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Assessment of test initiation method on hydraulic tomography resolution.

Paradis, Daniel; Lefebvre, René; Gloaguen, Erwan; Giroux, Bernard (2015). Assessment of test initiation method on hydraulic tomography resolution. In: NovCare 2015: Novel Methods for Subsurface Characterization and Monitoring: From Theory to Practice, 19-21 mai 2015, Lawrence, États-Unis.

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Résumé

Hydraulic tomography is receiving increasing attention to characterize the heterogeneity of hydraulic properties at local scale. While recent advances have been promising, few efforts have been dedicated to understanding the impact of fundamental controls on hydraulic property resolution, such as the magnitude and anisotropy of hydraulic properties and experimental configuration. In this presentation, we focus on the impact of the test initiation method used to induce head perturbations in an aquifer, on the resolution of hydraulic properties that can be obtained from hydraulic tomography experiments. In particular, we compare respective sensitivities and associated resolution for tomographic experiments carried out with pumping and slug tests. Tomographic pumping and slug tests are indeed expected to present distinct resolution characteristics due to their different sensitivity patterns. To have a common basis of comparison for both tests, an equivalent constant pumping rate is first calculated to obtain identical steady-state drawdowns and initial heads in stressed intervals. Then, a resolution analysis based on truncated singular value decomposition of the sensitivity matrix with a noise level representative of typical field measurements is computed using synthetic data mimicking a known littoral aquifer composed of sand and silt To approximate the hydraulic behavior of a real aquifer system, synthetic tomographic experiments and their associated sensitivity matrix are generated using a radial flow model accounting for wellbore storage in a plane encompassing a stressed and an observation wells. Results of synthetic experiments lead to several important conclusions. First, while drawdowns induced by pumping in observation intervals are three times larger than head changes from slug tests in the same intervals, sensitivity magnitudes for Kh, Kv/Kh and Ss are comparable. Therefore, resolution for the two different test initiation methods are similar, except for Ss where pumping tests have much smaller resolution than slug tests due to wellbore storage effects that hinder Ss aquifer responses recorded in stressed intervals. However, head responses in observation intervals for slug tests must be larger than the noise level of measurements to ensure proper resolution, which may require closer test wells or large initial heads. Secondly, tomographic slug tests require up to 50 times less water (9 L/slug test versus 450 L/pumping test) to perform a tomographic experiment with the same resolution. This could be a huge benefit for projects where disposal or injection of water is an issue. Even though the pumping rate could be lowered to reduce the water used for testing, lowering drawdowns in observation intervals would lead to poorer resolution. For identical drawdowns and heads in observation intervals, resolution from tomographic pumping tests is indeed half the resolution of tomographic slug tests. This study shows that slug tests provide an interesting alternative to commonly used pumping tests for hydraulic tomography experiments.

Type de document: Document issu d'une conférence ou d'un atelier
Mots-clés libres: hydrogeology; hydraulic tomography
Centre: Centre Eau Terre Environnement
Date de dépôt: 10 mai 2018 17:40
Dernière modification: 10 mai 2018 17:40
URI: http://espace.inrs.ca/id/eprint/3907

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