Although the mechanisms by which induced magnetization can arise are rather complex, the field generated by these mechanisms can be quantified by a single, simple parameter known as the susceptibility, k. As we will show below, the determination of a material type through a knowledge of its susceptibility is an extremely difficult proposition, even more so than by determining a material type through a knowledge of its density.
The susceptibilities of various rocks and minerals are shown below.
| Material | Susceptibility x 10^3 (SI)* |
|---|---|
| Air | ~0 |
| Quartz | -0.01 |
| Rock Salt | -0.01 |
| Calcite | -0.001 - 0.01 |
| Sphalerite | 0.4 |
| Pyrite | 0.05 - 5 |
| Hematite | 0.5 - 35 |
| Illmenite | 300 - 3500 |
| Magnetite | 1200 - 19,200 |
| Limestones | 0 - 3 |
| Sandstones | 0 - 20 |
| Shales | 0.01 - 15 |
| Schist | 0.3 - 3 |
| Gneiss | 0.1 - 25 |
| Slate | 0 - 35 |
| Granite | 0 - 50 |
| Gabbro | 1 - 90 |
| Basalt | 0.2 - 175 |
| Peridotite | 90 - 200 |
Unlike density, notice the large range of susceptibilities not only between varying rocks and minerals but also within rocks of the same type. It is not uncommon to see variations in susceptibility of several orders of magnitude for different igneous rock samples. In addition, like density, there is considerable overlap in the measured susceptibilities. Hence, a knowledge of susceptibility alone will not be sufficient to determine rock type, and, alternately, a knowledge of rock type is often not sufficient to estimate the expected susceptibility.
This wide range in susceptibilities implies that spatial variations in the observed magnetic field may be readily related to geologic structure. Because variations within any given rock type are also large, however, it will be difficult to construct corrections to our observed magnetic field on assumed susceptibilities as was done in constructing some of the fundamental gravitational corrections (Bouguer slab correction and Topographic corrections).