Читать книгу Geophysical Monitoring for Geologic Carbon Storage - Группа авторов - Страница 44

Active absorption spectroscopy is the simplest method to monitor changes in the CO₂ flux (Repasky et al., 2006; Humphries et al., 2008; Soukup et al., 2014; Barr et al., 2011). This is usually accomplished with one of the many commercially available instruments available from LICOR. This technique involves directing a laser or monochromatic light source through the gaseous sample. The light is absorbed by the CO₂ in the sample and a reduction in the light is observed. This change in light intensity is directly proportional to the CO₂ concentration based on the Beer‐Lambert law,

(3.1)

where e is the extinction coefficient, c is the concentration, l is the pathlength, and I/I_o is the change in light intensity.

The Beer‐Lambert law shows that the change in intensity is proportional to the path length of the light source and the extinction coefficient. The sensitivity of the instrument can be improved by the selection of the light source now that new quantum cascade lasers that operate in the midinfrared spectral region (MIR) are increasingly available. The tunable diode lasers that operate in the near‐infrared are very reliable light sources, but the CO₂ extinction coefficient is significantly lower than the optical transitions in the MIR. Finally, the fundamental sensitivity of absorption spectroscopy depends on the optical path length. Consequently, instruments that are capable of very long path lengths are more sensitive at any given laser wavelength. One can increase the path length by collecting the sample within a multipass optical cell such as a Herriott cell or a White cell.

Unfortunately, the interpretation of the simple absorption results can be compromised by the diurnal changes in CO₂ flux. The CO₂ concentration will diurnally change by 20% –50% (depending upon location and season) and any method that depends strictly on the CO₂ flux will be trying to distinguish changes in CO₂ concentration due to seepage to the surface from diurnal changes. This fundamentally limits absorption techniques to relatively large changes in flux due to seepage.