Analysis of the near-source error in TEM due to the dipole hypothesis

• Direct-dipole approximation in near-source zone is analyzed.
• Superposing-dipole approximation in near-source zone is analyzed.
• An alternative way of solving the near-source problem is introduced.
• Direct time-domain solution of damped wave equation for point charge is given.
• The response of loop based on point charge is given.

The dipole hypothesis commonly used in the description of electromagnetic fields associated with TEM loop antennas or grounded wire source is not accurate enough in the near-source zones where the response is also of significant importance for clearly mapping subtle structures embedded in shallow settings with geologic complexities. In order to better understand these near-source errors, we investigate the scale of errors resulting from the direct-dipole and superposing-dipole approximation of the large loop or long grounded wire sources. The results demonstrate that both the direct-dipole approximation and the superposing-dipole approximation fail to give accurate results in the near-source zone. Not only the direct-dipole approximation but also superposing-dipole approximation will arouse some errors. Therefore, a ‘point-charge’ solution of the EM damped wave equation is proposed and derived to calculate the near source responses. Forward modeling and field data analysis demonstrate that the proposed method provides more accurate results in the near source areas and can be effectively used to calculate the TEM response in the near source region.