Downhole Raman Analyzer for Soil Gases at Discrete Depths

Soil gas concentration data are critically importance for assessing regions of the surface and subsurface as part of developing a more accurate understanding of subsurface biogeochemical processes, production of gaseous end products, their relationship to microbe-catalyzed pathways of interest, and eventual release to the atmosphere across the subsurface-land surface interface. Mesa Photonics proposes development of a real-time, analyzer for soil gases at discrete depths based on Raman spectroscopy for simultaneous measurements of carbon dioxide, methane, oxygen, nitrogen, water vapor and hydrogen. Previous work has required gas sampling from monitoring wells. In this project Raman probes will be miniaturized for real-time downhole measurements. The probes are built entirely from fiber optics and we envision small diameter probes that are conceptually similar to medical imaging catheters. They will be flexible and easily positioned in porous soils or narrow diameter monitoring wells. Implement and test compact side-view fiber optic light collection for Raman spectroscopy. Assemble, test, and optimize a 532 nm laser source based on an amplified and frequency-doubled 1064 nm fiber laser. Compare performance sensitivity, signal- to-noise ratios, interference due to background light) of Raman gas detection using the frequency- doubled 1064 nm laser and using a commercial free-space 532 nm laser. Direct measurement of multiple soil gases including nitrogen now requires multiple analyzers. Commercial gas analyzers will be suitable for field studies including long-term continuous monitoring of nitrogen-cycle and carbon-cycle processes. Specific end user applications include characterizing biologic respiration, CH4 oxidation, dissolution of CO2, reaction with soil carbonate, and atmospheric mixing. Instruments will be easily transported for field work, fully automated, and self-calibrating.