Electrical Resistivity Imaging (ERI) is a fantastic geophysical method for subsurface exploration and delineation (it certainly is our favorite!). In this article, we want to focus more on the “geo” part of geophysics and take a look at some resistivity values for common materials.
You see, ERI works best when you have a good knowledge of the geology of your survey area. In fact, when we’re helping new customers design their survey, or helping someone in the field—the geology of the area is one of our first questions.
Furthermore, if you have a good idea of resistivity ranges of the materials you’re looking at, you can easily put together the story of your survey. Something that may look suspicious in your survey data could actually just be normal geology (and vice versa). Knowing this information up front could save you a lot of time later!
There are many factors that affect a material’s resistivity. Resistivity is affected by natural geology, hydrogeology, mineralogical composition, porosity, temperature, pressure, water saturation, dissolved electrolytes, and other factors.
| Typical Ranges of Resistivities for Common Materials | |
|---|---|
| Rock/Material Type | Resistivity Range (Ωm) |
| Alluvium | 1 - 1,000 |
| Basalt | 10 - 1.3x107 (dry) |
| Clay (Including wet clay) | 1 -100 |
| Cobalt | 5.6x10-8 |
| Copper | 0.0000002 (native) - 1.7x10-8 |
| Drill Mud or Hydraul-EZ | 4.5 |
| Fresh Water | 10 - 100 |
| Gabbro | 103 - 106 |
| Gneiss (various) | 6.8x104 (wet) - 3x106 (dry) |
| Gold | 2.4x10-8 |
| Graphite Porphyry | 4.5x103 (wet) - 1.3x106 (dry) |
| Gravel | 100 - 10,000 |
| Igneous | 100 - 1,000,000 |
| Limestone | 100 - 10,000 |
| Marble | 102 - 2.5x108 (dry) |
| Mica | 9x102 - 1014 |
| Nickel | 7x10-8 |
| Salt Water | 0.1 - 1 |
| Sand (Both dry and wet) | 1 - 10,000 |
| Sandstone | 100 - 10,000 |
| Schist (Calcareous and Mica) | 20 - 104 |
| Schist (Graphite) | 10 -102 |
| Silver | 1.6x10-8 |
| Soil | 1 - 10 |
| Tuffs | 2x103 (wet) - 105 (dry) |
Reference: (Modified, Telford W.M-1990)
