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| Titel |
Monitoring of CO2-induced geochemical changes in a shallow aquifer by time domain spectral induced polarization |
| VerfasserIn |
Joseph Doetsch, Gianluca Fiandaca, Esben Auken, Anders V. Christiansen, Aaron G. Cahill, Rasmus Jakobsen |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250094862
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| Publikation (Nr.) |
 EGU/EGU2014-10294.pdf |
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| Zusammenfassung |
| Contamination of potable groundwater by leaking CO2 is a potential risk of carbon
sequestration. With the help of a field experiment, we investigate if surface monitoring of
electrical resistivity and induced polarization can detect geochemical changes induced by
CO2 in a shallow aquifer. For this purpose, we injected CO2 at a depth of 5 and 10 m and
monitored its migration using 320 electrodes on a 126 m x 25 m surface grid. A fully
automated acquisition system continuously collected direct current (DC) resistivity and
full-decay induced polarization (IP) data and uploaded it into an online database. CO2 was
injected for a period of 72 days and DC/IP monitoring started 20 days before and continued
until 120 days after the beginning of the injection. The DC/IP data were supplemented by
chemical analysis of water samples collected in 29 wells at time intervals of approximately
10 days.
DC/IP data are inverted using a 2-D algorithm (AarhusInv) that incorporates the full
voltage decay of the IP response to resolve DC resistivity, intrinsic chargeability and spectral
IP content parameterized using the Cole-Cole model. Borehole information and a baseline
inversion reveals the geology at the site consisting of aeolian sands near the surface, glacial
sands between 5 and 10 m depth and marine sands below 10 m depth. Following the
injection, we use a time-lapse inversion where differences in the DC/IP data are inverted for
changes to the Cole-Cole parameters. Two different geochemical signatures that occur due
to the injected CO2 are evident both in the geophysical inversions and the water
samples. The first and clearest subsurface signal is a decrease in DC resistivity that
moves advectively with the groundwater. The area of resistivity decrease expands
with time in the direction of the groundwater flow and there is good correlation
between geophysical and geochemical results. A chargeability decrease after the
injection has been also observed, but in contrast to the decrease in DC resistivity, the
chargeability anomaly remains localized around and slightly downstream of the
injection wells throughout the experiment. This chargeability decrease correlates in
space and time with a decrease in pH, which is observed in the water samples.
Consequently, we attribute this change in chargeability to pH-induced changes to the grains
surfaces.
These results highlight the potential for monitoring of field scale geochemical changes by
means of surface DC/IP measurements. Especially the different development of the DC
resistivity and chargeability anomalies and the different associated geochemical processes,
highlight the added value of induced polarization to resistivity monitoring. |
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