Magnetic Cleanliness and Interference
*
When making total field measurements from which estimates of the subsurface
distribution of magnetic susceptibility or the presence of subsurface magnetized bodies are made, it
is imperative that factors affecting the recorded field other than these be eliminated or
isolated so that they can be removed.
We have already discussed several of these added complications, including spatial variations
of the Earth's main magnetic field and
temporal
variations mostly associated with the
external magnetic field.
In addition to these factors which we can not control, there are other sources of noise
that we can control.
Because any ferromagnetic substance can produce an induced magnetic field in the presence of the Earth's main field and because modern magnetometers are very sensitive (0.1 nT), the field crew running the magnetic survey must divest itself of all ferrous objects. This includes, but is not limited to, belt buckles, knives, wire-rimmed glasses, etc. As a result of this, proton precession magnetometers are typically placed on two to three meter poles to remove them from potential noise sources worn by the operators.
In addition to noise sources carried by the operators, many sources of magnetic noise may be found in the environment. These can include any ferrous objects such as houses, fences, railroad rails, cars, rebar in concrete foundations, etc. Finally, when using a proton precession magnetometer, reliable readings will be difficult to obtain near sources of AC power such as utility lines and transformers.
*Figure from Introduction to Geophysical Prospecting, M. Dobrin and C. Savit