Transient Electromagnetic (TEM) Surveys as a First Approach for Characterizing a Regional Aquifer: The Case of the Saint-Narcisse Moraine, Quebec, Canada

Abstract

Geological contexts that lack minimal stratigraphic and piezometric information can be challenging to produce an initial hydrogeological map in remote territories. This study proposes an approach to characterize a regional aquifer using transient electromagnetic (TEM) surveys. Given the presence of randomly dispersed boreholes, the Saint-Narcisse moraine in the Mauricie region of Quebec (Canada) is an appropriate site for collecting the required geophysical data, correlating the stratigraphic and piezometric information, and characterizing regional granular aquifers in terms of stratigraphy, geometry, thickness, and extent. In order to use all TEM results (i.e., 47 stations) acquired in the moraine area, we also correlated 13 TEM stations, 7 boreholes, and 6 stratigraphic cross-sections to derive an empirical and local petrophysical relationship and to establish a calibration chart of the sediments. Our TEM data, combined with piezometric mapping and the sedimentary records from boreholes and stratigraphic cross-sections, revealed the compartmentalization of a multi-kilometer morainic system and indicated the presence of two large unconfined granular aquifers overlying the bedrock. These aquifers extend more than 12 km east to west across the study area and are between 25 and >94 m thick. The TEM method provides critical information on groundwater at a regional scale by acquiring information from multiple stations within a short time span to a degree not possible with other existing methodologies.

Using geophysical data to assess groundwater levels and the accuracy of a regional numerical flow model

“…The GPR data were collected in a continuous recording mode with a real-time interpretation from MALÅ AI at two-way travel-time settings that varied between 50 and 200 ns. All radargrams were processed using the MALÅ Vision program, and mean velocity was assumed on the basis of the interpretation of the sedimentary facies described by Lévesque et al (2021) for this area of the Saint-Narcisse moraine, v = 0.065 m•ns − ¹ for saturated sand, and v = 0.1 m•ns − ¹ for unsaturated sand. In fact, Lévesque et al (2021) propose a stratigraphic calibration chart that links the sedimentary facies (i.e., clay, sand, sandgravel), the associated electrical resistivity, and water content of the Saint-Narcisse moraine in Eastern-Mauricie.…”

“…All radargrams were processed using the MALÅ Vision program, and mean velocity was assumed on the basis of the interpretation of the sedimentary facies described by Lévesque et al (2021) for this area of the Saint-Narcisse moraine, v = 0.065 m•ns − ¹ for saturated sand, and v = 0.1 m•ns − ¹ for unsaturated sand. In fact, Lévesque et al (2021) propose a stratigraphic calibration chart that links the sedimentary facies (i.e., clay, sand, sandgravel), the associated electrical resistivity, and water content of the Saint-Narcisse moraine in Eastern-Mauricie. This chart, combined with the electrical resistivity values acquired using the TEM and the ERT, gives us a good overview of the type of sediment located on the subsurface.…”

“…Furthermore, geophysical surveys can efficaciously investigate subsurface sediments and provide a non-invasive, inexpensive, and effective means of characterizing the internal dimensions of the aquifers and their stratigraphic variability. Geophysical techniques have proven their ability to improve the geological framework and hydrostratigraphic characterization of aquifers, including hydraulic properties, spatial extension, and flow paths (McClymont et al 2010;Marker et al 2015;Greggio et al 2018;Kalisperi et al 2018;Pondthai et al 2020;Lévesque et al 2021). Over the past decade, many studies have been conducted using geophysical data to improve the accuracy of numerical modeling by incorporating additional data.…”

“…The ERT and GPR methods can also provide accurate hydrogeophysical parametrization for flooding events (Huisman et al 2010) or capture heterogeneous soil properties and system states to assess and predict subsurface flow and contaminant transport (Kowalsky et al 2005). Finally, some authors simply convert geophysical properties to observed hydrologic properties (e.g., water content) through a petrophysical relationship (Hinnell et al 2010;Tran et al 2014;Lévesque et al 2021). Few of these studies apply multiple combined geophysical approaches to improve the accuracy of numerical modeling and, to the best of the authors' knowledge, water levels derived from multiple geophysical techniques have yet to be used to validate the reliability of a numerical hydrogeological model.…”

“…Few of these studies apply multiple combined geophysical approaches to improve the accuracy of numerical modeling and, to the best of the authors' knowledge, water levels derived from multiple geophysical techniques have yet to be used to validate the reliability of a numerical hydrogeological model. Lévesque et al (2021Lévesque et al ( , 2022 recently developed methods to locate the water table more accurately by improving the geophysical interpretation of regional stratigraphy and piezometric levels. These new methods represent an effective means of augmenting the amount of data available to validate the numerical model's performance.…”