Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.
Yesterday was the last day of the 2017 season at Gorhambury. Apologies for the slight delay in posting… we went down the pub for supper! We have completed 54 days of survey at Gorhambury over the three seasons. As well as the 35.7ha of magnetometry we completed last season, we have now completed 14ha of GPR survey. Just pushing the machine along the strings is about 280km. We didn’t manage any usable Earth Resistance survey in 2015, but we have done quite a bit in 2016 and 2017. Last year we had terrible problems with the very hard and dry soil. As a result, many grid squares have been done twice. We have, however, completed a 5.6ha survey at 0.5m intervals. That is 224,000 resistance readings, or sticking the machine into the soil 112,000 times. That doesn’t include three days with the beast which is a further 9,600 survey points resulting in 67,200 readings. Here is the story in pictures.
Fig. 1: crude mosaic of GPR time slices showing the extent of the survey so far.
Fig. 2: the 2016 and 2017 Earth Resistance surveys at Gorhambury.
Figure 1 shows a (very crude) mosaic of time slices for Gorhambury just to show the entire extent of the survey. There is going be a great deal of work reprocessing these to get the best out of them and to get the various blocks to match. I also need a faster computer. I tried out the kriging option last night and it took several hours to process the data, but the images were no doubt sharper.
Figure 2 shows the 2016 and 2017 Earth Resistance surveys. At the moment the two seasons are just images put together in Google Earth. I need to see if I can join the two into one big survey and get the edges to match properly. I also need to see if I can get rid of the line caused by the deluge this season.
Firstly, let us look at the Earth Resistance survey results. On the last day we redid four squares from last year, and then completed five awkward partials around the corner of the enclosure for the theatre. Why did we redo those four? Figure 3 shows last year’s survey with the block marked. The hot dry conditions gave very noisy and unsatisfactory results. I thought it was worth a morning’s effort to get those re-done.
Fig. 3: the 2016 resistance survey showing the duff grids.
Now the improved grids. Note that the slight difference between the two surveys is due to minor differences in how I processed the data. I will produce a more standardised plot.
Fig. 4: detail of the area surveyed at the end of the 2017 season.
Fig. 5: the Earth Resistance survey with the blocks from 2017 high-pass filtered.
There are some interesting things happening at the north-eastern corner of the plots, and into the area to the north we haven’t surveyed. My guess is that the stratigraphy is probably deep and complex in this area. Let us compare this area to the magnetic data (Fig. 6) and the GPR data (Fig. 7).
Fig 6: the magnetic survey with the area of the Earth Resistance survey from days 17 and 18 indicated by the cyan outline.
Fig. 7: the GPR survey with the 2017 Earth Resistance survey area indicated in red.
Fig. 8: the GPR survey with the res data overlain on it.
From all three data sets we can see that there is a lot going on in that bit of the field near the drive and the theatre, but it also appears there has been a good deal of robbing to add confusion to the picture.
How about the GPR team? A month of nice weather with a bit of rain has made the grass green and lush. Lovely for sheep, but a pain to push the GPR through. They completed two 40x40m grids on a hot humid day, excellent progress in the conditions.
Firstly, here are a set of time slices (Fig. 9).
Fig. 9: GPR time slices from the area surveyed on Day 18.
The vast majority of the area surveyed appears to be empty. There are hints of earlier agricultural practice but not much else apart from the top edge where part of a building can be seen. This connects to the area surveyed earlier in the season. I reprocessed the earlier version of that block today using kriging to give a sharper image. Here are the slices together (Fig. 10).
Fig. 10: the GPR survey from day 18, along with a re-processed block from day 12.
We clearly have a nice L-shaped building. I suspect this is Insula XXX, Building 4, Niblett and Thompson Monument No. 461 which is known from aerial photographs from 1976. They only have part of the plan, however, and ours looks quite different (hence my doubts). The mag data shown in Fig. 11 shows some slighter, more ephemeral buildings to the SW along the line of the SW-NE street 25 which can be seen very easily.
Fig. 11: the mag data in the area of the GPR survey.
The SSW–NNE running street 25 which runs along the NW side of the building we have been discussing, shows very clearly indeed but street 10, which is supposed to have run WNW–ESE just to the south of our new building and the “House on the Hill”, does not show at all in either the mag or the GPR data. It was observed in excavations by Frere near the modern road, but not this far west. In terms of the town plan, we seem to have two lines of buildings running SW-NE, one along Street 25, and another along street 23, but a large open area with nothing very much it in apart from a few quite large pits. One can almost see some alignments in those pits. Are we seeing backyard areas divided into blocks?
Although our season at Gorhambury has come to an end, we will be undertaking surveys elsewhere, and probably in Verulamium Park once more. I started this posting with some numbers, so I thought I would end with some as well. This is the 131st posting on the blog. Those postings take-up 583mgb of our 3gb free allowance and include 693 images. There have been 68 comments, but we have been protected from 9,402 pieces of spam! The blog has been viewed 32,150 times by 11,016 separate visitors (in practice, this means that number of IP addresses). Our best month was the first season at Gorhambury in 2015. This August has been down on the previous two (2015: 1.8k, 2016: 1.7k and 2017: 1.4k) but the average number of visitors per day has gone-up over the year and we are likely to reach 9,000+ views by year’s end.
I would like to thank everyone who helped this season once more, both with pushing machines, moving strings, laying-tapes and moving equipment. You are all stars in my eyes, and I think we have created a stunning survey. We all got a bit tired towards the end, especially in the rather hot and humid conditions over the couple of days, including CAGGs loyal follower, Fergus (Fig. 12).
Fig. 12: Fergus sleeps off a busy day on site.
If anyone is interested in joining in with some of CAGG’s activities, drop us an email. We are a friendly bunch, and on-the-job training is given.