Electrical properties of the subsurface

Electrical conductivity is the ability of a material to conduct (transmit) an electrical current. The inverse of electrical conductivity is electrical resistivity. Electrical conductivity is generally expressed in either Siemens per meter (S/m) or (more commonly) milliSiemens per meter (mS/m). Electrical Resistivity is generally expressed in Ohm.m.

Current can be conducted through a rock through three different mechanisms. These are electrolytic, electronic and interfacial conduction.

• Electrolytic conduction occurs by the movement of ions within an electrolyte. This is the dominant mode of conduction in soils and rocks for the majority of cases.
• In Electronic conduction the current is carried by electrons. For most rocks (which are near perfect insulators) such conduction will not occur, but if the rock contains minerals that act as semi conductors or metallic conductors this can contribute to the overall conductivity. Examples of this are e.g. pyrite and most other sulfides, magnetite and some other oxides as well as metals (both native and anthropogenic (pipelines).
• Grain surface properties can give rise to a so called interfacial conductivity (which in turn allows for interfacial conduction). This conductivity originates with the EDL (electrical double layer) which is formed at the interface between the electrolyte and grain surfaces. Interfacial conductivity is complex (i.e. it has a real and imaginary part)

It should be noted here that this description of electrical properties only starts to scratch the surface of these electrical properties, and many books and articles have been written (and are being written) on this topic. A more in depth description of this topic can be found e.g. in this  overview article by Gary Olhoeft or in Chapter 2 of the PhD thesis of Andreas Kemna. Especially,

In the electrical resistivity method we  we measure the total electrical resistivity (which is a result of the  three conductivities listed above). The electrical resistivity that we measure depends on the electrical properties of all the constituent materials of these soils and their geometric relationships.

There is a good understanding of the values of electrical properties of different rocks and soils (and of the changes caused in these electrical properties due to changes in moisture, precipitation and so on). This understanding (discussed below) allows us to interpret the results of an electrical resistivity survey  in terms of geology, hydrology and subsurface processes.

Dependency of electrical properties on common subsurface properties

On the right a non exhaustive list is shown of properties which influence electrical conductivity.

The fact that this list is so long is both good and bad. It is good because – at least in theory – we can relate electrical property distributions to many soil properties of interest. It is bad because the fact that electrical property depends on so many  properties means that we can rarely say anything definitely about a single soil property. As discussed in more detail in the section on Timelapse Electrical Resistivity one exception to this is in timelapse geophysical studies: in such studies we can (for cases where the geological matrix, mineralogy and microbiology remain the same) sometimes make statements about the behavior of a single property of interest.

Subsurface properties which influence electrical conductivity

• Porosity
• Permeability
• Grainsize Distribution
• Mineralogy
• Lithology
• Saturation
• Water chemistry/ Ionic strength of liquids
• Percentage Clay
• Temperature
• Microbiological processes
• Metallic infrastructure

Relating electrical properties to subsurface properties of interest

A common wish is for a way in which we can transform electrical properties into rock physics properties. Such a transformation (known as a petrophysics transform) can be generated if we have sufficient point measurements of electrical properties in the subsurface (generally from cores) in addition to the electrical resistivity image.

However, it turns out that – under certain conditions – we can use an experimentally derived relationship called Archie’s law. This law states that if no metals are present the resistivity of a formation will relate to the volume and conductivity of water in the soil. As the groundwater conducts through its ions, its conductivity will depends strongly on the total dissolved solids. Archie’s law holds for a porous, clay-free medium whose matrix is non-conducting.

Ranges of subsurface electrical properties

The electrical conductivity of rocks has one of the greatest ranges of all physical properties – about 24 orders of magnitude. The table below gives ranges of the electrical conductivity and electrical resistivity for some commonly encountered materials. Values for other materials can be found on Wikipedia. Note that Electrical Conductivity is a parameter which is often measured on a liquid sample using a commercial EC (electrical conductivity) meter. EC meters typically report in μS/cm (microSiemens/cm). In some cases the fluid conductivity and porosity can be used using Archie’s law to predict bulk electrical properties and it is this often valuable to know fluid conductivity.

While resistivity is typically given in Ohmm, electrical conductivity is reported in several different ways:

• Siemens (SI unit symbol S) is the unit of electric conductance. It is the reciprocal of electrical resistance (SI unit ohm (Ω)).
• Electrical conductivity and resistivity, are given in S/m and Ohm.m, which are each others reciprocal.
• Siemens is also referred to as mho (so we would have mho/m for the conductivity).
• While resistivity is typically given in Ohm.m, conductivity can be given in different manners:
• S/m (Siemens/meter)
• mS/m (milliSiemens/meter). 1000 mS/m = 1 S/m. This is the unit generally used in electrical geophysics.
• mS/cm (milliSiemens/centimeter). 1 mS/cm = 100 mS/m =0.1 S/m. This unit is relatively rarely used.
• μS/cm (microSiemens/cm). 1 μS/cm = 0.1 mS/m = 0.0001 S/m.

The values reported below (in different units) are broad ranges found in the literature which are provided as guidelines. Higher and lower values can and will occur.

 Material Electrical Conductivity (mS/m) Electrical Conductivity (μS/cm) Electrical Resistivity (Ohm.m) Comment Surface & groundwater 5-150 50-1500 6-200 Large variability Seawater 3000-5000 30000-50000 0.2-0.33 Mean: 3270 mS/m Unconsolidated materials 10-1000 100-10000 1-100 saturation & mineralogy dependence Sedimentary Rocks 1-200 10-2000 5-1000 – Igneous & Metamorphic Rocks 0.1-10 1-100 100-10000 Weathering decreases resistivity Clays 25-250 250-2500 4-40 Claytype and moisture important Clayey Soil (40 % clay) 125 1250 8 Clay % decrease increases resistivity