What is resistivity?
Resistivity is an intrinsic electrical property of a given material, defining how it resists the flow of electrical current. The term “intrinsic property” indicates that the resistivity value is independent of how much material you have; having more or less material will alter the resistance but the resistivity of it remains unchanged (similar concept to the relationship between weight and density of a material). Resistivity is expressed in ohm-m or Ωm and is the inverse of conductivity.
If the resistivity of a material is low, current will pass easily but if the resistivity is high, current flow will be more restricted.
Resistivity values can vary from fractions of Ohm-m to several tens of thousands of ohm-m. The resistivity of a material is mainly dependent upon:
- Degree of water saturation
- Amount of dissolved solids
- Content of organic matter
- Grain size
- Grain shape of the soil matrix
- Chemical composition.
The resistivity values for a selection of soil types are listed in the table below; these represent saturated conditions but for dry conditions the resistivity can be an order of magnitude higher.
|Material||Resistivity (ohm-m)||Conductivity (mS/m)|
The resistivity of other geological materials and water in various states is presented in the following diagram. Note the large overlap in resistivity values which can make an absolute interpretation from just a single dataset quite difficult. Often interpretation is made in relative terms, i.e., looking for a change in resistivity from the expected or experienced ‘background’ values due to the presence of some other material or feature such as mineralization, groundwater or voiding.
During resistivity surveys we can normally only record Apparent Resistivity whilst in the field and it is necessary to run the dataset through inversion software to produce a model of the true magnitude and distribution of resistivity values below the surface.