What is Bathymetric LiDAR? by Peter Hobson

Bathymetric LiDAR is an adaption of terrestrial (land) LiDAR which uses the focussed, coherent, light pulse to penetrate below water in shallow, clear, rivers, lakes, and seas. Bathymetric LiDAR Systems use pulses of 2 wavelengths of light, one (Green) penetrates the water column, and the other (Red - standard terrestrial LiDAR wavelength) is reflected from the water surface. 
The difference in range between the 2 co-axial pulses gives the raw (uncorrected for Tide and Waves) depth. 
Combination of returns generates a digital terrain model of the sea bed, and further information about the nature of the seabed and the water column can be derived from analysis of the returned pulses. 
Bathymetric LiDAR Systems have economic advantages over conventional survey methods along coastlines, and in areas which are difficult and expensive for vessel based survey systems to operate. 
They are limited by the turbidity (clarity) of water columns, and thus not efficient everywhere.

Peter Hobson is founder and Director of Waywiser Hydrographic Consultants Limited, based in Plymouth, UK. Over the last 12 years he set up 2 Bathy LiDAR companies - Admiralty Coastal Surveys AB and PELYDRYN Limited.