Quantitative Tracking of a Chloride Tracer Front Progression with 3D ERT and TDR
Abstract
Electrical resistivity tomography (ERT) has proved to be an valuable tool to image solute transport processes through the subsurface. However, a quantitative interpretation is aggravated by the non-uniqueness of the ERT data. In the vadose zone, additional problems arise in terms of distinguishing between contributions of water content and solute concentration to the bulk electrical conductivity. In this study, we developed an experimental setup to quantitatively monitor a chloride tracer front through a large undisturbed unsaturated soil column by means of 3D ERT and time domain reflectometry (TDR) probes. A smoothness constraint inversion algorithm was used for the ERT inversion. A comparison of bulk electrical conductivity obtained from ERT and TDR measurements showed that deviations between both methods were largely dependent of the degree of smoothness imposed on the ERT image. The deviations reached a minimum when the smoothness in ERT image was similar to values which are empirically known to yield qualitatively reasonable results. Additionally, by means of the TDR data, we were able to estimate confidence intervals for ERT derived apparent transport model parameters. We therefore conclude that TDR measurements can be used as a ground truth data for ERT measurements. As an additional validation of our approach, we compared the apparent transport model parameters for the effluent chloride flux concentrations to corresponding parameters for the flux-averaged resident concentrations derived from ERT. Here, the match between the two values was similar to and consistent with the ones found at the TDR probe locations. Furthermore, the bulk electrical conductivity changes were related to chloride concentration by calibrating a petrophysical model with spatial resolution. The validity of the petrophysical parameters was appraised by comparison with TDR data and data gathered from soil samples. A good agreement was found. Consequently, the ERT derived chloride concentrations exhibited no mass balance problems due to change of apparent petrophysical parameters with change of ERT sensitivity.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- Bibcode:
- 2007AGUFM.H41H..04K
- Keywords:
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- 0550 Model verification and validation;
- 1865 Soils (0486);
- 1875 Vadose zone;
- 5109 Magnetic and electrical properties (0925)