Subsurface Electrical Properties

Dependency of electrical properties on common soil properties

As briefly discussed before the value of the electrical conductivity of a soil depends on a large number of soil properties. Soil properties which influence electrical conductivity include

  • Porosity
  • Permeability
  • Grainsize Distribution
  • Mineralogy
  • Saturation
  • Ionic strength of liquids
  • Percentage Clays
  • Metallic infrastructure

Archies law 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 soil properties means that we can rarely say anything 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 behaviour of a single property 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 Archies 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. Archies law holds for a porous, clay-free medium whose matrix is non-conducting.