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REMOTE SENSING FEATURE


‘We still do not yet have a full


understanding of the response of lidar signals to changes in the optical and biogeochemical properties of marine particles,’ he said. As such, researchers like Schulien, Collister and many others are now working hard to expand what lidar can tell us about the oceans. Lidar emits a laser pulse and measures light scattered back to the detector. Perhaps the best-established ocean science lidar application is bathymetry, mapping the sea bed and objects on it. For this, Stockholm-headquartered Hexagon supplies two different lidar products. The first, Leica Chiroptera 4X, integrates one topographic lidar, one bathymetric lidar, and a red-green-blue-near-infrared camera, explains Anders Ekelund, vice-president of bathymetric lidar at the company. The second, Leica Hawkeye 4X, adds another bathymetric lidar designed to collect data from greater depths. Both are intended to be carried on light aircraft or helicopters. The difference between mapping


terrain topographically on land and bathymetrically under the sea is that light interacts with water much more than it does with air, Ekelund noted. Using 1,064nm wavelength lasers, topographic


‘The applications for using measures of light attenuation and phytoplankton biomass in the ocean are seemingly boundless’


lidars only penetrate a few centimetres into the water. The 515nm wavelength lasers in bathymetric lidars penetrate down to the sea-bed. On that journey, photons interact with


the water surface, molecules and particles in the water volume, objects in water, sea-bed vegetation and seabed itself. ‘A lot of waters around the world are turbid, and therefore systems need to work in those environments,’ Ekelund said. ‘High turbidity may cause you to get so much backscatter from the water, it hides the bathymetric signals.’ In general there is an exponential


www.electrooptics.com | @electrooptics


Stephanie Cayula and Damien Josset, of US Naval Research Laboratory’s Ocean Sciences Division, teamed up with Scientific Development Squadron (VXS) 1 to conduct airborne lidar research off the south coast of Alaska in January in a specially modified aircraſt


loss of photons as a function of depth, said Ekelund, described by the diffuse attenuation coefficient for downwelling irradiance, Kd. Meanwhile, the coherent laser beam also diverges exponentially, changing direction and speed as it passes the water surface. Together, these factors combine to mean


that lidar bathymetry can’t resolve small objects at the full depth extinction of the lidar, explained Ekelund. Instead, the Leica lidar survey studio (LSS) software that Hexagon supplies with its lidars analyses the full waveform of laser light returned, classifying the likely origin of scattering. This helps distinguish noise from vegetation or other objects.


Scattering information Classification data can help map seafloor habitats, although Ekelund noted that this requires research and adaptation to local species and sea-bed types. One of Hexagon’s research partners, at the Nova Scotia Community College (NSCC) in Canada, is now extending the existing classifications to identify the plant eelgrass. ‘By monitoring eelgrass you measure ocean health, and of course it’s an important habitat for fish,’ said Ekelund. The automated classification process the NSCC team has used identifies lidar returns from many different levels in the water, and recognises a specific intensity signal, he added. This enables researchers


to distinguish between eelgrass and other seabed terrain. NSCC also uses a similar approach to quantify the amount of rockweed, a type of seaweed useful as a fertiliser, in the region. ‘They want to have a sustainable harvesting method, so they don’t harvest more rockweed than grows,’ Ekelund explained. Rockweed grows in tidal zones, enabling researchers to estimate biomass using bathymetric lidar when the tide is in. ‘They are not measuring every plant individually, but they have a notion of the area they cover,’ Ekelund said. ‘They also get a measure of the plant height, and plant weight is a function of that.’ While posing a challenge to Hexagon,


light attenuation can provide useful information for researchers like Collister and Schulien. They use lidar to count particles in water, and measure how reflective they are, Collister explained. ‘The time required for light to return to the detector can determine the depth, or range, of the reflecting particles,’ he said. ‘As the pulse propagates, light is scattered and absorbed by materials in water that cause the return signal to decay with depth.’ Here Kd provides key information, as it relates to the concentration of materials in the water. Oceanographers can also get


information from how particles change the orientation of linearly polarised laser light when they scatter it. ‘The extent to which


June 2021 Electro Optics 13 g


US Navy/Lt Alex Christie/Office of Naval Research


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