Lidar: light detection and ranging (think sonar, only with light!)
There are both land- and water-based applications of lidar.
The basic principle of bathymetric lidar is that a laser mounted on an airplane or helicopter generates both a green (532 nm) and infrared (1064 nm) light pulse. The infrared pulse, which essentially has no attenuation in water, reflects off the air-sea interface and provides a surface return. The green pulse, which does attenuate, penetrates the water and reflects off the seabed giving a bottom return. The difference in travel time between the surface return and the bottom return can be used to estimate water depth. Because lidar is optical it is constrained to the photic zone and is heavily dependent on water clarity. Therefore, lidar typically will only work in 50 meters of water or less. A couple of the tech specs claim 70 meters, but this has yet to be actually acquired in the field. If I learned anything at all during this tech review, it is to take company technical specifications with a huge grain of salt!
Here is a nice little schematic of the green energy pulse that I modified from (Guenther 2001 [pdf]).
When the green energy pulse hits the water, a small portion of it reflects off the surface. The remainder penetrates the water column, where it is subjected to refraction, absorption, and scattering. When the light wave hits the seabed, it reflects back towards the laser and is again subjected to refraction, absorption, and scattering. Only about 4% of the incident green energy reaches the seafloor.
If you are interested in lidar, the Guenther chapter is a great introduction.