Geophysical mapping of freshwater lens in Big Pine Key, Florida: Electromagnetic Induction Calibration and Application

Abstract

AbstractGeoelectric and electromagnetic (EM) methods are rapid and non‐invasive geophysical techniques for estimating groundwater properties and characterizing the spatial and temporal variability of subsurface formations. However, to quantitatively interpret the EM data, the systematic error due to calibration problems and random error must be considered. We conducted coincident EM and electrical resistivity tomography (ERT) surveys in January and December 2018 on Big Pine Key, FL. In this study, we used vertical electrical sounding (VES) data extracted from a 220 m long ERT profile to calibrate the EM data. The inverted VES data were used as input in the EM forward model to estimate the quadrature component response. Then, the observed offset between the calculated and observed quadrature data was corrected using a multiple linear regression model. Finally, the calibrated quadrature data were converted to apparent electrical conductivity and inverted as a 2‐layer model using both the full solution and the low induction approximation. These models were used to assess the temporal and spatial variations of conductivity on a 2.2 km long E–W trending profile across the island. The calibrated apparent electrical conductivity on the profile decreased between January and December 2018, with the largest decreases in the lower elevation regions of the profile. In addition, the 2‐layer models inverted using the full solution and low induction approximation showed the depth of the freshwater interface increased by December 2018. These observations suggest the recovery of the freshwater lens due to precipitation. Based on this study, we concluded the VES at pilot locations can be used for calibration purposes and verification of the accuracy of EM measurements.