Abstract
We demonstrate the efficacy of a commercial portable 2D laser scanner (operating at a wavelength close to 1,000 nm) deployed from a fixed-wing aircraft for measuring the sea surface topography and wave profiles over coastal waters. The LiDAR instrumentation enabled simultaneous measurements of the 2D laser scanner with two independent inertial navigation units, and also simultaneous measurements with a more advanced 2D laser scanner (operating at a wavelength near 1,500 nm). The latter scanner is used routinely for accurately measuring terrestrial topography and was used as a benchmark in this study. We present examples of sea surface topography and wave profiles based on low altitude surveys (< ~300 m) over coastal waters in the vicinity of Cape de Couedic, Kangaroo Island, South Australia and over the surf zone adjacent to the mouth of the Murray River, South Australia. Relative wave heights in the former survey are shown to be consistent with relative wave heights observed from a waverider buoy located near Cape de Couedic during the LiDAR survey. The sea surface topography of waves in the surf zone was successfully mapped with both laser scanners resolving relative wave height variations and fine structure of the sea surface to within approximately 10 cm. A topographic map of the sea surface referenced to the airborne sensor frame transforms to an accurate altimetry map which may be used with airborne electromagnetic instrumentation to provide an averaged altimetry covering a portion of the larger electromagnetic footprint. This averaged altimetry is deemed to be significantly more reliable as a measurement of altimetry than spot measurements using a nadir-looking laser altimeter and would therefore improve upon the use of airborne electromagnetic methods for bathymetric mapping in surf-zone waters. The aperture range of the scanner does not necessarily determine the swath. We observed that instead, the maximum swath at a given altitude was limited by the angle of incidence of the laser at the water surface.
Highlights
Application of airborne LiDAR (Light Detection and Ranging) techniques over coastal waters has focused to a large extent on airborne laser bathymetry to map water depth from reflections off the sea floor using blue-green visible light (532 nm) to penetrate the seawater column [1,2]
We evaluated the efficacy of a 2D laser scanner (Riegl model Q240i-80) for measuring the topographic surface of waves in shallow coastal waters and in the surf zone in two study areas shown in Figure 1, the mouth of the Murray River, South Australia (SA) and in surrounding waters of Kangaroo Island, SA, where relative wave heights were compared with wave rider buoy data located near Cape De Couedic on the south-west tip of Kangaroo Island
We show a comparison of sea surface topography and wave profiles obtained from Q240 and Q560 data
Summary
Application of airborne LiDAR (Light Detection and Ranging) techniques over coastal waters has focused to a large extent on airborne laser bathymetry to map water depth (i.e., the sea floor topography) from reflections off the sea floor using blue-green visible light (532 nm) to penetrate the seawater column [1,2]. Other coastal applications of airborne and terrestrial LiDAR, not involving direct measurements over seawater, include measurements of, for example, shoreline variations [14] and sea cliff erosion [15,16] Many of these airborne studies are based on relatively large, fully integrated and expensive LiDAR systems, which may require the use of larger aircraft. A comparison was made with a second, more complex 2D laser scanner (Riegl model Q560) of significantly higher weight and power supply requirements, simultaneously operating at a longer wavelength with a much higher pulse repetition rate As this unit has been routinely used and ground-truthed during accurate measurements of terrestrial topographic features [22], its data can be considered to be of benchmark quality for the present study. LiDAR systems operating simultaneously, over coastal waters and over a surf zone region (to obtain wave profiles) and the use of LiDAR to obtain an altimetry map, which can be used to assist AEM methods for bathymetric mapping by providing an averaged altimetry within an area contained by the larger footprint (relative to the LiDAR system) of the AEM system
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