## Glossary of geophysical terms relating to soil electromagnetic properties.

Absorption - The conversion of electromagnetic energy to heat energy due to the inefficiency of charge storage, during reorientation of particles with respect to an electromagnetic field. Elevated phases of absorption accompany dispersion during relaxation.

Adsorbed Water - Water attached to the surface of a solid fraction of a soil that is considered to have reduced potential to rotate in response to an electromagnetic field, due to surface charges on a solid. Such water is generally considered to be within the Diffuse Double Layer and in engineering terms to have elastic, rather than viscous, properties. Also known as bound water, to contrast with free water, adsorbed water is considered to have a very low permittivity compared to other forms of liquid water.

Attenuation - Reduction in amplitude of an electromagnetic wave, as a function of distance propagated through a material.

Attenuation Coefficient - Reduction in electromagnetic wave energy, per unit distance, used to determine attenuation.

Conductivity - The ability of charges (e.g. ionic and electronic) to move through a material. Conductivity is the inverse of resistivity.

Dielectric - An insulating material, such as used between capacitor plates to control the build-up of charge on them. Lossy dielectrics are those where the insulating properties are not ideal, causing loss of charge.

Dielectric Constant - The relative permittivity of a material (generally referring to the value at low frequencies). The dielectric constant is often not a constant at all, and so the term relative permittivity is preferable.

Diffuse Double Layer - A theory that the water phase around charged mineral particles will comprise a thin bound layer of ion-rich water (the Gouy layer) adjacent to mineral particles, which are themselves surrounded by a diffuse layer (the Chapman layer) that is influenced by the surface charge of the minerals in a manner that reduces with distance.

Dipolar Molecules - Dipolar molecules can be considered to be those which have net charges separated by a small distance, such that the molecule will attempt to reorient itself to an applied electromagnetic field. In soils, water forms the dominant source of dipolar molecules.

Dispersion - In the context of soil electromagnetics is a relaxation in the permittivity of a material with frequency, named due to the dispersion of light that has passed through a prism. However, electromagnetic dispersion should not be confused with the same term used in geotechnical engineering.

Free Space - A complete vacuum with no properties that may influence the propagation of an electromagnetic wave. Therefore, free space is a theoretical construct that differs only slightly from the concept of a vacuum.

Free Water - In the context of soil pores, free water is considered to be water which has little dependence on solid surface charges and boundaries for its properties and may act as a viscous liquid.

Impedance - The equivalent of resistivity for alternating signals, impedance defines the degree to which a material impedes the propagation of electromagnetic signals through it. It is defined by the permittivity and magnetic permeability of the material, as well as its means of containment. In simple terms it is the ratio of the electric to magnetic field strength in a material, akin to the resistance expressed in Ohms Law.

Impedance of Free Space - The ratio of the electric to magnetic field strength of an electromagnetic signal in Free Space. The impedance of free space is 120 PI ohms.

Magnetic Permeability - The ability of a material to store energy due to the lining up of atomic and electronic spins in response to an electromagnetic wave. Magnetically permeable materials may exhibit dispersion and absorption responses with frequency in a similar manner to permittivity.

Permittivity - The ability of a material to store energy due to the separation of charged particles (e.g. ions, atoms, electrons) in response to an electromagnetic wave.

Phase Constant - A measure of the phase change, generally measured in radians, occurring for an electromagnetic wave over unit distance.

Polarization - The term polarization is used in two different ways. In geophysics it generally refers to the degree to which energy has been stored through separation of charges. It can also be used to mean the orientation of an electromagnetic wave.

Reflection Coefficient - The amount of electromagnetic energy that is reflected from an interface between materials of differing impedance, expressed as a proportion of the incident energy.

Relative Permittivity - The ratio of the permittivity of a material to that of free space (8.854x10-12 F.m-1).

Relative Magnetic Permeability - The ratio of the magnetic permeability of a material to that of free space (4 PI x10-7 Hm-1).

Relaxation - A reduction in the permittivity of a material, over a dispersion spectrum, centred on a relaxation frequency, with an accompanying phase of elevated absorption. The probability of relaxation occurring at a particular frequency is described by the dispersion model for the material in question.

Resistivity - The degree to which the ability of charges to move through a material is impeded. Resistivity is the inverse of conductivity.

Signal - For the purposes of geophysics a signal can be considered an electromagnetic wave purposely created to propagate through a material. Noise is therefore not a signal, but impacts on the detection of signals, the degree of impact generally being expressed as the signal-to-noise ratio.

Transmission Coefficient - The amount of electromagnetic energy that is able to pass through an interface between materials of differing impedance, expressed as a proportion of the incident energy.

Velocity of light (c) - The velocity of an electromagnetic wave in free space (2.9979x108 m.s-1) and so also the maximum velocity that can be achieved by an electromagnetic signal.

Wavenumber - See phase constant.

Wavelength - The scalar distance through which an electromagnetic wave travels during a phase change of two PI radians.