Carbon Dioxide Stable Isotope Detection of Geological Sequestration Seepage

A priority for geological sequestration measurement, mitigation and verification (MMV) is a means of tracking seepage at concentrations at or below ambient CO2 concentrations. The carbon stable isotope ratio (13C16O2/12C16O2) is a sensitive diagnostic signature of anthropogenic and natural sources of CO2. However, the concentration of 13CO2 is approximately 100 times smaller than 12CO2 and sensitive analytical tools are required to measure the ratio in the field. Frequency modulated spectroscopy (FMS) is an ultra sensitive means of detecting the stable isotopes of CO2 that is conservatively 100x more sensitive than standard absorption spectroscopy. FMS involves directing a tunable diode laser (TDL) through an electro- optical modulator operating in the radio frequency regime producing the original carrier frequency from the TDL (wc) and evenly spaced sidebands (wc plus,minus wm). The species of interest is detected by tuning the TDL and the modulation frequency such that one of the sidebands (wc) interacts with a specific spectral feature. This paper will include experiments involving field measurements using both an in situ and remote FMS prototype instruments. The field site is located in a remote location on the Los Alamos National Laboratory campus were the instruments could monitor natural fluctuations. The in situ instrument was placed in the field site next to the remote instrument. The remote instrument was directed towards a retroreflector located 50m from the laser source signal and back to detector positioned next to the laser. The stable isotope ratio is monitored as the carrier frequency is scanned and the sidebands interact with both CO2 isotopes.