Laser Swath Mapping System for Hydrothermal Plumes

Rob Reves-Sohn, Geology and Geophysics
Al Bradley, Applied Ocean Physics & Engineering

DOEI Project Funded: 2004

What are the primary questions you are trying to address with this research?
The method of hydrothermal plume detection by optical backscatter (OBS) is a highly effective part of a suite of plume detection techniques currently used on the AUV platform. The OBS essentially looks for cloudy water. A shortcoming of the current OBS sensor is that it is essentially a point source measurement: the AUV must pass directly through a plume in order to for the optical backscatter instrument to sing. Thus the resolution of the current OBS is limited to the separation between track lines in the AUV survey, and a narrow plume stem could "fall through the cracks". We propose to design an improved swath-type OBS instrument capable of detecting a plume several meters to either side of the vehicle, greatly enhancing plume detection. The design incorporates a pair of green lasers, one projecting to each side of the vehicle. Set back from the lasers are an optical detector and telescope which scan along the beams, looking for scattered light.

What is the significance of this research for others working in this field of inquiry and for the broader scientific community?
While this technology would prove universally useful to hydrothermal vent research, it is essential for plume mapping in challenging environments where an AUV is the only feasible survey method, such as under Arctic ice. Most importantly, it would allow systematic, robust searches for plume stems from an AUV platform. This stand-alone AUV capability is essential for the NASA-funded Arctic initiative. More broadly, any advance in optics in the deep ocean is exciting, and new adaptations of the laser swath system may arise.

What is the significance of this research for society?
The impact this technology will have on society will largely be determined by the scientific research it enables or facilitates via AUV-based hydrothermal vent surveys.

When and where will this investigation be conducted?
A preliminary investigation into the properties of an eye safe green laser in ocean water was conducted in the summer of 2003 by Johanna Mathieu, a summer student fellow. Encouraged by her findings, a further study into the quality of optical backscatter in vent plumes is to be conducted in November 2004 at the TAG hydrothermal mound. The third phase of the project will involve designing a photodetector based upon data gathered from the TAG voyage, with implementation of the laser/detector system in surveys to follow.

What are the key tools or instruments needed to conduct this research?
Apart from the elements of the instrument itself (two lasers and an optical detector in pressure housings), the Jason ROV and its high sensitivity monochrome camera will prove essential to the development of the laser swath system. Jason makes the TAG phase of the project possible: Jason is going to the TAG mound for another study, so the laser test fits seamlessly into the dive schedule. Not only does Jason give access to the deep ocean vents, the ROV is maneuverable enough to position the laser at will around vent plumes, allowing better qualification of backscatter characteristics. The camera will function as a light detector for this phase of the project by providing an image of the laser beam scattering off of plume sediments. The final detector for implementation on an AUV platform will be designed specifically to the purpose of backscatter detection, rather than image formation.

What are the greatest challenges - physical or intellectual - to conducting this investigation?
Selection and design of components of this system each rely on accurate values for light attenuation and backscatter signal strength. Such values will be obtained experimentally on the TAG mound. One challenge will be to design the instrument to work in conditions of varied water clarity and backscatter strength, not just the conditions on TAG. Keeping power consumption to a minimum is always paramount for an instrument to be deployed on an AUV, so power is an ever-present challenge.

Is this research part of a larger project or program?
As previously mentioned, the laser swath detector should facilitate AUV-based Arctic vent field research. Additionally, this technology can feed into an adaptive vent field search algorithm, under development by Mike Jakuba. Specifically, the laser system’s large swath width lends new flexibility in track line planning, which is welcome in an AUV that is essentially planning part of its survey.