Dipole source and conversion problem in transient electromagnetic sounding

The dipole source is the basic research object of electromagnetic method and the precondition for constitution of more complex detection devices and data interpretation, and also the theoretical basis for the development and innovation of technology. In this paper, starting from the concept of dipole source, the main transient electromagnetic field formulas of the dipole source of uniform earth surface are introduced or derived, and the similarities and differences between transient electromagnetic field formulas for positive and negative vertical step pulse excitation are indicated to deepen theoretical understanding. It illustrates that the electrical field components corresponding to the electrical and magnetic source can be converted to each other according to the reciprocity theorem of electromagnetic field, convenient for derivation formula. The paper described emphatically that more complicated formulas of electromagnetic field can be derived from the field intensity formulas of the dipole source by using superposition principle, such as transient electromagnetic field formulas for a large loop and any points, the application of the transient field is activated.