Lets get geo-physical!
I finished up my week of work on a Roman mile-fort on the Antonine wall. As it turns out carrying out a geophysical survey is very tiring, which is why I did not post sooner. Archaeologists are supposed to be big drinkers and partiers but I was in bed by 11 this past week. At least I have a beard and cool hat so hopefully I am still in the club.
What is geophysics you may ask? Geophysics is namely used to look for valuable resources such as mineral, water and oil. The same techniques and technologies have been modified to ignore the ‘deeper stuff’ and focus on depths ranging to about 2 meters as most archaeological deposits do not go any deeper than that. Well when used for archaeological purposes a type of ‘remote sensing’ (as you can see what is beneath without having to disturb it). Geophysics is a series of methods that investigate subsurface features through their interaction with the surrounding geology.
The two types of geophysics we employed on the fort was resistivity and magnetometry. Resistivity (Rez) finds ‘anomalies’ that interferes with an electrical current that is passed through the earth. If something solid is in the path of the current, like a Roman wall, the resistance level will increase. Those who want to know more about this technique we employed the ‘twin probe’ configuration.
Magnetometry (Mag) find magnetic differences in the geology. Obviously it is very good at picking up metal but it also picks up very hot fires, such as kilns, as the high temperature may have reached the ‘Currie point’ which causes all the susceptible material to magnetically point North (unlike the surrounding area). Those who want to know more about this technique we employed a gradiometer.
Both techniques have their advantages over the other but they are actually complementary as one process along will miss a feature that the other will likely detect. Here are just a few considerations.
Rez survey requires you to physically place probes in the ground, take a reading, move forward and repeat making it much slower. Mag, however, can ‘hover’ above the ground to take measurements, which makes the process much quicker.
Mag reacts to metal so you have to strip yourself down. Apart from the obviously jewelry consideration, think of how much outdoor clothing has metal zippers, grommets or is metal toed. Rez has no such qualms,
Rez has cords which can get tangled and snagged.
Mag cannot operate near metal fences or garbage/rubbish as it will give very high readings. It also picks up modern utility pipes
All in all I have had a really interesting time and I have learned a lot. I can state that we as a team have proven that a Roman fortlet exists within a larger mile-fort along the Antonine wall. I will post the visual data later, and perhaps a link to a published report when it is finished, however I currently have to withhold the sites name for privacy’s sake.