A similar seafloor mapping technique to multibeam sonar is the use of airborne laser bathymetry, or Light Detection and Ranging (lidar), which use low-flying aircraft to scan pulsed laser beams across the seafloor and generate a swath of depth soundings.
For example, the Laser Airborne Depth Sounder (LADS) uses a narrow infrared beam fired directly at the sea surface beneath the aircraft but does not penetrate the water, and thus reflects directly back to aircraft for calculating the aircraft’s height above the water. A scanning mirror reflects a second green laser that tracks back and forth across the flight path in a grid pattern.
The scanned green laser penetrates the water column to a limit of about 50-70 m (in clear water) that reflects off the seabed, if lying within this depth limit. The airborne receiver detects any reflected green laser signals and converts the returned signals into multiple depth points after subtracting the aircraft height value obtained from the infrared laser.
Data density and acquisition rates vary between lidar instruments but typically collect about 1000 depth soundings per second, with a swath width of 200 m while flying at a height of about 500 m. At this height, the green laser beam footprint at the sea surface is about 2 m while the actual grid spacing can vary from 2-10 m apart.
With aircraft speeds of about 150-175 kn, lidar can deliver fast, high-resolution, shallow water surveying capabilities compared to vessel-mounted multibeam sonar systems. The major disadvantage of lidar compared to multibeam is that laser signals are highly attenuated in turbid water and therefore lidar is ideally suited for surveying only in clear shallow waters.
However, tropical coral reef areas provide additional navigational dangers to vessels conducting multibeam surveys, and so lidar has been used to successfully map large areas of continental shelf where vessel surveys would be impracticable.
The Gallery images and Movie below are from a LADS survey conducted on the northern Great Barrier Reef in 2009. The aircraft is a Fokker F27 which has been used for many years by the Royal Australian Navy LADS Flight team to conduct lidar hydrographic surveying. Many of the navigational charts in use for the Great Barrier Reef were surveyed using this aircraft. In late 2009, a newer Dash 8 aircraft will replace the Fokker F27.