Terrestrial Geophysics

Getting out to the site of the Chiseldon Cauldrons to start our geophysical survey was a bit disorienting. The entire field was covered in a blanket of thick fog, and once we were driving along the path through the field the hedges that marked the edges of the field were out of view. When we reached the site and got out of the cars the ground was frozen, slippery and covered in patches of snow. Those of us who had been out to the site for the field walking were grateful that the wind that was blowing that day wasn’t blowing now… It was cold enough in the field. There was work to be done, however, so we disappeared into the fog to set up a grid. With that done, one of the volunteers and I retreated to the vehicles and our flasks of soup and tea. A geophysics team of two from Wessex Archaeology started working on the survey. By lunch time they were finished work on the north side of the field boundary that’s near the cauldrons site. The results for the first half of the day were uploaded onto a laptop.

Setting Up the Grids

After lunch a group of volunteers joined us to take a look at the survey process and the equipment. After learning how everything worked they had the chance to try doing the geophysics themselves. Before anyone could have a go, however, they had to make sure they were metal-free. The results produced by the equipment we were using (fluxgate gradiometers) can be distorted by any metal objects near their sensors. It’s a challenge to find clothing - and particularly warm clothing – without any metal on it. Take a moment to think about your own wardrobe – how many articles of clothing do you have without any metal on them? (Even small items like zip-pulls count.) Unfortunately some of our volunteers had metal pieces on their boots and their coats. Far from being disappointed at not having the opportunity to try out the equipment, they just seemed thankful for their warm winter clothes.

Chatting about the kit

Throughout the afternoon volunteers arrived on site to have a look at the survey in process, and left the site in search of somewhere warmer. A few hardy people stayed to see the results at the end of the day.

Giving It a try

Once the work on the south side of the boundary was finished the second half of the results could be uploaded onto the computer and put together with the results from the morning. No Iron Age enclosures popped out at us once the results were one the screen… but that doesn’t mean that the results weren’t interesting. We’ve found out some new information about that field boundary, and we have maybe found some other interesting features as well. However, it’s hard to interpret a black and white picture when your laptop is on the back of a truck and there’s snow falling all over the screen. Now that we’re back in the office the results will get a good looking over. I’ll be able to tell you more about what we have learned once that has happened.

Taking a look at the results

If you would like to know the results of our geophysical survey please keep checking this blog. The results will be posted here.

Gradiometer survey with barrowsThe magnetometer is the standard piece of kit that gets used on most of the sites we look at; we use one made by Bartington (one of the pictures on their website shows Ben in a yellow jacket!). It can cover large areas quickly, and can detect most kinds of archaeology from most periods. It works by sensing tiny differences in the Earth's magnetic field, which is much larger than the tiny signals produced by archaeological features, but the instruments we use are able to ignore this background field and are very good at picking up the relatively weak archaeology.

We can only detect features when there is a difference between the magnetic field occurring naturally and the fields caused by archaeology. Imagine a ditch being dug through the natural soil and rock; the ditch slowly fills up as soil from the site slides in and people throw rubbish into it; and finally the ditch becomes completely full. It is this change in the fill of the ditch from its surroundings that causes a difference in the magnetic field, which we see as an anomaly in the survey data.

This is a very different technique to metal detecting, mainly because we can look for archaeological features such as ditches and pits, but also because the only metal the magnetometer detects is iron. These ferrous objects act just like magnets and have 'north' and 'south' ends; they also tend to produce much stronger anomalies than most other things, and show as spikes from positive to negative over a short distance. Sometimes being able to detect iron can be useful, but mostly the ferrous anomalies we find are just bits of tractors and old steel cans! This also means that our clothes and shoes must be completely free of any zips and press-studs; shopping can be challenge...

We display the survey data so that negative readings are white, through shades of grey, to positive readings are black; we choose the values so that we get good contrast throughout the survey data. The magnetic background of the site shows as grey because the values lie around the middle of the display range.

The image shows part of a survey we conducted earlier this year. The two circular anomalies are barrows, or burial mounds; the larger one is about 40m wide, and the smaller one 33m wide. After we processed the GPS data we collected as part of the survey, it became clear that the barrows sit on top of a low ridge that was barely visible in the field.

Lots of positive anomalies appear inside the upper barrow and look like pits, but we can't tell from the geophysical survey alone how they are related. The slightly curved line near the bottom of the lower barrow is probably a ditch or old field boundary, but we can't say if it's from the same period. Faint stripes running at a slight angle from top to bottom are the result of ploughing; the field was under a young wheat crop at the time, but may have been ploughed for centuries. There are also some ferrous anomalies sprinkled throughout the data; these stand out because of their characteristic magnetic response.