Who on Earth goes out on midwinter’s day to do fieldwork? Well, we do! Mike, Ruth, Peter and myself met up with Rex and Nick from FRAG to extend the magnetometry and Earth Resistance surveys at Durobrivae. It was a foggy morning but at least the temperature was well above freezing and, in fact, it was quite pleasant working conditions… for midwinter!
Unfortunately, the mag developed some problems and although we managed two grid squares before it died completely, I am unable to download them at the moment. Fingers crossed I can rescue the data in due course.
The Earth Resistance survey was more successful. Given the short day I was only aiming for four grids to extend the previous survey to see if the outer circular feature did actually join-up on the eastern side. Figure 2 shows the results.
As can be seen, the “outer circle” does indeed form a continuous curved feature on the eastern side. There also seems to be two features running off this curved edge eastwards. I have marked these in Figure 3.
We have clipped the corner of a building in the bottom right hand corner of the survey.
How do these match up with the mag data? Figure 4 shows it for the same area.
The northern linear feature picked-out in Figure 3 shows clearly in Figure 4 as being strongly magnetic (it ranges from -10 to +15nT). It is probably a ditch full of rich organics and/or burnt material. The southern linear feature is harder to see in the mag data so in Figure 5 the lines from Figure 3 have been superimposed on the image.
The northern line in Figure 5 fits nicely on the magnetic feature as we expected. The southern line joins-up three magnetic anomalies ending up with a big one which is about 10m by 5m. It looks like parts of the ditch were filled in with non-magnetic material, and parts with highly magnetic material suggesting several phases of infilling.
It clearly worth continuing to expand the Earth Resistance survey yet further. It would be good to see what is happening on the western side, but also to see if we can pick-up the plan of the building to the south where we have just got a single corner.
For those of you interested in this sort of thing… Since getting the new RM85 I have been using a “pole-pole” configuration for the probes. That means having the remote probes 20m or more away from the edge of the survey area, and separated by at least 10m. This contrasts with the more usual set-up (called “twin probe”) where the remote probes are often only half a meter apart. The reason for doing this is that is removes, usually, the need for grid-matching, i.e., getting an image where one cannot see the joins between grid squares. If the survey is done over only a few days, this works well. If, however, the two blocks of survey are separated by several weeks of winter weather, the differences can still be seen (Fig. 6). This requires a bit of extra processing, but still not as bad as having to get the grids to match every time one moves the remote probes.
The CAGG team are a sociable group, and we get to sit and eat lunch in entertaining places (Fig. 7). If you would like to come and join us during our surveys in 2018, please get in touch. We provide training, and commitment is minimal. We simply add everyone to a mailing list, and when there is a survey coming-up, we ask who can make it. Often, we do not want a team bigger than seven people apart from the big survey season at Verulamium in August, and maybe one or two other sites where we would like to run more than one machine.
I hope you all have a very Merry Christmas, and wish us all a successful year of surveys in 2018.