Читать книгу Geophysical Monitoring for Geologic Carbon Storage - Группа авторов - Страница 45

CRS is an experimental method that is designed to significantly improve sensitivity by greatly increasing path length within a compact instrument (Zalicki & Zare, 1995; Lindenmaier et al., 2014). CRS involves directing a laser or monochromatic light source into a highly reflective cavity containing the gaseous sample. The cavity consists of two mirrors that reflect the light through the sample. One of the mirrors is designed to produce maximum reflectively while the second mirror is designed to leak a very small amount of light, ~99.99% reflective, 0.01% transmission. The transmitted light is recorded with a detector as a function of time. As time increases, the number of times the light passes through the sample increases, more of the light is absorbed, and the amount of light that escapes the cavity decreases. This configuration fundamentally produces a very long, ~10–100 km path length and the concentration is determined by Beer‐Lambert law.

The increase in sensitivity achieved from the very long CRS path length also enables the detection of the minor ¹³CO₂ stable isotope. As discussed in the introduction, one can distinguish the sequestered CO₂ from natural emissions by shifts in the stable isotope ratio from natural background levels that are insensitive to the diurnal variations.

Finally, CRS is fundamentally limited to in situ analysis and standoff or remote analysis is impossible. The technique requires the precise alignment of the mirrors to achieve the required sensitivity. Some have proposed using a remotely placed CRS instrument and using meteorological analysis to track the source of the sample. While this can be accomplished, this configuration generally eliminates the capability of directly probing the surface leak locations without moving the instrument.