Field testing of a polarization-based bathymetric LiDAR

Abstract

Atmospheric & Space Technology Research Associates Lidar Technologies (ASTRALiTe) has developed a polarization-based bathymetric lidar, called the edge™. The polarization approach differs from conventional lidar bathymeters that utilize waveform digitization and complex algorithms to separate laser reflections from the water surface and bottom. While discriminating and identifying these reflections in shallow depths can prove problematic for these conventional systems, a polarization-based lidar is designed to separately detect the reflection from the surface, which is polarized, and the bottom reflection which is depolarized. The U.S. Geological Survey is interested in defining the range of conditions under which the edge™ lidar can provide useful, accurate information on river depths. Potential riverine applications include collecting bathymetric data for non-contact streamflow measurement and mapping shallow water areas that pose a hazard to acoustic instruments deployed from boats. A critical factor influencing the success of a bathymetric lidar survey is water clarity. Suspended sediment and/or organic matter in the water column can scatter laser pulses and impede detection of the river bed. To gain a better understanding of how the edge™ lidar performs in various field settings, we selected several rivers with a range of water clarities. At each field site in situ measurements of the inherent optical properties of the water column (e.g., the absorption and attenuation coefficients at 532 nm, the wavelength of the edge™ laser) were collected as well as measurements of turbidity and chlorophyll and colored dissolved organic matter concentrations. The Colorado River near Lees Ferry, AZ, was selected because the turbidity is relatively low, 0.5 NTU, the water surface is generally smooth, and the substrate is predominantly sand. These conditions, along with the wide range of depths at this site, allowed the maximum depth penetration of the edge™ to be quantified. Preliminary results indicate that depths up to 5 m can be measured under such ideal conditions. The slightly more turbid (1.9 NTU) Blue River near Kremmling, CO, has a sand and gravel bed and had a smooth water surface at the time of survey. The edge™ was able to detect the bed up to the maximum depth of transects surveyed at this site, 1.2 m. The Colorado River near Parshall, CO, was also surveyed with the edge™ during a relatively high flow period in late spring. At the time of the survey, riffles were present at the water surface, the turbidity was higher than the Blue River (6.2 NTU), and the bed was composed of cobbles. Two transects surveyed with the edge™ at this site had a maximum depth of approximately 0.9 m. Despite a similar range in depths, one of the transects displayed a lower point density along the deepest portion of the transect. The reduced river bed coverage at this transect could be attributed to the elevated turbidity levels at this field site. A final field evaluation is planned for the Niobrara River near Norden, NE, to quantify the maximum turbidity under which the lidar can provide reliable bathymetry. The turbidity of the Niobrara River often exceeds 10 NTU, making this river a critical end member for evaluating the performance of the edge™ across a wide range of water clarities.