leighandgill

Archaeology in East Oxford

Geophysics Week

A while ago (apologies for the gap in posts, I’ve been getting my head around a new GIS package, ESRI ArcGIS, and working on the Cowley Enclosure map – more about that in a later blog) Olaf organised a geophysics week. We knew it was in the pipeline, but he has had to do a lot of work behind the scenes to get all the necessary permissions for access; some sites were public recreations grounds and one was a school, so a lot of negotiation was in order.

I missed the first few days as my car had given up the ghost (Oh Woe, Oh decreased bank account)

The first site to be surveyed

Olaf started off by surveying the public recreation ground next to the sports field at Rose Hill Primary School that I talked about in an earlier blog (09 May – Geophysics Weekend) – this was done while I was busy. I met up with everyone after lunch on Wednesday at Donnington Sports Ground.

Donnington Sports Ground (to the west) & Larkrise sports ground (to the East of Iffley road

I found a couple of things had changed since the Geophysics Weekend, both for the better.

First Olaf had sorted out the GPS; it turns out that you have to explicitly log on to the OS network. Survey grade GPS works by being basically the same as your off-the-shelf satnav, but it has a built-in mobile which dials up a network of stations maintained by the Ordnance Survey which give the unit a whole load of extra data to refine the information coming down from the satellites. Why anyone would spend about £12,000 and then not want to log on, which then drops the accuracy back to a £150 satnav, is beyond me – but that’s the way it goes. This was why we were getting such a lot of problems with the GPS on the previous weekend.

The second change was that we have upgraded the gradiometer to the two tube version.

Two tube gradiometer.

This has two advantages – first, the obvious one, is that you collect the information twice as fast, or to put it another way, you walk half the distance to collect the same amount of info. The other is that being quite a bit heavier, it comes with a harness, which makes it a lot easier to use.

So while the gradiometers were settling down (the first thing that happens when we arrive on site is that the gradiometers are assembled and left to come up to ambient temperature) we went and marked up the grid of squares to be surveyed – with the GPS now working properly this is pretty quick; we marked the vertices of the squares with an aerosol as the sports ground is a public space, we didn’t want bamboo sticks sprinkled around (Health & Safety).

We had two gradiometers with us, the upgraded Project one, and another one which we had had lent to us by the Continuing Education department at Rewley House. This meant we were able to run two teams (it takes three people in a team – one to do the actual survey and two others to move the trapeze) so we could really get going! Well, in theory, but as this was a training exercise, not all of us were zipping along at the same speed as Olaf, even when Olaf, David Griffiths and Jane were doing the tricky bits – it’s bad enough doing the grid squares, but doing the odd, non-square bits round the edge need a bit more confidence. You have to have the basics down pat before starting on the bells-and-whistles.

We ran into another problem then, also to do with it being a public open space – a load of guys turned up to use the football pitch. We will have to come back really early to finish off the remaining grid points, though I’m not sure what sort of results we are going to get around the goal posts as the gradiometer is thrown out by large lumps of iron (Olaf said it could have an effect as much as 10m away).

Next day we went off to the Larkrise Sports Ground – this was a different kettle of fish as it was part of (confusingly) Gregory the Great school so we had to sign in and get issued with ID tags – in fact, as we thought we might have some of the sixth-form students coming along, we had to have one person with a CRB (Criminal Records Bureau) check present; though as it turned out, they had something else to do so never turned up.

Olaf left Tim, Steve and I to do the GPS part of the survey – setting up the grid squares – so we each did about 6 grid points each. You really start to get the hang of it with a bit of repetition! I had to shoot off at that point (I was booked in on the next Animal Bones workshop) but Jane arrived just as I left to take over and make up the numbers, so they were going to have no problems with finishing off.

Slightly depressingly, one of Olaf’s next-door neighbours turned up and was telling us that he had planted the hedge and assorted trees around the edge of the field – he said the digging was awful;it looked like the level had been made up with some modern “soil”, so we weren’t at all confident of finding anything. This was a bit of a blow as we knew that this was part of the medieval field system, and I had found a couple of marker stones shown on the Cowley Enclosure map, but with a load of modern stuff laid on top of the older surface, any archaeology was most probably going to be masked.

Saturday turned out to be the last day; Olaf had planned on doing Sunday as well, but it looked like we were going to finish off so he felt like cancelling Sunday and having a bit of a (well deserved) lie-in. I managed to set up the GPS and check if we could set up another grid point – we couldn’t; trees caused me to lose the signal about 6 metres away from the grid point – but it was good experience doing it from scratch on my own.

I did a couple of grids and then I had to do one of these partial grids. Instead of walking the full 30 metres, one stops short at, say, 28 metres, then starts again at 28 metres to go. Sounds simple, but you do have to be fairly confident of walking at a steady pace to do the return walk and get to the end marker at the correct time.

I’ve been chatting to Olaf about getting the results to show here – we’re having problems emailing it (we don’t know which end is limiting the file size) – so I’m going to call it a day now and do another blog when we have managed get the files across, and when I’ve worked out how to display them!

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