On the Radar

We haven’t done any excavation on the project this summer but we have been working hard on making sense of everything we have found over the last five years. In particular, we have been trying to understand how the Middle Iron Age timber features and bank and ditch of the New Laund Enclosure fit together.

Gradiometer details 2011

Alex is working on this for his Masters dissertation. He is re-examining the existing geophysics, like this gradiometer plot of the features from 2011, looking at the ceramics and metalworking slag from the topsoil and re-evaluating the excavations from 2012 and 2013.

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However, he has also been doing some fieldwork. We now have a(nother) new toy. This is the Ground Penetrating Rader set in action on the enclosure interior last Friday. (obviously the only damp day of last week). It looks like a lawnmower† but inside the bit that looks as if it should be the grass-collecting box are three different frequency rader antennae. GPR works by detecting these radar waves as they bounce off any sub-surface features.

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This is the driver’s eye view, taken as I paused during one of my traverses. The tablet screen shows three vertical slices through the ground directly beneath the machine, one for each frequency of radar in use. We recorded 130 of these traverses on Friday, and therefore 390 scans in total, covering a 28 by 18 metre area just to the west of trench D from 2012 and trench H from 2013.

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This is only the third time we have used the GPR but I’ve already learnt that, laborious although the data collection feels while you are doing it, the real work will start next week when we download the data and begin the computer processing. The vertical slices you see on the screen as you are recording the data are extremely confusing to look at. They show the time the radar waves take to travel through the ground, rather than any scale measurement of what is beneath you. They also show multiple reflections from any target. To get anything which will be comparable to the existing ‘plan view’ geophysical results Alex is going to have to process all these effects and reflections out and then stack all the vertical traverses up together in their right geographical location next to each other.

Once this is done then it will be possible to virtually ‘slice’ through these at right angles and produce many plan views at different depths. Some of these will hopefully show the post-holes and foundation slots of the timber features. Of course the great strength of the GPR is that, unlike other geophysical techniques, you get this three dimensional view. Hopefully, not only will we be able to see if there are features in the area we surveyed but we should also be able to tell whether they are at the right depth to be part of the same structures as the features we dug in this area in 2012 and 2013.

Rick

†On Friday I kept thinking that actual lawnmower blades on the front of the antennae set would be a good after-market accessory. We had to push through some very stubborn tussocks of reeds to do the survey.

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