DETERMINING ECA-DEPTH PROFILES FROM ELECTROMAGNETIC INDUCTION MEASUREMENTS

The response of the EM-38 instrument to a wide range of simulated apparent electrical conductivity (ECa) profiles of different shape and mean ECa was modelled using depth response functions derived for uniform ECa profiles. Relationships between the calculated EM-38 readings and average profile ECa for depths to 1 m were determined using linear regression. These relationships were used to predict ECa profiles measured in the field using a 4-electrode probe. The mean profile ECa of the simulated profiles was strongly correlated with sinple linear combinations of the calculated vertical (EMV) and horizontal (EMh) dipole measurements. The model calculations indicated that the depth to which the EM-38 measures may vary from 1-65 to 5 m, depending on the ECa of these layers and the instrument range setting. Apparent electrical conductivity profiles measured with a 4-electrode probe were more closely predicted from EM-38 measurements using this approach than the established coefficient approach of D. L. Corwin and J. D. Rhoades. The results indicate that the depth response functions, derived for homogeneous profiles, also apply to heterogeneous profiles. © CSIRO 1990.