attenuation-length-of-electromagnetic-radiation-in-water-vs_fig1_258254992

Much of the EM radiation spectrum in general wouldn’t be very useful. (This is why they have to use ELF to send messages to subs to tell them get near enough to the surface to allow for easier communication)

Thank you. Well, learned something else today. Infrared based.

Can lidar be conducted over/across air in visible light ? It would be nearly plain photography then ?

4 minutes ago, Externet said:

Thank you. Well, learned something else today. Infrared based.

Can lidar be conducted over/across air in visible light ? It would be nearly plain photography then ?

It could, but IR has more efficient detectors and it more readily penetrates hazy and foggy atmosphere. Plus you don’t have bright lights flashing, which might bother (or alert) people

It’s not photography, though. You are measuring time-of-flight signal of a light pulse to measure a distance.

To amend my previous post, in checking something I saw a reference to bathymetric lidar, which uses green light to scan shallow water, like riverbeds and coastline.

Can you see the bottom of the sea?
If you can, then LIDAR would work.
If not, then... not.

Thanks.

What differs on... If you cannot see the terrain but only the forest above it, airborne lidar gives you the topography contours without eyeing the soil surface.

If cannot visually see the ocean floor; submerged lidar scanning will not perform ?

2 hours ago, Externet said:

Thanks.

What differs on... If you cannot see the terrain but only the forest above it, airborne lidar gives you the topography contours without eyeing the soil surface.

Not quite - canopy coverage generally has gaps, which is what lidar leverages. Unlike a photo, you don’t need to sample all of the surface to get a map. If you got one data point per square meter you could still make a contour map of reasonable precision.

2 hours ago, Externet said:

If cannot visually see the ocean floor; submerged lidar scanning will not perform ?

Not unless it’s really close to the bottom to begin with, owing to the strong attenuation, and if you’re doing that you don’t need lidar. You’re basically doing an old-fashioned sounding