FEATURE MAPPING
Left: Oso 2014 lidar survey: Colours show lost material in red, to emphasise that there is less buttressing on the upper slope; Right: Aerial image of Oso landslide ridge, March 2014
➤ shoreline habitats change over time. A visible spectrum laser (490nm in the blue open ocean; and 532nm in green coastal areas) is necessary to see through a column of water. Laser pulses shorter than those used above ground enhance the accuracy with which newer bathymetric lidar systems can detect the bottom, resulting in better vertical measurements through light-attenuating, coastal water environments. In addition, airplane flight passes need to be lower and slower for bathymetric lidar systems, to ensure they can detect the return signal.
‘The photons from your laser will be absorbed and scattered by the water. Your signal is from the photons coming back to your receiver after they hit the sea floor. The more photons you can get to the receiver, the better your chance of detecting [the signal],’ explained Kirk Waters, scientist at the US National Oceanic and Atmospheric Administration (NOAA). In order to differentiate signal from noise, transmitting more power into the water increases the depth that can be detected for a given water clarity and receiver. Another user of this technology is
USACE’s Jennifer Wozencraft, who notes that improvements in optical filters further reduce noise, and systems with larger field-of-view receivers collect more light from each laser shot.
20 ELECTRO OPTICS l OCTOBER 2016 As with most laser
systems, the recent advances in laser diodes and solid state lasers were important for our [lidar] technology
‘Sensors that have a large field of view and low noise characteristics are letting us survey in areas that we couldn’t before,’ she said, speaking in her role as project manager at the National Coastal Mapping Programme. Today’s smaller, less expensive components can be flown in the same aircraft for topographic and bathymetric surveys, thus allowing mapping of an entire coastline – from sand dunes out to shallow water – all in a single pass. With two iterations of the US coastal mapping project complete, Wozencraft and colleagues can quantify changes in sediment volumes and provide better data with which to evaluate complex geomorphological models. Work towards realising the potential benefits of bathymetric lidar continues. Airborne systems using
broad-beam, high-power lasers can only reach 60m water depth (some low-power versions only reach 10m). Looking deeper requires scanning from within the water itself. While several companies offer subsea 3D laser systems, many are triangulation- or camera- based, so have a shorter range and are more sensitive to background light and other noise. 3D at Depth, based in Boulder, Colorado, designs and builds lidar scanners for integration with remotely operated vehicles, autonomously operated vehicles, and tripods on the sea floor.
‘As with most laser systems, the recent advances in laser diodes and solid state lasers were important for our [lidar] technology,’ commented managing director Carl Embry. One challenge to overcome was designing a system that could withstand the harsh sea environment. Recent advances in optical coatings have helped increase the longevity of their systems. Also, motion sensors calibrate pitch and roll to create real world reference frames, thus overcoming the struggle to level a sea floor sensor. 3D at Depth supports a wide range of industries and applications, such as: reducing time between initial surveys and the start of extraction in the oil and gas industry; investigating sea life down to 3,000m off the Californian coast; preserving a portion of the submerged USS Arizona in Pearl Harbour, Hawaii; and improving water allocations and conservation.
And, the applications are only expected to
grow as new techniques and hardware become available.
How the wind blows
Not too long ago, wind speed and direction were measured just twice a day at selected locations around the globe using helium filled balloons that were assumed to move with the wind. However, lidar can also be used to measure wind velocity, and scientists are building on existing technology to prepare an Earth-orbiting lidar system that will measure winds over the entire planet.
@electrooptics |
www.electrooptics.com
➤
Harvey Greenberg, University of Washington/Puget Sound Lidar Consortium
Spc. Matthew Sissel, 122D PAOC; public domain/Wiki
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