CHAPTER 5: ELECTROMAGNETIC SOUNDING
Electromagnetic soundings are made to determine variations in the electrical conductivity of the earth with depth. Electromagnetic (EM) sounding methods include natural-field methods such as magnetotellurics (MT) as well as high-frequency radiation techniques such as radar probing. This chapter is concerned with controlled-source induction methods of sounding, so-called because an artificial source is used to generate the EM field, and because the frequencies are low enough that the measurement distance is less than the free-space wavelength. This is the quasi-static range, where conduction currents rather than displacement currents predominate. In the following discussing the general term EM sounding will be used for simplicity and to follow convention.