Hurricanes wreak havoc not just on manmade infrastructre, but also on coastal shorelines, shifting large amounts of soil (through erosion or accretion) that can dramatically reshape coastlines, sometimes with serious environmental impacts. The US Navy worries constantly about avoiding collisions with other watercraft, not to mention avoiding underwater mines. And marine biologists would love more precise mapping and imaging methods to better delineate the structure and development of coral reefs.
Increasingly, researchers are turning to LIDAR imaging systems to produce the data-rich 3D images they need for all of these applications. LIDAR — which stands for LIght Detection and Ranging — is an optical remote sensing technology that exploits the same basic principle as radar and sonar. The system sends out pulses that bounce off objects and analyzes the returning signals to determine an object’s distance from the source — except it uses light wave pulses instead of radio waves.
A LIDAR instrument transmits pulses of light to a target, and the parts of the spectra that are not absorbed by the target are reflected back to the system, which then are detected, stored and analyzed. It’s the changes in the properties of the light when it scatters back that enable scientists to measure specific properties of the target. The more frequent the light pulses emitted in a LIDAR system, the more information is gathered, and the more accurately a target area can be mapped.
Usually, a LIDAR system is mounted onto an aircraft equipped with a GPS receiver to track its exact location and altitude. It also needs a high-accuracy inertial measurement unit (IMU) to track the pitch and roll of the airplane so that movement can be accounted for in the final analysis. All the data collected from the various instruments, when combined, can give an elevation that is accurate to within 6 inches.
There’s more than one kind of LIDAR system, and which one to use depends on the application. If you want to map a shallow river bed underwater, or use the system for mine detection, you’re better off using differential absorption LIDAR. Underwater imaging in particular can be difficult using infrared and near-infrared preferred for terrestrial mapping, since water absorbs those wavelengths; only the blue-green end of the visible spectrum can penetrate water, for the most part.
Arete’s Bruce Hubbard reported at Tuesday morning’s IPF session on the development of his company’s streak-tube imaging LIDAR (STIL). STIL uses a combination of air and underwater-borne platforms to produce very high resolution 3D images of ocean scenes from a remote platform. In particular, says Hubbard, it enables “unrivaled object detection and classification in turbid media” — that is, the cloudy waters along coastal shorelines, or especially choppy waves. Arete uses STILL as the basis for its collision avoidance system, which allows high-speed water craft to detect and avoid floating obstacles, shallow bottoms, or submerged (or floating) military mines.
Another speaker at the session, Jennifer Wozencraft, is a scientist with the US Army and LIDAR Bathymetry Technical COE. She has been conducting various surveys around the world using a system developed in the 1980s known as SHOALS (Scanning Hydrographic Operational Airborne Lidar Survey). Applications include shoreline mapping — important before and after major storm surges like hurricanes to measure any topographical changes like soil erosion or accretion — as well as mapping coral reefs, nautical charting, and flood water modeling.
Bathymetric LIDAR systems like SHOALS transmit two light waves, one in the infrared and one in the green spectrum, and are capable of detecting two returns that delineate the water surface and seabed. The infrared band is quickly absorbed, so it’s perfect for detection on the surface of the water, while the green band is used as the optimum color to achieve maximum penetration in shallow water.
In 2003, Wozencraft began working with a new and improved system, called Compact Hydrographic Airborne Rapid Total Survey (CHARTS), which fused multiple sensors into one remote sensing system to better characterize coastal dynamics and update shallow water charts. Specifically, CHARTS incorporates a hydrographic laser system, topographic laser system and digital camera, with a future option being a hyperspectral imager.
Wozencraft has deployed these systems to map coastlines all over the globe. Her latest project, Coastal Zone Mapping and Imaging Lidar (CZMIL) employs a new state-of-the-art optical system for improved depth measurements, as well as streamlined data processing. The latter is particularly important to fill in key gaps in data — the result of things like bubbles in breaking waves, or particles of sand stirred up in shallow coastline waters.
But the most striking part of Wozencraft’s presentation were her before-and-after LIDAR images of shorelines post-Hurricane Ivan (2004) and Katrina (2005):
