A new approach to measuring subsurface water temperature using Raman spectroscopy and LIDAR methods
Information about subsurface water temperature is valuable to the defence, oceanography and environmental science sectors, with particular applications relating to underwater sound propagation, environmental monitoring and predicting algal bloom events. The mainstream technology for measuring subsurface water temperature is to deploy strings of thermocouples from ships or buoys, and accordingly there exists an opportunity for a new technology that can be used to systematically map subsurface water temperature on a local or regional scale.
Raman scattering is an inelastic scattering process, and in the case of water, the spectral content and the polarisation properties of the Raman signal both vary in a systematic manner with temperature. The use of Raman spectroscopy to predict water temperature was first proposed in the 1970s, and the potential for extending this to determine depth-resolved temperature profiles using LIDAR methods has been investigated by ourselves and others. When used in combination with LIDAR methods and fast, sensitive detection by photomultipliers, there are strong prospects for developing a relatively-simple optical system to make depth-resolved measurements of water temperature, and potentially other parameters such as salinity, turbidity and chlorophyll concentration.
I will present the design principles and operation of a custom-built LIDAR-compatible, four-channel Raman spectrometer integrated to a 532 nm pulsed laser. The multichannel design allowed for simultaneous collection of polarised Raman photons at two spectral regions identified as highly sensitive to changes in water temperature. Four independent temperature "markers" could be calculated from the Raman signals, and each of these was evaluated with regards to the accuracy with which water temperature could be predicted. When multiple linear regression models were constructed using a linear combination of the simultaneously acquired temperature markers, accuracies of ±0.3°C to ±0.7°C were achieved for a range of natural water samples from Sydney Harbour. I will also present data from preliminary field trials and discuss prospects for measuring parameters additional to temperature, using similar apparatus and methods.