Results from 3 seasons of surveys in maritime Canada using the Leica Chiroptera II shallow water topo-bathymetric lidar sensor

摘要

The coastal zone is a challenging place to map, techniques for mapping the land cover and elevation do not work under water and methods for mapping the seabed cover and elevation work well in deep water but are dangerous and expensive in shallow water. As a result, detailed information in the shallow coastal zone is lacking. This presentation will focus on the Chiroptera II, a new shallow water topo-bathymetric lidar sensor that is designed to collect seamless elevation data from land to the submerged terrain under coastal or freshwater. The sensor is equipped with a 1064 nm laser capable of a laser scanning frequency up to 500 kHz and a 515 nm green laser capable of a scanning frequency of 35 kHz. The lasers pulses are directed to the surface using a palmer scanner that results in an elliptical scan pattern with a 14 degree forward and back scan angle and a 20 degree left and right scan angle. The NIR laser is used to map details on the land and to assist in defining the water surface, which is critical in order to compensate for the 515 nm laser beam diffraction angle and the change of speed from air to water, thus accurately positioning the reflected seabed return. In addition to collecting the elevation at high resolution ~ 1m and vertical precision ~ 15 cm, the intensity of the reflected green laser also provides insights into what is on the seabed. A 60 MPIX RCD30 camera capable of capturing NIR and RGB bands of imagery is co-aligned with the lidar sensor to provide additional data that can be directly georeferenced with a ca 5 cm resolution. Full waveforms of the green laser are recorded and used to calculate discrete points representing the water surface, objects in the water column and the seabed bathymetry. Standard point attributes include the x, y, z positioning, echo information and intensity or amplitude of the reflected signal. An empirical method has been developed to compensate for the exponential energy loss of the green laser energy with water depth. This method results in a “depth normalized” seabed intensity image which provides details on the seabed cover materials. Tools have been built to calculate waveform metrics of the seabed return including: total area under the curve, skewness, kurtosis, the left and right distance and angle to the peak. These waveform metrics can then be used to construct grids representing the metrics which can be used for subsequent bottom cover classification. NSCC-AGRG has conducted topo-bathy lidar surveys throughout Maritime Canada for the past 3 years using this sensor. Since the sensor relies on the penetration of 515 nm light to detect the seabed, water clarity and the reflectivity of the seabed are constraints on the depth of penetration of the lidar. Coastal waters have variable turbidity, often dominated by wind induced wave action that suspends near shore sediments causing poor survey conditions. A turbidity management system has been designed that relies on meteorological forecasts and observed wind conditions as well as shoreline orientation along the coast to predict suitable periods for bathymetric lidar surveys. The turbidity management system will be presented as well as the results of several surveys. Various applications of these data will be presented including mapping submerged aquatic vegetation, providing input to hydrodynamic model simulations to support aquaculture site selection and improve oil spill preparedness, and shoreline sensitivity mapping. In areas of repeat surveys, the sensor has demonstrated the capability of mapping subtle changes in the dynamic near shore environment.