Electrical Resistivity Primer

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Ranges of soil 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 here 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