Deriving TDS values in coarse sediments from long normal and electromagnetic logs.

Area of Science:

• Geophysics
• Hydrogeology
• Environmental Science

Background:

• Accurate groundwater salinity assessment is crucial for coastal resource management.
• Traditional logging methods like long normal (LN) logs have limitations in monitoring and data acquisition.
• Electromagnetic (EM) logging offers potential advantages for continuous and detailed subsurface characterization.

Purpose of the Study:

• To develop an empirical relationship between EM borehole recordings and total dissolved solids (TDS) in pore water.
• To adapt existing resistivity-based TDS estimation methods for EM log data.
• To establish a framework for enhanced groundwater salinity monitoring in the Belgian coastal plain.

Main Methods:

• Linking long normal (LN) logs to electromagnetic (EM) logs.
• Rewriting established relationships between LN resistivity and TDS for EM recordings.
• Utilizing Archie's Law to derive the formation factor from groundwater analyses and LN logs.

Main Results:

• An empirical relationship was established, enabling TDS estimation from EM borehole recordings.
• EM logging demonstrated advantages including measurement in PVC-cased wells and high vertical resolution.
• EM logging is not hindered by the invasion zone and allows for time-dependent data collection.

Conclusions:

• The developed technique provides a robust method for estimating groundwater TDS using EM logs.
• This approach facilitates the establishment of effective salinity monitoring networks in coastal areas.
• The method allows for historical salinity comparison and monitoring of freshwater-saltwater evolution over time.