IEEE Std 356-2020 pdf download – IEEE Guide for Measurements of  Electromagnetic Properties of  Earth Media.
For more complex earth structures, the calculation of the apparent conductivity from such measurements is quite difficult analytically (Macnae and Lamontagne [B133], Macnae and McGowan [B134], Raiche and Spies [B136], Spies and Frischknecht [B137], van Blaricom [B138], Ward and Hohmann [B141]). Data interpretation of the apparent conductivity, determined as a function of frequency, is often based on sets of master parametric curves, which differ for each loop configuration. One approach with such curves is to normalize the data to the conductivity of the top layer (Spied and Frischknecht [B137], van Blaricom [B138]). A highly effective conjugate-gradient method of data inversion has been developed (Wang et al. [B140]). 8.2 Field procedures The ground-based transmitter is normally supplied current of approximately 5 A although the higher-powered units can achieve 50 A (Raiche and Spies [B136]). A number of different transmitter pulse techniques are used. They include the following (see Figure 8):  Pure impulses (alternating positive and negative square pulses with a zero resting position between each pulse)  Half-sinusoidal pulses (alternating positive and negative sinusoids with a zero resting position between each pulse)  Linear impulse ramps (alternating positive and negative going trapezoidal pulses with a zero resting position between each pulse)  Linear ramps (triangular waveform) The field strength is then monitored for periods of up to several 100 ms, usually in the form of a set of predefined windows (Nabighian and Macnae [B135]). The field at each time-step window is averaged; up to 32 windows are used. The window arrangements are different for each commercially available system and are used to resolve different subsurface features. In airborne applications, both the transmitter and receiver are located on or towed behind the aircraft. Most commercial systems run at audio frequencies (Barringer [B131]).
8.3 Borehole transient electromagnetic measurements The current source for a TEM system can be located in a borehole, and the surface of the earth can be surveyed using a staked-dipole receiving system (Wait [B139]). From these measurements, one can apply a simple analytical formula to calculate the apparent conductivity of the equivalent half-space (Wait [B139]). Also under consideration is the location of a fixed receiver in a borehole and a moving transmitter located on the surface (Coggin and Wait [B132]). This clearly allows a much higher transmitter power to be used.