Magnetometry in Kaga

Originally posted 19 September from my handheld via the cell network and e-mail to my old site.

Drove to Linköping this morning listening to the Digital Planet podcast, M Coast's latest album and a Povel Ramel hits collection. On site in Kaga I was greeted by my friendly National Heritage Board colleages Immo Trinks and Per Karlsson. They were busy assembling Scandinavian archaeology's first motorised magnetometer setup, and informed me that my site would see the equipment's first non-trial run.

The setup consists of a long trailer made of aluminium and held together by bronze and plastic bolts, pulled by a four-wheel off-road vehicle. Both the materials used and the length of the trailer are intended to keep the magnetometers away from anything magnetic above ground. The instruments, four of them today, are mounted at the rear of the trailer, where they measure magnetism at about 30 cm and 100 cm above the ground surface. As magnetism's strength decreases with the cube of the distance, the lower sensors pick up the planet's magnetic field plus any subsoil anomalies, while the upper sensors only get the planet's field. Subtract one from the other and you have a measurement with archaeological relevance. Take measurements 25 times a second as you drive across a field, with each measurement being tagged with GPS position data, and you get a dataset that allows you to map what's under the topsoil. Drive to and fro across a field as if you were ploughing it, covering its entire surface, and you can map 15 hectares in one day's work.

As always with newly developed gear, there were glitches, so we didn't cover quite so much ground. But I'm confident that Immo will iron out the wrinkles and get the job done as he has at Birka, Ales stenar, Gamla Uppsala and other high-profile sites. At those sites, he used a manual setup with simpler positioning tech.

The reason that I asked the guys to come to Kaga was the metalwork myself and the Gothenburg metal detectorists have found there in the past two Easters -- the gold foil figure die, the many brooches, the copper-alloy casting debris. I believe we're dealing with an elite settlement site of the mid- to later 1st Millennium AD, and before I ask Chris the farmer for permission to dig in his field I want to know what the site's layout is like. Hopefully the magnetometry will be informative enough to allow me to open up 300 square meters or so right over the remains of the mead hall. Blindly machine-stripping the place would be a clunky, destructive and expensive way to map the settlement.

I wasn't much use to Immo and Per, so I spent the afternoon metal-detecting around the spot where Kenth Lärk found the c AD 500 relief brooch back in April. Compared to the Gothenburgers I'm crap at detecting, so I only found one datable piece of prehistoric metalwork in four hours: the handle of a 10th century key. Also a coin of mid-17th century Queen Christina and a lot of later odds and ends. I think this was the fourth It's the sixth quarter-öre of Christina we've found in that field. Per has also noticed in his urban excavations that those coins are bloody everywhere.

Having failed to make contact with Dear Reader Hans to invite myself to his casa in Malmslätt, I am spending the evening alone in my room at a hostel by the swimming centre in Ljungsbro. Rain pours, making me fear for what the site will be like tomorrow. But I have books and chocolate, and so need not feel too sorry for myself.

[More blog entries about archaeology, magnetometry, fieldwork, Sweden, metaldetecting; arkeologi, magnetometri, Östergötland, metallsökare]