Electrical Properties of Rocks and Soils
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 nice overview article by Gary Olhoeft or in Chapter 2 of the PhD thesis of Andreas Kemna.
When we measure the electrical conductivity of soils or rocks (details on how we do this is provided in subsequent sections) we measure the total electrical conductivity (which is a combination of the three conductivities listed above). This electrical conductivity 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 in some detail in the next section) allows us to interpret an electrical resistivity image in terms of geology, hydrology and processes (and is thus the reason why we use electrical resistivity to investigate the subsurface).