12 July 2014

Messing around with GPR

I'm still learning about my GPR system and what it is capable of, so recently I have been trying out a few things to find out what the results are like.

Whilst you wont see much of a cut in clay with GPR, since the geology and clay soil have similar EM properties, I wanted to try out what a cut in chalk looked like, since chalk bedrock is going to have a different response to chalk soil. Firstly I visited the BHAS dig at Ovingdean where they are excavating a medieval manor complex. They had a barn like structure with large square post holes, which showed up nicely. While the normal soil-chalk boundary was fuzzy due to the top layers of the underlying geology breaking down into a soil, the bottom of the post holes were a much cleaner cut with a much clearer change on the radar.

Pleased with this success, and happy that I now have something that will quickly assess chalk sites (unlike magnetometry, which is rubbish on chalk without tertiary geology), I decided to try it out on another chalk site. This time, a neolithic causewayed enclosure. There were sights (and sites) to see on the way. Firstly, climbing the hill itself was illuminating. Below you can see layers of flint within the chalk. To the left and in the middle are two layers of nodular flint, which look broken up. To the right, you can see a more solid looking layer of tabular flint. These layers seem to be diving down into the ground, but as I was climbing the hill, they are actually pretty level.

Flint Layers. Click to Enlarge

Reaching the top of the hill was a Bronze Age barrow, unfortunately looted in antiquity. Again, because there is no height data, it looks like a pit. The layer visible is the underlying chalk. It seems there is not much solid chalk in the mound of the barrow.

Bronze Age Barrow. Click to Enlarge

Finally, I reached the neolithic causewayed enclosure, and was rather disappointed. I couldn't make out much difference in the ditch cuts. It didn't help that flint layers kept getting in the way, pretending to be archaeology and becoming the latest in a long line of 'annoying geology'. I later saw an old picture of a ditch section from Whitehawk Hill, which ASE are digging at the moment, and it looked like the ditch fill was mostly chalk rubble rather than soil, which would explain the lack of difference in EM response and why I couldn't see much.

Another thing I wanted to try out was a bit of Roman road hunting (of course). A lot of the Roman road network has unfortunately been built on, making it difficult examine. Unlike with earth resistance and magnetometry, it is possible to use GPR to examine built-up but flat surfaces, such as a modern road or pavement. I took the opportunity to visit the excavation at Ewell, where Stane Street and associated roadside settlement is being excavated in a small field (church meadow) near the church. The course of the road to the south-west is somewhat unclear, as there is a turn in the road somewhere in the area. With the help of the local archaeologists, we found the Roman road crossing the more modern Church Street. You can see the curve of the agger in the centre of the image below.

Stane Street. Click to Enlarge

I've still got a lot more to learn about GPR. I hope to learn a lot more when I do a week of GPR surveying at Bridge Farm shortly. Watch this space.