Characterization of granular aquifer heterogeneity using oscillatory hydraulic tomography (St-Lambert, Canada)

Aymen Nefzi, Rene Lefebvre, Daniel Paradis, Olivier Bour

Dans les comptes rendus d’articles de la conférence: GeoNiagara 2021: 74th Canadian Geotechnical Conference; 14th joint with IAH-CNC

ABSTRACT: Heterogeneity controls the transport of solutes in aquifers, but the characterization of the heterogeneous spatial distribution of hydraulic properties represents a major challenge. Hydraulic tomography (HT) has been developed to define the heterogeneous distribution of hydraulic conductivity (K). HT involves the emission of a series of hydraulic head perturbations in a stressed well and the recording of this signal at several levels in observation wells. The simultaneous analysis of the multiple hydraulic responses through numerical inversion can thus be done to estimate the heterogeneity of K in 2D or 3D. This communication reports on a tomographic experiment carried out in a highly anisotropic unconsolidated aquifer at the Saint-Lambert research site near south of Quebec City, which has been already the object of a detailed characterization. A relatively new approach named oscillatory hydraulic tomography (OHT) was tested, in which multi-frequency oscillatory head perturbations are induced by a submerged rod in the stressed well that is numerically controlled by a winch system. The objective of the study was to test the feasibility of making an OHT experiment in a highly anisotropic unconsolidated aquifer where signal propagation is known to be an issue and also to assess if multiple controlled frequencies can provide a better spatial resolution of K compared to slug or pumping HT. Results show that the field experiment provided frank hydraulic responses with distinct change in amplitude and phase according to the geological material and relative position of the observation intervals with respect to the stressed interval. Comparison was made of inversion results using a single and up to three different frequencies (2, 4 and 8 min/cycle), and OHT results compared to slug HT results and other hydraulic tests. This experiment is likely the first successful OHT implementation aiming at defining K heterogeneity and its anisotropy in unconsolidated aquifers.

Please include this code when submitting a data update: GEO2021_301

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