describe the virtues of in situ laboratory and field analysis versus remote methods capable of wide area monitoring. Finally, the chapter will describe frequency modulated spectroscopy (FMS) as a means of in situ and remote surface monitoring of geological sequestration sites with stable isotope sensitivity.
Figure 3.1 The CO2 produced from several anthropogenic and natural processes fractionates the carbon isotope. Here, the range of δ13C produced from several processes is depicted (Fessenden et al., 2010).
3.2. CURRENT STATE OF THE ART
The monitoring, verification, and accounting of a sequestration site will likely require the deployment of multiple instruments making measurements on different scales. Ideally, one or more instruments could be deployed to conduct wide area, remote monitoring and identify seepage in a general area. When seepage is identified remotely, in situ instruments could be deployed to verify the source location and determine the seepage mechanism. Alternatively, short range or in situ instruments could be deployed to strategic locations such as around likely seepage pathways. In this section, the current‐state‐of‐the‐art analytical techniques are briefly discussed starting with point source approaches and ending with standoff or remote techniques.
3.2.1. Point Source or In Situ Measurements
Point source or in situ CO2 measurements are by far the most common methods of seepage detection at the surface and there are many options with various levels of complexity and sensitivities. Absorption spectroscopy is the simplest method to measure CO2 flux but it is generally the least sensitive (Barr et al., 2011; Fessenden et al., 2010; Humphries et al., 2008; Repasky et al., 2006; Soukup et al., 2014). Cavity ringdown spectroscopy (CRS) and FMS are significantly more complex optical methods to measure CO2 flux that have the sensitivity to monitor carbon stable isotope ratios (Zalicki & Zare, 1995; Lindenmaier et al., 2014; Fessenden et al., 2010). There are other optical and imaging methods of identifying the impact of CO2 injection and seepage without directly measuring the change in CO2 flux. Interferometric synthetic‐aperture radar (INSAR) has been used to monitor surface distortions during injection (Yang et al., 2015). Plant stresses due to exposure to increased CO2 flux have been monitored by imagery (Costa et al., 2013). Here, each method capable of measuring the CO2 flux will be briefly discussed.
Passive Absorption Spectroscopy
Passive absorption spectroscopy is perhaps the simplest approach to measuring CO2 where the Sun is the light source (Lindenmaier et al., 2014). A Fourier Transform Infrared (FTIR) spectrometer can be used to record the CO2 spectrum for the atmospheric column. The instrumentation required for this approach tends to be simpler than other techniques discussed later because it does not require the use of a laser or other light sources that need to remain optically stable in the field. However, passive absorption spectroscopy has several limitations that limit utility as a method of monitoring CO2 seepage to the surface. First, passive spectroscopy depends on the Sun and, therefore, cannot be used at night. The technique is also limited to looking in the direction of the Sun and is a column average that is well beyond the region above the sequestration reservoir. FTIR spectrometers can be used to measure the carbon isotopes of CO2 but the fundamental physics of this approach requires a large instrument footprint.
Active Absorption Spectroscopy
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,
where e is the extinction coefficient, c is the concentration, l is the pathlength, and I/Io 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.
Cavity Ringdown Spectroscopy
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.
