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

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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.

StatusUdgivet - 2 jun. 2023

Bibliografisk note

Funding Information:
This work was supported by funding from Innovation Fund Denmark-under SuperTEM project (grant number 0177-00085B). The first author would like to thank to Consejo Nacional de Ciencia y Tecnología (Mexico) for the PhD grant (No. 769715).

Publisher Copyright:
© 2023 Society of Exploration Geophysicists.

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