Calibrating a salt water intrusion model with time-domain electromagnetic data.

Area of Science:

• Earth Science
• Hydrogeology
• Geophysics

Background:

• Saltwater intrusion models are crucial for coastal groundwater management but often suffer from sparse calibration data.
• Electromagnetic surveys provide high-resolution subsurface resistivity, useful for mapping geology and salt concentrations.
• Previous models often used inverted resistivity, introducing errors due to scale inconsistencies and inversion constraints.

Purpose of the Study:

• To develop and test a novel coupled hydrogeophysical inversion (CHI) method for saltwater intrusion modeling.
• To directly integrate measured geophysical data into hydrologic model calibration, overcoming limitations of previous approaches.
• To improve the accuracy and reliability of groundwater resource management in coastal areas.

Main Methods:

• Developed a Coupled Hydrogeophysical Inversion-State (CHI-S) methodology.
• Used a saltwater intrusion model to interpret and guide geophysical inversion.
• Transformed simulated salt concentrations into electrical resistivity models for forward response calculation.
• Applied the method to a field site in Santa Cruz County, California, using time-domain electromagnetic (TDEM) data.

Main Results:

• Successfully applied the CHI-S method to a 2D cross-sectional model.
• Estimated six parameters (five aquifer properties and one geophysical parameter) with good resolution.
• Achieved a root-mean-square error close to 1 for over 300 apparent resistivities from the TDEM dataset.
• Identified potential reasons for poor data fit at three sounding locations, including model simplifications.

Conclusions:

• The CHI-S approach effectively integrates hydrologic and geophysical data for improved saltwater intrusion modeling.
• This method reduces errors associated with scale mismatches and inversion constraints.
• The study demonstrates the potential of CHI-S for enhanced coastal groundwater resource management.