29 August 2013

Latest Results: Oaklands Park

I did buy the Groundvue 3A ground penetrating radar cart that I discussed in my earlier post, and have been giving it a go at Oaklands Park, where I did a large magnetometer survey with IHRG a while back. There were a few areas where we thought that there might be a possibility of buildings. The results have been very positive. The two survey areas we covered, outlined in red in the image below, are 40x40m and overlap as we found that the building was on the edge of the first grid we did. The first area, to the north-east, was recorded down to 70ns. The second area, to the south-west, was recorded down to 50ns.

GPR survey areas. Click to enlarge.

There are interesting features at various levels, and to get a full idea of them all, you need to look at different depths. Below are features from 10ns and 13ns followed by an interpretation. To get a better picture of what is going on, you can look at the whole lot on youtube. The video for the first survey is here and for the second survey is here.

Features at 10ns. Click to enlarge.

Features at 13ns. Click to enlarge.

Interpretation over magnetometry. Click to enlarge.

You can see in the above image the location of features on the GPR surveys overlaid on the magnetometry results. The most important feature is the building in dark blue. This shows up fully on the magnetometry, but the west wall and a second north wall are missing on the GPR results. This may be due to robbing. The green feature seem to be metalling on a trackway, most likely iron slag. It doesn't extend all of the way along the track, but seems to have been placed outside of the building, which may indicate that this section of the track was busy and in need of repair. The two light blue features are very strong both magnetically and very dense on the GPR results. They may be where the iron was processed. The yellow lines are boundary features, which mark out an enclosure around the building and a further enclosure to the north-east, the other side of the track. The orange areas are large pits. On the GPR results, these can be seen to reduce in size as they go deeper. Light purple areas seem to be dumps of material with no pit. Finally, the small dark purple feature to the north-east seems to be a well. It is circular, with a small L shaped feature attached at higher levels, which disappears further down. While all other features disappear by 30ns, the well keeps going all the way down to the 70ns recording limit.

These GPR results are great. I'm looking forward to using it on a lot more sites in the future.

19 August 2013

The FM256 Problem

A while back, I attempted to write an import for the Geoscan FM256 in Snuffler based on the documentation alone, as I didn't have a machine to test with. Unfortunately, I had a number of my users reporting that the import didn't work. Despite extensive testing with a number of them, for which I am very grateful for their time, I couldn't resolve the problem. An opportunity arose recently to borrow a machine from the Surrey Archaeological Society (many thanks to them), so I was able to do some testing with a physical machine. I tried the download myself and all appeared fine. I couldn't work out what the problem was. I then realised that I do something slightly different to most people. I use a PCI serial port card rather than a USB to serial converter. I tried a couple of these converters and the results came out as garbage. It wasn't just Snuffler either, it looked like junk in Hyperterminal. What I can't understand is how people are using Geoplot and getting it to work with this data. It is a mystery which I have little inclination to solve. The upshot of all this is that if you want to download from an FM256, get rid of the USB to serial converter and get a PCI serial card. That would be no use to you if you have a laptop of course. You would either need to buy a tower computer, or better yet, a Bartington.

14 August 2013

Digging up the Geophysics at Barcombe

Despite what I said in my last post, I always enjoy it when somebody else digs up a site based on my geophysics. I get to learn so much by comparing the geophys results to what is excavated, yet someone else has to deal with running the thing and paying for it all. This happened at Barcombe this year, on the site of the fortified Roman settlement I had previously surveyed. I even took a week off work to have a dig for a change, which is not something the local archaeologists are used to seeing.

The Culver Project was of course running the dig, with the help of AOC Archaeology in a giant community dig funded by the Heritage Lottery Fund. The excavation was carried out in a pasture at the southern end of the site. The Culver guys also did an earth resistance survey to the west of the excavations, which shows the trackway and the old river bank nicely. Here is an annotated zoomed in image (click to enlarge) with descriptions of the various features described below.

A) This is a very strong feature on the survey, and it was expected to be industrial. The feature didn't disappoint, as it turned out to be a large circular pit with strong in-situ burning. There was no pottery wasters, but there was some tile wasters, suggesting that it might have been used as a tile kiln. This is not certain though, as Roman tile kilns are usually rectangular.

B) This curving ditch feature surrounding feature A contained a lot of tile wasters, reinforcing the idea that it was a tile kiln.

C) This small feature on the geophysics turned out to be very interesting indeed. It was a rectangular pit lined with tiles. A large blob of concrete was in the bottom, seemingly put in wet, as it had stuck to some of the tiles at the bottom. Nobody knows what this feature was used for.

D) This is one of the many tracks going through the site. The side ditches were quite substantial, being nearly as large as you would expect on a normal Roman road. The surface, where it showed, was river gravel, and had cart ruts running through it. Being a Roman road nerd, this feature is the one that got me most excited.

E) Half of a large pit was excavated here, containing occupation debris. These sort of features are common across the site, especially in the centre of the settlement, where there hardly seems to be a spot free of them.

F) This trench was put in to examine the relationship between the double ditched enclosure and the trackway as they crossed, and did not disappoint. The later enclosure clearly cut the earlier trackway. An almost complete quern stone was found towards the eastern end.

G) This trench was put in to see how the archaeology was affected by ploughing in the arable field to the north. Fortunately, the archaeology seems to be mostly below plough depth. The western ditch of the trackway was found as expected.

I) The eastern ditch of this trackway could not be found. Instead, the two features shown on the geophysics turned out to be two large pits. The southern one was square with vertical sides.

J) The enclosure was examined in greater detail here, by means of a hole cut through the hedge. The layers were not as clear as at F, with three ditches seemingly crossing the trench. The ditches were very deep, heading well down into the water table.

K) The ditch crossing here was again found to be cut by the main enclosure ditch. An intact (before being clipped by the bucket) cremation burial was found here and block lifted to be excavated elsewhere.

If you want to know more details about what happened, you can visit the excavation diary.