The purpose of this guide is to describe the measurement principles of the electrical properties of naturally occurring solid materials@ although it will also serve as a guide for the measurement of any solid material. These properties are the conductivity ??(in S/m) and the permittivity (or dielectric constant) ??(in F/m). The magnetic permeability @??(in H/m) will not be considered except where it impacts on the interpretation. This is because ?????o(to within less than 5%) for most naturally occurring materials. Even dry@ pure magnetite sand only increases @?r to 1.09. In free space (vacuum)@ the permittivity is ??= 8.854 x 10-12 F/m and the magnetic permeability is @?o = 4??x lO-7 H/m. The conductivity of a material is defined by Ohm??s Law; i.e.@ the current I (in A) passing through a block of the material (length I in m and cross-section A in m2) is related to the voltage V applied to plate electrodes covering two parallel faces of the cube@ as given by Equation (1):where R is the resistance in ohms. The conductivity ??can be determined using Equation (2): Similarly@ the permittivity of the material can be determined by the same physical arrangement of a block (area A and length I ) between two parallel electrodes by measuring the voltage V between the plates and the charge stored on the plates Q (in coulombs) by the capacitance C (in F/m)@ as shown by Equation (3) and Equation (4): and where the effects of fringing fields have been ignored. The similarity between the measurements of R and C@ and consequently ??and ??@ result in very similar methods being used to determine the two parameters. In particular@ by using Equation (5)@ one can write [B1.3]: When the material has conduction and displacement currents@ the relationship between the applied voltage and the current can be written as shown in Equation (6): When the material has conduction and displacement currents@ the relationship between the applied voltage and the current can be written as shown in Equation (6): where ??#39;r is the real part (also called the effective relative permittivity) and ??quot;r ?accounts for losses. The conductivity ??can range in value from 10-6S/m for very dry sand@ rock@ or fresh water ice to 10 S/m for highly saline soils. The effective relative permittivity ??#39;r can range from 3 to 100 for isotropic materials@ but the effective relative permittivity can assume much larger values (or even be negative) in the case of mixed and/or anisotropic media [B1.1@ B1.21. For example@ a clay-loam soil with 10% water content was found to vary continuously from ??= l0-2 S/m and ??#39;r = l04 at 100 Hz to ??= 5 S/m and ??#39;r = 10 at 1010Hz [B1.11]. It is well known that the physical and electromagnetic properties of the earth are highly nonuniform. Consequently@ the use of parameters ??and ??to describe the earth has to take into account the fact that they will be a function of spatial dimensions or will represent a composite value@ which is directly affected by the nonuniformity of the sample. In rock mechanics@ these differences are described by the terms ??rock mass?? to represent the nonuniform composite structure@ and ??rock material?? to represent the uniform material. 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 guide does not cover electrical or electromagnetic geophysical methods [Bl. 101 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@ and many airborne and satellite remote sensing techniques. The guide does include the methods used to provide ??ground truth?? for these mapping methods. The frequency of measurement@ the water content of the sample@ the temperature of the sample@ the pressure on the sample@ and the degree of fracture of the sample will all affect measurements [B1.1]. 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 [B 1.1 O]. 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) [B1.19]. In-situ measurements are commonly made from 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 shall be 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@ however@ 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 [Bl .18]. The mathematical techniques of numerical modeling for solving the forward problem (i.e.@ assuming a particular earth profile to calculate expected measurement results)@ and stimulated 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 shall be made to numerical methods for forward modeling and data inversion for two- and three-dimensional structures.
IEEE 356-2001 history
1970IEEE 356-2020 IEEE Guide for Measurements of Electromagnetic Properties of Earth Media
2010IEEE 356-2010 Guide for Measurements of Electromagnetic Properties of Earth Media
2002IEEE 356-2002 Guide for Measurements of Electromagnetic Properties of Earth Media
2001IEEE 356-2001 IEEE Guide for Measurements of Electromagnetic Properties of Earth Media
1974IEEE 356-1974 Guide for radio methods of measuring earth conductivity