80 hectares and counting

Anyone new to this blog or geophysics in archaeology is recommended to read the material on the “Geophysical survey in archaeology” page.

At the end of week 3 of the 2018 season we have now collected over 80 hectares of magnetometry data, that’s about 16 million readings. It would be traditional to cite the area in terms of soccer pitches, but soccer pitches do not have a standard size! It is, give or take a bit of variation, roughly 106 pitches in size.  Figure 1 shows the entire area surveyed with the mag so far.

Figure 1: the total area surveyed with the mag at the end of Week 3.

The mag crew have had to deal with some awkward corners lately (Figure 2).

Figure 2: In the corner. The mag crew negotiate an awkward corner with the tower of St Alban’s Abbey in the distance.

The next two images show the whole of the survey in Mobbs Hole (Figure 3) and a zoomed-in view of the southern area completed in recent days (Figure 4).

Figure 3: the Mobbs Hole survey at the end of week 3.

Figure 4: the southern area of the Mobbs Hole survey.

The first thing that catches one’s eye are the blocks of stripes.  Those are the result of the dreaded “sensor freeze”.  Normally the team catch these in the field, but Friday and Saturday appears to have been bad days for it.  I wish Foerster would sort this problem out, it is such a monumental waste of our time to have to go back and redo those grids (and reprocess the data!).

In broad stroke terms, it is interesting to note that the “texture” of the survey is very different to the east than the west.  Looking at the superficial geology map there are gravel deposits a little way to the south, but the area of the survey has not been mapped.  The difference in texture is very similar to what we have seen elsewhere, e.g., between the “theatre” and macellum” fields, and I would argue that this represents some superficial gravel deposits in this area.  The large high-magnetic features towards the middle of Figure 4, along the edge of the texture change, could be quarries into this gravel?

Just to the west of the lone square of frozen data is a feature which looks like a semicircle on a square.  A building perhaps, or wishful thinking?

Running three teams, as well as the impact of flag-eating sheep, requires the GPS to travel widely (Figure 5). (My feet ache…!)

Figure 5: GPS and sheep (photo © Mike Smith).

We managed to extend the area surveyed by the Earth Resistance meter by another six 20x20m grid squares. Thanks to Ellen, Rhian and Anne for all their hard work (Figure 6).

Figure 6: the res team.

Figures 7 to 9 show the results of the Earth Resistance survey at the end of Sunday.  Remember this is only a day and half’s worth of survey.

Figure 7: the Earth Resistance survey overlain on the GPR survey.

Figure 8: a crude photo-mosaic of the GPR data in the same area as Figure 7.

Figure 9: the mag data in the same area as figures 7 and 8. The yellow square shows the location of the 2018 resistance survey.

There is a lot going on here.  The stone building which shows so clearly on the southern side of the resistance survey and in the GPR survey barely shows in the mag data, although to be fair it is partly obscured by the highly magnetic building. The aqueduct which shows clearly cutting across the mag data (the sinuous long line of high readings), barely shows in the resistance or GPR data.  The curious kink in the aqueduct does seem to have a high resistance feature along the southern edge, probably a wall.  Are we looking at the aqueduct avoiding some earlier feature, or is that some sort of water management feature like a sluice?

Comparison of the corner of the building in the right hand side of the resistance plot with the GPR data shows that each method has picked-up similar features, but some show better in the GPR and some show better in the resistance.

The GPR team faced the alpine challenge (Figure 10).

Figure 10: Nigel earns his Alpine GPR badge (photo © Mike Smith).

They completed 80x80m on Saturday and Sunday.  The land flattens off a little bit now!  Figure 11 shows the time slices from Saturday.

Figure 11: Six time slices from the GPR survey on Day 13.

In the fourth time slice (Figure 11, top-right corner), I have indicated a nice building showing with a red arrow. It lies short-end to Street 11 which is running east-west and is marked in slice five with a blue arrow.  Also in slice five is a curious curving feature marked with a red arrow.  I’ve no idea what that might be.

Figure 12 shows the time slices from day 14.

Figure 12: six time slices from Day 14.

I have started with slice 2 for a change.  This is essentially the ploughsoil.  I have marked in red (Figure 12, top-left hand corner) some curious circular features.  Long term readers of this blog will know that these are mushroom rings!  We’ve not had many mushrooms showing on the surface in the dry weather but after recent rains there are starting to appear.

In slice four I have marked an obvious building (red arrow) and a much more subtle one (purple arrow).  They are, again, lying short-axis to Street 11 (shown in slice five with a purple arrow).  Buildings like this are common for more modest dwellings in the Roman world.  We had similar buildings at Durobrivae along Ermine Street.

Figure 13 shows the GPR data and the mag data. (It is worth clicking on this and looking at it full-size.)

Figure 13: mag data (above) and GPR data (below). The yellow box indicates the location of the days 13 and 14’s GPR data.

Some of the buildings show in the mag data and the GPR data, but some do not show in the mag data at all.  This is not surprising as the flint used for the foundations is not magnetic.

I have spent the greater part of today processing data and writing this blog, so I am going to shut-up now, have a cup of tea and head for bed.  I dislike ending-up on Figure 13, however, and so Figure 14 is an entirely gratuitous photograph of the abbey taken on Sunday from the site.

Figure 14: St Alban’s Abbey.

We will back back on site on Wednesday for the last week of the 2018 season.  Many thanks to everyone who turned out even when faced with Mount Gorhambury and a GPR!



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