Mid-Infrared Spectroscopic Sensors for In-Situ Monitoring of Methane Dissolved in Sea Water
Authors: Vogt Frank , Pennington Neil , Mizaikoff Boris 
Published: October 2008

A mid-infrared spectroscopic measurement technique is presented for determination of the concentration of methane and other short-chained aliphatic hydrocarbons dissolved in sea water. This approach will be used for studying gas hydrate reservoirs providing analytical information on the amount of methane, ethane, and propane in the vicinity of hydrate fields.

On account of their methane content gas hydrates are considered a potential energy resource for the future. Furthermore, environmental concerns are related to methane as greenhouse gas. If large amounts of methane are released from hydrate sites through the water column into the atmosphere major impact on the climate is expected. Furthermore, gas hydrates are stabilizing the sea floor and disturbances e.g. by oil exploration drillings can result in release of large amounts of gas posing serious risks for oil rigs.

The proposed target analytes are detected at low concentrations due to their molecule specific absorption spectra in the mid-infrared spectral range (3-20 Âµm). As the water matrix itself is a strong IR absorber, conventional transmission spectroscopy can not be applied. The presented approach is based on attenuated total reflection (ATR) spectroscopy utilizing a polymer coated waveguide as sensor head. This measurement principle is based on IR radiation evanescently guided as exponentially decaying electromagnetic field at the waveguide-polymer interface. Hence, resonant absorptions of IR-active molecules present at the waveguide-polymer interface result in spectra similar to conventional transmission absorption experiments. Hydrophobic polymer coatings at the waveguide surface prevent water from being in contact with the evanescent field and simultaneously enrich analytes at the waveguide-polymer interface for sensitivity enhancement.

The deep sea sensor will be based on a compact Fourier transform infrared (FT-IR) spectrometer and a high-pressure sensor head for ATR spectroscopy encapsulated into a spherical glass pressure vessel.

Concentrations of multiple analytes dissolved in sea water with overlapping spectral features are evaluated using chemometric1 data evaluation algorithms, which are modified to handle spectroscopic difficulties encountered in deep sea applications.

Introduction

Gas hydrates ([ 1 ] - [ 5 ] and further citations given there) are of growing interest mainly due to three reasons:Since gas hydrates are a major source of methane and natural gas, they are considered as an important future energy resource. The economic impact of gas hydrates has been discussed recently [ 6 ]. It was estimated that there is more than twice as much carbon bound in gas hydrates than in crude oil, natural gas, and coal deposits together [ 5 ], [ 7 ].Sub sea gas hydrate fields are very sensitive to environmental conditions, in particular temperature and pressure. If these condition change, gaseous methane is released in large quantities into the atmosphere [ 8]. Methane, however, acts as green house gas with a warming effect 30 times stronger than CO2. Moreover, methane is slowly oxidized to CO2 and resides in the atmosphere as green house gas for a long time.