A feasibility study on time-lapse transient electromagnetic for monitoring groundwater dynamics

Juan Carlos Zamora-Luria; Paul McLachlan; Pradip Kumar Maurya; Lichao Liu; Denys Grombacher; Anders Vest Christiansen

doi:10.1190/geo2022-0532.1

A feasibility study on time-lapse transient electromagnetic for monitoring groundwater dynamics

Juan Carlos Zamora-Luria^*, Paul McLachlan, Pradip Kumar Maurya, Lichao Liu, Denys Grombacher, Anders Vest Christiansen

^*Corresponding author for this work

Department of Geoscience

Research output: Contribution to journal/Conference contribution in journal/Contribution to newspaper › Journal article › Research › peer-review

Abstract

Time-lapse geophysical studies have been used as noninvasive tools to study subsurface processes. However, compared to direct current (DC) resistivity methods, the transient electromagnetic (TEM) method has not been explored in much detail for time-lapse investigations. Here, we propose the use of a monitoring TEM (mTEM) system and methodology for long-term groundwater monitoring purposes. The system can run measurements automatically and the modelling approach uses a time-constrained inversion (TCI) scheme based on an existing laterally constrained inversion scheme. Instead of lateral constraints that traditionally ensure lateral coherency between measurements, temporal constraints are used to control the temporal coherency between measurements. Synthetic analysis demonstrated that very tight time constraints on resistivities and a time-constraint on the depth to the water table fixed at a fraction of the expected annual water table variation is needed to track water table changes at the decimeter level. Further, the synthetic analysis showed that the mTEM method can track annual variations in the water table with amplitudes as low as 0.6 m with a mean absolute error of 0.2 at depths between 16 and 30 m. A field example consisting of 233 1D TEM measurements acquired over ten months the mTEM instrument supports the synthetic results. The estimated water table depth from the TCI scheme reproduces the variation of the water table accurately, as confirmed by measurements from a water level data logger in a nearby well. The differences between the results from the TEM monitoring system and the diver measurements are in the range of +/- 0.1 m with an average absolute difference of 0.05 m. This study demonstrates the potential to monitor changes in the groundwater level non-invasively.

Original language	English
Journal	Geophysics
Volume	88
Issue	5
ISSN	0016-8033
DOIs	https://doi.org/10.1190/geo2022-0532.1
Publication status	Published - 2 Jun 2023

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title = "A feasibility study on time-lapse transient electromagnetic for monitoring groundwater dynamics",

abstract = "Time-lapse geophysical studies have been used as noninvasive tools to study subsurface processes. However, compared to direct current (DC) resistivity methods, the transient electromagnetic (TEM) method has not been explored in much detail for time-lapse investigations. Here, we propose the use of a monitoring TEM (mTEM) system and methodology for long-term groundwater monitoring purposes. The system can run measurements automatically and the modelling approach uses a time-constrained inversion (TCI) scheme based on an existing laterally constrained inversion scheme. Instead of lateral constraints that traditionally ensure lateral coherency between measurements, temporal constraints are used to control the temporal coherency between measurements. Synthetic analysis demonstrated that very tight time constraints on resistivities and a time-constraint on the depth to the water table fixed at a fraction of the expected annual water table variation is needed to track water table changes at the decimeter level. Further, the synthetic analysis showed that the mTEM method can track annual variations in the water table with amplitudes as low as 0.6 m with a mean absolute error of 0.2 at depths between 16 and 30 m. A field example consisting of 233 1D TEM measurements acquired over ten months the mTEM instrument supports the synthetic results. The estimated water table depth from the TCI scheme reproduces the variation of the water table accurately, as confirmed by measurements from a water level data logger in a nearby well. The differences between the results from the TEM monitoring system and the diver measurements are in the range of +/- 0.1 m with an average absolute difference of 0.05 m. This study demonstrates the potential to monitor changes in the groundwater level non-invasively.",

author = "Zamora-Luria, {Juan Carlos} and Paul McLachlan and Maurya, {Pradip Kumar} and Lichao Liu and Denys Grombacher and Christiansen, {Anders Vest}",

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AU - Zamora-Luria, Juan Carlos

AU - McLachlan, Paul

AU - Maurya, Pradip Kumar

AU - Liu, Lichao

AU - Grombacher, Denys

AU - Christiansen, Anders Vest

PY - 2023/6/2

Y1 - 2023/6/2

N2 - Time-lapse geophysical studies have been used as noninvasive tools to study subsurface processes. However, compared to direct current (DC) resistivity methods, the transient electromagnetic (TEM) method has not been explored in much detail for time-lapse investigations. Here, we propose the use of a monitoring TEM (mTEM) system and methodology for long-term groundwater monitoring purposes. The system can run measurements automatically and the modelling approach uses a time-constrained inversion (TCI) scheme based on an existing laterally constrained inversion scheme. Instead of lateral constraints that traditionally ensure lateral coherency between measurements, temporal constraints are used to control the temporal coherency between measurements. Synthetic analysis demonstrated that very tight time constraints on resistivities and a time-constraint on the depth to the water table fixed at a fraction of the expected annual water table variation is needed to track water table changes at the decimeter level. Further, the synthetic analysis showed that the mTEM method can track annual variations in the water table with amplitudes as low as 0.6 m with a mean absolute error of 0.2 at depths between 16 and 30 m. A field example consisting of 233 1D TEM measurements acquired over ten months the mTEM instrument supports the synthetic results. The estimated water table depth from the TCI scheme reproduces the variation of the water table accurately, as confirmed by measurements from a water level data logger in a nearby well. The differences between the results from the TEM monitoring system and the diver measurements are in the range of +/- 0.1 m with an average absolute difference of 0.05 m. This study demonstrates the potential to monitor changes in the groundwater level non-invasively.

AB - Time-lapse geophysical studies have been used as noninvasive tools to study subsurface processes. However, compared to direct current (DC) resistivity methods, the transient electromagnetic (TEM) method has not been explored in much detail for time-lapse investigations. Here, we propose the use of a monitoring TEM (mTEM) system and methodology for long-term groundwater monitoring purposes. The system can run measurements automatically and the modelling approach uses a time-constrained inversion (TCI) scheme based on an existing laterally constrained inversion scheme. Instead of lateral constraints that traditionally ensure lateral coherency between measurements, temporal constraints are used to control the temporal coherency between measurements. Synthetic analysis demonstrated that very tight time constraints on resistivities and a time-constraint on the depth to the water table fixed at a fraction of the expected annual water table variation is needed to track water table changes at the decimeter level. Further, the synthetic analysis showed that the mTEM method can track annual variations in the water table with amplitudes as low as 0.6 m with a mean absolute error of 0.2 at depths between 16 and 30 m. A field example consisting of 233 1D TEM measurements acquired over ten months the mTEM instrument supports the synthetic results. The estimated water table depth from the TCI scheme reproduces the variation of the water table accurately, as confirmed by measurements from a water level data logger in a nearby well. The differences between the results from the TEM monitoring system and the diver measurements are in the range of +/- 0.1 m with an average absolute difference of 0.05 m. This study demonstrates the potential to monitor changes in the groundwater level non-invasively.

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JO - Geophysics

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A feasibility study on time-lapse transient electromagnetic for monitoring groundwater dynamics

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