| 1. |
- Gonzales Amaya, A.
(författare)
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Geophysical surveys for improving the hydrogeological conceptual models in the semi-arid region of Valle Alto (Bolivia)
- 2018
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Ingår i: 24th European Meeting of Environmental and Engineering Geophysics. - : European Association of Geoscientists & Engineers. - 9789462822634
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Konferensbidrag (refereegranskat)abstract
- Geophysical methods provide important information for mapping the subsurface. In semiarid areas, a detailed knowledge of the aquifers geometry can assist to a better management of groundwater. In the Valle Alto (Bolivia), which is a semiarid region, there is gap in the knowledge about the aquifer system geometry and hydrogeological processes. Local reports indicated problems in some wells such as very low production and bad water quality (i.e. high concentrations of salts). In order to improve the knowledge about the aquifer system and identify the depth of the layers with saline water, the geophysical methods Electrical Resistivity Tomography (ERT) and Time Domain Electromagnetic (TEM) were applied. The main objective of this study is to refine the hydrogeological conceptual models and to delimitate the depth and thickness of saline layers. The TEM soundings are performed in a grid of 150 m separation, which provide significant information for proposing 2-D models that displays the lateral/vertical extent of the saline layers. While the ERT surveys were performed across the Valle Alto in order to obtain long cross sections where the aquifer geometry can be exposed. With the refined conceptual models, further planning and policies can be proposed for protection and sustainable groundwater exploitation.
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| 2. |
- Loke, M. H., et al.
(författare)
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Time-lapse 4-D resistivity imaging inversion with positivity constraints
- 2018
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Ingår i: 24th European Meeting of Environmental and Engineering Geophysics. - : European Association of Geoscientists & Engineers. - 9789462822634
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Konferensbidrag (refereegranskat)abstract
- Time-lapse resistivity surveys are used to monitor changes in the subsurface. In some situations, it is known that the resistivity will only decrease (or increase) with time. The 4-D ERT smoothness-constrained inversion method, that includes temporal smoothness constraint, has proved to be a robust method that reduces artefacts due to noise. However, in some cases, the time-lapse inverse models might show an increase in the resistivity with time where it is only expected to decrease. We modify the 4-D ERT inverse method to include a constraint that removes this artefact. The standard 4-D ERT inversion algorithm is first used to generate an initial model. If the resistivity is expected to decrease with time, for the model cells that show a resistivity increase with time, a truncation procedure is used where the resistivities of the different time models are reset to the mean value (corresponding to zero change with time). We then use the method of transformations in the inversion method that ensures the resistivities of the later time models are always less than the first model. The constraints can be modified so that they are only applied to selected regions in the model in cases where additional information is available.
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| 3. |
- Nivorlis, A., et al.
(författare)
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Monitoring of in situ remediation with the direct current time-domain induced polarization method
- 2018
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Ingår i: 24th European Meeting of Environmental and Engineering Geophysics. - : European Association of Geoscientists & Engineers. - 9789462822634
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Konferensbidrag (refereegranskat)abstract
- In Alingsås, a dry-cleaning facility was operated for many years, and huge amounts of the solvent PCE was spilled into the ground. This contributed to an increasing concentration of PCE over the years until the use of PCE was stopped, resulting in the formation of a DNAPL plume beneath the building. Treatment of contaminated soils in Sweden often includes excavation and landfilling, however in Alingsås this is not applicable. In situ remediation methods (thermal, biological, chemical) are the only alternative however, there is a need for tools to monitor the effectiveness of those methods. One method of particular interest in this context is the Direct Current time-domain Induced Polarization (DCIP). For that purpose, a fully autonomous and automatic monitoring system was installed in Alingsås, to perform frequent automated measurements and to provide information about the changes in the subsurface. The geophysical data should be ideally acquired, analyzed and verified with automated routines as part of a larger monitoring system. It is of great importance, especially in the early stage, to verify events that appear to show interesting changes with sampling data to evaluate the level of reliability of the system.
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| 4. |
- Rejkjær, S., et al.
(författare)
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A simplified approach to array optimization for the DC resistivity and induced polarization method
- 2018
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Ingår i: 24th European Meeting of Environmental and Engineering Geophysics. - : European Association of Geoscientists & Engineers. - 9789462822634
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Konferensbidrag (refereegranskat)abstract
- In this work a simplified approach to array optimization for the DC resistivity and induced polarization method have been created and tested. The optimization method uses the centre of the sensitivity function for the individual electrode configurations, called the mass centre, which is spread over the area of investigation, wherefrom the name Spread of Mass Centre (SMC). This array has been tested both in theory through forward modelling and in the field. The theoretical results have shown that the SMC array generally measure higher voltages than the common multiple gradient array, which could indicate an improvement in signal to noise ratio when using the SMC array. For the field test it was found that the SMC array achieved comparable resistivity results with slightly higher mean residual in the inversion than the multiple gradient array, while the integral chargeability results show a more significant difference with the SMC array having significantly higher mean residuals. In general, the SMC array show promising results for a simple optimized array which is computationally light and highly compatible with multichannel optimization.
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| 5. |
- Rejkjær, S., et al.
(författare)
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Comparison between DC ERT and moving multi-depth electrostatic arrays in an urban context
- 2018
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Ingår i: 24th European Meeting of Environmental and Engineering Geophysics. - : European Association of Geoscientists & Engineers. - 9789462822634
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Konferensbidrag (refereegranskat)abstract
- A series of experiments was undertaken in the province Scania in southern Sweden with the aim to acquire a better knowledge about the electrical resistivity of the soil surrounding heating and water distribution pipes, in order to assess the risk of corrosion. Electrical Resistivity Tomography (ERT) and moving multi-depth electrostatic arrays have been applied to determine the resistivity distribution in an urban context along selected pipes. Implimentation of the ERT was done by drilling holes, in the mostly paved surfaces, and inserting electrodes with a gel to decrease electrode contact resistance. The moving multi-depth electrostatic arrays was implimented in a “sliding” carpet dragged along the lines. Direct comparison between ERT and “sliding” carpet results show a good coherence. This confirms the high interest of the second solution for in-town measurements due to its easy implementation. The interpretation of the data proved to be complicated and was not simply explained as a pipe embedded in a homogeneous layer, which show that a careful attention must be paid to other external information about the underground work history. One practical application might be the mean resistivity values of the medium surrounding the pipe(s), to be used as a tool by the pipe owner.
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