Although some native metals and graphite conduct electricity, most rock-forming minerals are electrical insulators. Measured resistivities in Earth materials are primarily controlled by the movement of charged ions in pore fluids. Although water itself is not a good conductor of electricity, ground water generally contains dissolved compounds that greatly enhance its ability to conduct electricity. Hence, porosity and fluid saturation tend to dominate electrical resistivity measurements. In addition to pores, fractures within crystalline rock can lead to low resistivities if they are filled with fluids.
The resistivities of various earth materials are shown below.
| Material | Resistivity (Ohm-meter) |
|---|---|
| Air | ¥ |
| Pyrite | 3 x 10^-1 |
| Galena | 2 x 10^-3 |
| Quartz | 4 x 10^10 - 2 x 10^14 |
| Calcite | 1 x 10^12 - 1 x 10^13 |
| Rock Salt | 30 - 1 x 10^13 |
| Mica | 9 x 10^12 - 1 x 10^14 |
| Granite | 100 - 1 x 10^6 |
| Gabbro | 1 x 10^3 - 1 x 10^6 |
| Basalt | 10 - 1 x 10^7 |
| Limestones | 50 - 1 x 10^7 |
| Sandstones | 1 - 1 x 10^8 |
| Shales | 20 - 2 x 10^3 |
| Dolomite | 100 - 10,000 |
| Sand | 1 - 1,000 |
| Clay | 1 - 100 |
| Ground Water | 0.5 - 300 |
| Sea Water | 0.2 |
Like susceptibilities, there is a large range of resistivities, not only between varying rocks and minerals but also within rocks of the same type. This range of resistivities, as described above, is primarily a function of fluid content. Thus, a common target for electrical surveys is the identification of fluid saturated zones. For example, resistivity methods are commonly used in engineering and environmental studies for the identification of the water table.