Characterization of a C O2 gas vent using various geophysical and geochemical methods

Abstract

An understanding of gas migration along faults is important in many geologic research fields, such as geothermal exploration, risk assessment, and, more recently, the geologic storage of man-made carbon dioxide [Formula: see text]. If these gases reach the surface, they typically are discharged to the atmosphere from small areas known as gas vents. In a study of an individual gas vent located in the extinct Latera caldera, central Italy, near-surface geochemical and geophysical surveys were conducted to define the spatial distribution of gas-induced effects in the first few meters of the soil and, by inference, the 3D structure and geometry of the associated gas-permeable fault. Grid surveys and detailed profiles were performed across this vent using time-domain reflectometry (TDR), ground-penetrating radar (GPR), frequency-domain electromagnetics (FDEM), electrical resistivity tomography (ERT), and gas geochemistry measurements. Detailed profilesurveys indicate that the leaking [Formula: see text] has changed the physical, chemical, and biological soil environment of the vent, resulting in significant spatial variations in parameters (e.g., water content and soil electric/dielectric properties) that influence geophysical measurement results. Despite the strong difference in vertical and lateral resolution and depth of investigation, all methods show the same general trends and similar relative variations in the measured physical parameters. TDR and GPR data highlight anomalous shallow lateral variations, whereas FDEM and ERT measurements identify the vertical extension of the anomalous zone. All methods highlight a north-northwest–south-southeast anomaly alignment that we associate with the main fault; FDEM and, to a lesser extent, [Formula: see text] flux also show elongation orthogonal to this direction, implying that the vent may occur at the intersection of two structures. Thus, different near-surface geophysical and geochemical methods can provide important information on faults and their gas-migration characteristics.

Developments since 2005 in understanding potential environmental impacts of CO2 leakage from geological storage

“…Finally geophysical methods show promise for imaging the shallow sub-surface over larger areas to look for leakage-induced plumes. For example the higher conductivity associated with such plumes, especially those from brine leakage, can potentially be delineated using electrical resistivity methods (Pettinelli et al, 2010;Wagner et al, 2013), although there is a risk of false positives (Trainor-Guitton et al, 2013).…”

Comparison of the impacts of elevated CO 2 soil gas concentrations on selected European terrestrial environments

“…The field contains a number of vents, one of which has been extensively characterised in previous studies using geochemical, geophysical and remote sensing tools (e.g. Annunziatellis et al, 2008;Bateson et al, 2008;Pettinelli et al, 2008Pettinelli et al, , 2010. This vent is estimated to have been active for more than 75 years, perhaps much longer and was selected for further analyses.…”

Looking for leakage or monitoring for public assurance?

“…Small variations in the soil properties could lead to preferential pathways. The opportunity to compare the behaviour at these small scale release experiments with larger leakages in the near surface from deep geological storage has not arisen, but results from natural leaks display similar "patchy" surface expression [16][17][18][19][20][21][22][23][24]. Since results from controlled releases and natural releases to date suggest that CO 2 leaks do take preferential pathways, it is important to examine the implications if a similar type of behaviour occurred during an actual leak from deep engineered storage.…”