Electromagnetic methods are sensitive principally to electrical
conductivity\(\sigma\)
(Siemens/meter). We sometimes work with the inverse of conductivity which is
electrical resistivity \(\rho\) which has units \(\Omega m\) (ohm-
meters). Electrical conductivity characterizes the ease that current flows
through the material when an electrical force is applied. Electric current
(units of Amperes) quantifies the amount of charge that is moving by an
observer in one second. The electrical conductivity of Earth’s materials
varies over many orders of magnitude. It depends upon many factors, including:
rock type, porosity, connectivity of pores, nature of the fluid, and metallic
content of the solid matrix. A very rough indication of the range of
conductivity for rocks and minerals is presented in Fig. 140.
While conductivity is the generally the main physical property of interest, EM induction methods also depend on the magnetic permeability and electrical permitivity of rocks. Their importance depends upon the frequency of the EM signal used for given a system.
