Long-term monitoring of water table and saltwater intrusion dynamics through time-lapse transient electromagnetic

Understanding hydrogeological processes often requires continuous measurements, which in many cases are not cost-efficient. Time-lapse geophysical studies are invaluable for investigating dynamic subsurface processes. They offer several advantages over traditional borehole-based monitoring methods, such as being cost-efficient and non-intrusive. However, to be effective as an early warning tool, these studies must provide real-time information on the processes. This is often not feasible with traditional geophysical methods, which typically do not offer real-time monitoring. In this study, we present a windowed inversion approach for time-lapse transient electromagnetic (TEM) monitoring. The advantages of this approach include: (i) immediate and continuous results as each time-step dataset is acquired; (ii) no limitations on the size of the monitoring period or dataset; and (iii) the use of model information from previous inversion results for robust and continuous outcomes. The results from a synthetic study are followed by two field case studies, demonstrating the advantages of the windowed inversion compared to the inversion of the full dataset. We monitored the water table dynamics of a shallow unconfined aquifer over 8 months and the saltwater intrusion in a confined aquifer over 10 months using a monitoring TEM (mTEM) system while collecting nearly continuous daily measurements. With the windowed approach, the water table dynamics was recovered with an accuracy of 10 cm, and the changes in salinity were tracked with results comparable to those of an EC data logger.