IEEE Std 356-2020 pdf free download – IEEE Guide for Measurements of  Electromagnetic Properties of  Earth Media.
It is well known that the physical and electromagnetic properties of the earth are highly non-uniform. Consequently the use of parameters σ and ε to describe the earth should take into account the fact that they can be a function of spatial dimensions or can represent a composite value, which is directly affected by the non-uniformity of the sample. In rock mechanics, these differences are described by the terms “rock mass” to represent the non-uniform composite structure, and “rock material” to represent the uniform material (Brady and Brown [B2]). This distinction can also be made by differentiating between those methods of measurements that are made in situ and those that are made on rock samples in the laboratory. This distinction is also directly related to the wavelength of the radiation in the material under consideration and the size and separation of the contact electrodes used in the measurement.
This document does not cover electrical or electromagnetic geophysical methods reported by Keller and Frischknecht [B11] that rely on mapping anomalies in the earth’s structure, unless such information is directly related to determining the electrical properties of such materials. These geophysical techniques include magnetic tilt methods, magnetic surveys, most types of ground probing radar (GPR) and many airborne and satellite remote-sensing techniques.
This guide does include the methods used to provide “ground truth” for these mapping methods. The frequency of measurement, porosity of rock, water content, temperature, pressure, and the degree of fracture of the sample can affect measurement. Porosity is defined as the percentage of pore space in a unit volume of rock. Pores in a rock are often filled with fluids that give rise to finite resistance. While measuring the conductivity, there can be significant problems with probe contacts, both for in-situ measurements (probe impedance, conductive layers, etc.) and sample measurements (surface preparation, air gaps, etc.).
In addition, these materials can be highly inhomogeneous, anisotropic, layered, and fractured so that the orientation of the electrodes should play a significant part in determining the results obtained (Keller and Frischknecht [B11]). Anisotropy of a rock sample is scale-dependent. Consequently, electrode orientation as well as the separation of electrodes can influence the anisotropy measurement. The measurements on soil samples are particularly difficult as the removal of a sample can strongly affect soil compaction and water content (particularly the water concentration profile as a function of depth) (Sternberg and Levitskaya [B21]). In-situ measurements are commonly made on or above the earth’s surface, e.g., with inserted probes, from an airborne platform or from boreholes. In-situ measurements, where properly implemented, can avoid the problems resulting from changes in compaction and soil moisture content. An attempt was made to cover all of these techniques. Given the inhomogeneous and anisotropic nature of earth mass, the derivation of reliable data from field and laboratory measurements is difficult. It is possible to find analytical solutions to certain idealized earth structures.
The calculation of field results using analytical or numerical methods from a postulated earth structure is called forward modeling. Thus one can derive characteristic curves to deduce ground constants. More commonly, automated data inversion techniques are not available, as the number of unknown parameters (including their spatial distribution) is so large that least squared error minimization techniques do not converge to the correct answer (Oldenburg [B19]). The mathematical techniques of numerical modeling for solving the forward problem (i.e., assuming a particular earth profile to calculate expected measurement results), and simulated annealing and artificial neural networks for the inverse problem (i.e., determining the electrical properties of the earth from the measurements) require considerable computation time and effort. Therefore, only passing mention is made to numerical methods for forward modeling and data inversion for two- and three-dimensional structures.IEEE Std 356 pdf download.