Characteristic length measurement of a subsurface gas anomaly—A monitoring approach for heterogeneous flow path distributions

Approach for Self-Calibrating CO2 Measurements with Linear Membrane-Based Gas Sensors

“…A fit based on the experimentally determined mean inner slope ${\overline{k}}^i=0.105$ s/mbar results in a 2.5-fold smaller effective CO 2 permeability of $1.17\times {10}^{-9}$ m 2 /s, and a reduced effective selectivity of $f_{CO2,N2}\approx 4$ with respect to N 2 . Such a reduced selectivity has already been observed in previous mixed gas tests, and has been discussed, e.g., in [30]. In addition, the decrease in selectivity can be partly attributed to the non-zero permeabilities of the C-Flex-reference membranes [27].…”

“…The linearity between $\mathsf{\Delta }{\chi }_x^a$ and $a_1^I$ in Equation (3) has already been experimentally demonstrated for mixing oxygen and nitrogen within a large mole fraction range of ${\chi }_{O2}^a=0.01-1$ [28] and for CO 2 mixed with air for a range ${\chi }_{CO2}^a\le 1$ [30] for laboratory conditions. Within these linearity ranges, such a membrane-based sensor can be calibrated by a gas-specific linear characteristic.…”

Setup of a large-scale test field for distributed soil gas sensors and testing of a monitoring method based on tomography

“…A large number of potential technologies for monitoring CO 2 storage sites already exist [15]. A survey of these technologies can be found in [5,6,17,18].…”

“…At the small (meter sized) scale of a leakage gas mixing causes a highly variable dispersion and dilution of the escaping CO 2 [15] making it difficult to quantify the gas flow with techniques like local CO 2 detectors, LIDAR-systems, or eddy covariance. "There exist several sources of CO 2 emissions like soil and vegetation, and at the larger scale, e.g., urban traffic, combustion, and other industrial processes that may mask a leakage due to magnitude and temporal variability of these sources [6]" [15]. Furthermore, atmospheric monitoring does not permit forewarning, which would be required for improved risk management.…”

“…As a consequence, a cost-intensive, area-wide sensor network needs to be build-up (km scale) to supervise, e.g., critical regions of a geological repository. On the other hand, a number of advantages of shallow subsurface monitoring exist, e.g., they have a higher probability of detecting a leak due to a longer residence time of a gas anomaly in the shallow subsurface than in the atmosphere above the leak where-at, the expected leak position can be estimated already with a comparatively high accuracy with respect to a gas movement detected in the deeper ground, and shallow subsurface monitoring can enhance the time for a hazard management [15].…”