Make it so

As promised last week, on Tuesday and Wednesday Simon and I were back up on New Laund Hill to do some geophysical survey. After a very wet weekend the rain miraculously stopped on Monday night and we got through two dry days.

This work is part of Simon’s BSc dissertation: which is going to compare the results of three different non-destructive archaeological techniques carried out in the same area. For this reason we spent the first half of Tuesday carefully re-surveying the positions of the measurements Olaf, Jess and Mike took in the summer of 2011.

This picture shows that survey underway. They covered 10800 m2 on the top of the hill using a Bartington fluxgate gradiometer. This is the device like a small rugby goalpost Mike is wearing in the photo. Many people have tried to explain to me over the years how and why this works but at the end of all the explanations I am left with the vague knowledge that there is magic pixie dust in the tubes at the end of the frame. This detects buried sub-surface archaeology by measuring tiny changes in the earth’s magnetic field: midichlorians and/or dilithium crystals are probably involved somehow.

From an archaeological point of view the important part of the survey is that each reading that the data logger on the machine takes can be related to a precise spot on the surface of the site. This means that whatever it is that the gradiometer has detected can be plotted onto a map of the site (and you can find it if you want to dig it up). To achieve this all our geophysical surveys have been based on 30 x 30 m squares. The squares are marked out on the ground using tapes and garden canes. Each square is then divided with guidelines which have a plastic marker every metre. The surveyor can then walk along the guideline with the machine automatically recording the data as they pass the markers. Each line  and each grid is gradually built up from the individual readings to be stored on the data-logger.

After you have downloaded the data and processed it – and Olaf has cleverly fitted it into its right place on the overall map of the site – you get something like this. These results from the 2011 survey are what convinced us there was an enclosure on the hill which needed investigating. They also helped us to located the trench D on the internal feature. We would have had no idea where to dig inside the enclosure without this survey data.

The reason we are back up on the hill doing more survey is that there is more than one kind of geophysical survey technique and they all give you slightly different results. Although we got good results from the gradiometer last year, no bank and ditch showed on the southern and eastern side of the hill. I think there are two possible explanations for this. Either there never was a bank and ditch on that side of the hill, or it is there but it is so covered by the hillwash layer we found in the excavations that the gradiometer can’t see it. The plan is to try several different techniques over the same area to see if any of them, or a combination of them, give us a better picture

What we have been doing this week is earth resistance survey using a Geoscan RM15 resistivity meter. This works in a different way by measuring the electrical resistance of the soil. There are two metal probes stuck in the ground outside the survey grid which act as a control. The surveyor has a thing like a zimmer frame with a data logger on top and two spikes at the other end. Every time you want to take a reading you touch the spikes on the ground, the meter tries to pass a current between the spikes and compares the electrical resistance at that point with the control. When you measure the electrical resistance like this what you are mostly measuring is how wet the soil is; wet soil conducts much better than dry. This can identify buried archaeology because things like buried ditches hold more water and lower the resistance of the soil above. Rocky features like stone banks or walls tend to increase the drainage of the soil and therefore increase the resistance.

Here you can see Simon using the resistivity meter on a grid just outside the enclosure. As well as the technical differences between the two techniques and machines there are practical and what you might call stylistic differences too. Gradiometer survey needs the surveyor to be magnetically ‘clean’ – no metal anywhere about your person at all – so no boot eyelets, zips or other fasteners, no watches or jewellery.  It’s fussy to set up but once you are going it can be very quick to gather the data. The resistivity meter we used here is much quicker to set up but, because you have to physically touch the ground with it to log each point, it is a lot slower to gather the data. This is why we did the gradiometer survey first on this project and generally why gradiometer survey is such a powerful tool; it can cover a big area very quickly and give you a good sense of the shape of the whole site.

Olaf, Mike and Jess did the gradiometer survey in 2011 in two days, so far we have done slightly less than half of that area with the resistivity meter in the same time. However, there are downsides to the gradiometer too. The machine itself costs more than my car – more than any archaeologist’s car I should imagine – and if it stops working on you in the field then your options begin and end with sending it back to Bartington in Oxford. If the resistivity meter goes wrong in the field then a trip to Maplins and 20 minutes with a soldering iron will usually sort it out. If the different machines were rocket science the Bartington would be new style Star Trek (probably Voyager) while the Geoscan would be Patrick Troughton era Dr Who.

And now the results. So far, and we have only completed five grids, we can see more geology than archaeology in the resistivity results. The ditch is visible but it is slightly swamped by the very high resistance areas where the limestone is very close to the surface. We are also a bit concerned that the ground is too wet for fine detail to show up inside the enclosure. I grabbed this plot from Simon before he had chance to do any processing which might cancel out some of these effects. Hopefully, once we have covered a wider area and done some processing then a bit more detail may be forthcoming.

Next week Vicki and Michelle are taking the resistivity meter down to Wales to survey around some dolmens for the ‘Building the Great Dolmens’ project so we are going to concentrate on detailed topographic survey of the hilltop. This will mean we have to learn how to use the new robotic total station, which should keep us busy.

Wildlife of the day – we saw a hare on the way back to the car on Tuesday

Rick

Advertisements
3 comments

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: