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. In addition, proton precession magnetometers
are typically placed on two to three meter poles to remove them from potential
noise sources at ground level.
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. Good observational practice will see visible, non-geological magnetic
sources recorded as part of the field observations.
*Figure from Introduction to Geophysical Prospecting, M. Dobrin and C. Savit
