Shallow angle LIDAR for wave measurement

Abstract

The Exeter University Marine Dynamics Group has developed a shallow angle LIDAR for wave measurement purposes. This paper introduces the system and illustrates some metrology issues specific to this method. The LIDAR system uses a 532 nm (green) Q switched, high repetition rate, nanosecond pulsed laser. The system is designed to be scanned along a line or over an area. A 100 point square plot requires typically 0.5 seconds to complete. Whereas fixed-wing airborne LIDAR is continuously moving, the system described here is mounted in a fixed position and it is therefore possible to record sea surface shape and wave evolution over time. A plot comprises a set of surface elevation measurements determined by a combination of time of flight and high precision angle measurement. Unlike airborne LIDAR systems, the system is fitted to a shore or vessel mounted platform (with motion compensation in the case of vessel mounting) and the incident beam typically makes an angle with the sea surface of less than 10 degrees. The signal returns from airborne LIDARs, at approximately normal incidence, are strong compared to the very weak returns from the shallow angle system described in this paper. This fact has previously inhibited the development of such shallow angle systems. Some features of this system which make it viable are: a minimum field of view optical system, a very short pulse length high repetition rate laser system, an ultra high resolution beam scanner and what is effectively a one-dimensional image processing style coherent hybrid detection system operating with GHz data rates. The use of 532nm wavelength allows the scattering of light from calcareous plankton particles as well as other suspended particles and foam or aeration. The different nature of the light scattering from suspended particles or from foam or highly aerated water also allows the qualitative identification of breaking or foaming waves. The paper presents recordings of the real-time evolution of spatially resolved wave profile data obtained using this system. The shallow angle LIDAR system is shown to offer significant advantages over existing methods for wave measurement such as wavebuoys, wavestaffs, acoustic Doppler and radar, both in terms of the precision of the data returned and in the flexibility of deployment. Examples are presented of both near-shore and offshore applications. This system is one of the very few techniques capable of returning spatially resolved 'wave videos'. Another important feature of the new system for near-shore applications is that it avoids the vulnerability to damage of systems mounted in the surf and swash zones. The system can be installed at any state of the tide with no requirement to undertake installation work below the high water mark. The system is shown to be an ideal choice for most types of wave measurement applications and is eminently suited to the most extreme sea conditions. It is shown to be a portable remote sensing system capable of rapid temporary deployment, at sites of interest, during any sea conditions.