The stronger component known as the Earth’s poles, reverses direction at irregular intervals.
The weaker variations are the Earth’s magnetic map.
In the previous chapter, we described some of the desk-based research methods that can precede fieldwork.
We looked at mapping, archives and aerial photographs, and the ways in which these sources can be combined to respond to the desk-based assessment typically stipulated in a project ‘brief’. Of course, all of the methods associated with desk-based assessment can stand alone as a study in their own right, without any intention of following this up with fieldwork.
These artifacts of occupation can yield the magnetic declination from the last time they were fired or used.
Archaeomagnetic dating was described in the 1992 publication “Paleomagnetism: Magnetic Domains to Geologic Terranes.” By Robert F. Archaeomagnetic dating requires an undisturbed feature that has a high likelihood of containing a remnant magnetic moment from the last time it had passed through the Curie point.
Once the paleodirections of enough independently dated archaeological features are determined, they can be used to compile a secular variation record for a particular region, known as an SVC.
The Archaeomagnetic Laboratory at the Illinois State Museum has secular variation curves for the southwest, mid-continent and southeast United States.
Archaeomagnetic dating is the study and interpretation of the signatures of the Earth's magnetic field at past times recorded in archaeological materials.
These paleomagnetic signatures are fixed when ferromagnetic materials such as magnetite cool below the Curie point, freezing the magnetic moment of the material in the direction of the local magnetic field at that time.
Each of the samples is measured in a spinner magnetometer to determine the thermal remanent magnetism of each sample.
The results are statistically processed and an eigenvector is generated that shows the three-dimensional magnetic declination that will yield a location for the North Pole at the time of the last thermal event of the feature.
and the increased sensitivity of SQUID magnetometers has greatly promoted its use.