Читать книгу Geophysical Monitoring for Geologic Carbon Storage - Группа авторов - Страница 46

FMS is an ultrasensitive variation of absorption spectroscopy that has been developed to measure stable isotope ratios of CO₂ (Fessenden et al., 2010). FMS instruments capable of stable isotope field measurements for carbon sequestration (Fessenden et al., 2010) as well as measuring the carbon isotopes in CO₂ and CH₄ on Mars (Webster & Mahaffy, 2011). Most FMS applications are accomplished in an in situ configuration with a multipass cell (Webster & Mahaffy, 2011). However, FMS is also capable of operating in a remote configuration as discussed below. The technique involves directing a laser through a modulator that produce sidebands that are shifted from the laser wavelength by kilohertz to gigahertz frequencies as depicted in Figure 3.2. The entire system can be tuned, the laser wavelength and the frequency shift, so that one sideband interacts with an absorption feature while the carrier laser and the other sideband are unaltered. The detector senses the changes in the sideband and the derivative shaped function. As the CO₂ concentration increases, the peak‐to‐peak amplitude of the recorded response increases. This allows for very small changes in concentration to be measured, effectively increasing the signal to noise ratio where a large contrast in harmonic amplitude change can be seen on a dark background. This is in contrast to absorption spectroscopy where one is recording a relatively small change relative to a bright background. Consequently, one can theoretically achieve quantum noise levels, and it is conservatively a factor of 100 to 1,000 times more sensitive than absorption spectroscopy under identical conditions. Finally, this increased sensitivity also enables the detection of stable isotopes such as ¹³CO₂.

Figure 3.2 A FMS instrument produces sidebands (ω_c ± ω_m) shifted from the carrier (laser) wavelength (ωc) (based on Bjorklund & Levenson, 1983).