Fiber Optic Raman Spectroscopy for Detection of Methane Hydrates and Related Species

摘要

A capillary waveguide fiber optic probe system has been constructed which is capable of rapidly collecting Raman spectra of organic compounds in water. It consists of a dual fiber optic probe coupled to a capillary waveguide designed to increase the volume of sample interrogated by the excitation light. A Nd:YAG laser operating at 532 nm is used to generate Raman scatter at the sample in the capillary which is transmitted by the collection fiber to a notch filter holder and linear array detector. The feasibility of using this system for methane hydrate detection is evaluated through the use of organic surrogate molecules, due to the low solubility of methane in water at atmospheric pressures. The eventual application is the oceanic detection of methane gas, liquid, and solid hydrate at great depths (1000's of meters). Under these conditions, the solubility of methane increases significantly and solid hydrate may be present. The relative Raman cross sections of methane and other organic compounds relative to nitrogen are documented. The benefit of using a capillary waveguide with the dual fiber optic probe is quantitatively shown. The LOD of acetone (as a surrogate for methane) is determined and the LOD of methane under several experimental conditions is calculated. The ability to detect all three states (gas, liquid, and solid) is of great importance to the detection and characterization of ocean floor methane.