Tag Archives: Geoscan RM85

Just too claggy

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

A group of us headed out to Little Hadham today with the aim of extending both the Earth Resistance and magnetometry surveys.   By the time Nigel had pushed the mag across the field, the wheels had diameters several inches larger than they should.  Jim tried a line or two of data collection, but the odometer was over-running by two meters and the wheel needed trowelling clean every transect.  We decided, therefore, to abandon the mag survey, and concentrate on enlarging the area of the Earth Resistance survey.  We managed another six 20x20m squares giving us a total of 100m by 80m, some 32,000 readings in total.

Figure 1, below, shows the initial results.  As before, the data is dominated by striping caused by the cultivation pattern.  A 2D fast fourier transform (as implemented in TerraSurveyor), quickly removed these stripes.

Fig. 1: the Earth Resistance data overlain on the mag data.

In Figure 2 I have applied the filter to remove the striping.  To the right I have put the mag data for comparison.

Fig. 2: The resistance data after processing with the 2DFFT. The mag data of the same area is shown to the right.

Unusually, most of the features show in the res and mag data.  The res data has nicely picked-up many of the linear features more normally only clearly seen in mag data.  In Figure 3 I have labelled a few points.

Fig. 3: the res and mag data with labels.

Ditch features A and B show nicely in both the res and mag data.  What is clear from the res data, however, is that the ditch continues between the two and they are one distinct linear feature.  If one draws a straight line along A and B, it lines up perfectly with the linear feature C we found across the road in 2014.   Linear feature D shows equally well in both data sets.  At E, something complex is happening.  In the mag data it looks almost as if A is turning and runs alongside E, whereas in the res data is looks more like AB cuts across the linear to the west of E.  The parallel lines to the west of E show quite well in both, and are probably some form of trackway.

Many thanks to Jim (CVAHS; both for surveying and transporting the equipment and myself), Nigel (NHAS), Caroline, Peter, Amanda and Mark (BAG).  Hopefully we can get to do some more when the field is less claggy.

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Tilting at windmills?

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

Firstly, apologies to anyone waiting on the Durobrivae report.  I am trying to resolve a small problem with the GPR data and will post something soon.  We did get some good results, if not quite so spectacular as the temple we found last year.  The “tumulus” is proving very intriguing.

Back in April 2014 we surveyed part of a site in Little Hadham.  We had always intended to go back, but never quite managed to get our act together.  Last weekend we finally managed to plan another three days at the site, working in the field to the west of the road.  We were mainly intending to undertake magnetometry (Fig. 1), but as we had enough people we also did some Earth Resistance survey (Fig. 2).

Fig. 1: Jim West (CVAHS) moves the string for Ruth Halliwell (WAS).

Fig. 2: Peter Alley (WAS) ably uses the Earth Resistance meter aided by Caroline and Peter Baigent (Braughing Archaeological Group).

The three days were about as different as you could get.  The first day was quite nice, the second day wet, drizzly and foul (we had to keep trowelling the wheels of the mag clear of mud) and the last day was absolutely glorious.

The results from the magnetometry survey were excellent (Fig. 3).  The features are relatively subtle, however.  The image below is clipped to +/- 1.5nT.  In others words, all readings above 1.5nT are plotted solid black, and all readings below -1.5nT are plotted white.  At Gorhambury, I clip the images at +/- 5nT.  The pottery kilns are Verulamium have very strong values of -15 to +150nT.

Fig. 3: the magnetometry results from 2014 (east of the road) and 2017 (west of the road).

As can be seen from Fig. 3 there are lots of mainly linear features, some very straight, and some quite sinuous.  We are clearly dealing with a multi-period site.  The faint striping running west-nor-west to east-sou-east are a result of the harrowing of the field, made more visible by the extreme clipping of the image.  To make the discussion easier, I have labelled up the figure.

Fig. 4: the mag results.

One of the first things to note is how different all the linear features are.  The one indicated by blue arrows is quite straight and for some of its length, at least, very magnetic (-6nT to +11nT).  The one labelled with red arrows is, however, very sinuous and only faintly more magnetic than the background (about +/- 1nT).  That ditch seems to continue as indicated by the green arrows, which in places seems to break up into a series of linked “blobs”, either patches of more magnetic material dumped in the ditch, or perhaps pits within the line of the ditch.

The strength of the magnetic values is dependent on two things: firstly, the source of the magnetism.  Soils may be strongly magnetically enhanced by burning or intense occupation, for example, or may only be weakly magnetically enhanced if they contain just slightly more rotted organics than the background.  Secondly, size can also be a factor.  A large feature can contain more magnetically enhanced soil than a very small, shallow feature.

Features C and D are very straight, and are unlikely to be pre-Roman but they could be Roman or later.  I wonder if E could be a drove-way leading up from the valley to the west?  We need to do some work in the archives and see how much the field systems in this area have changed.

We have two circular features: A and B.  My initial quick thought is that these are both round barrows.  The majority of barrows are Bronze Age, but we do get barrows in the Roman and Saxon periods too.  Their location on a ridge with excellent views would support their interpretation as barrows.  The fact that the two features looked so different worried me, and then I remembered a lecture I used to give on aerial photography.  Could this be a windmill?  A quick comparison with an image published by Wilson (2000, Fig. 58) strongly suggested this interpretation.  Of course windmills also want to be up high!  We’ll come back to the putative windmill below.

Fig. 5: Members of BAG running the Earth Resistance meter.

We initially decided to use the Earth Resistance meter over a patch of the field where the farmer had noted it was difficult to plough, and where there were a large number of flints on the surface.  Over the three days we completed 14 20x20m grids.

Fig. 6: The Earth Resistance results overlain on the mag data.

As can be seen from Figure 6, we have detected the ring ditches quite clearly, and some of the linear features.  The stripes are plough scars.  This makes it quite difficult to see what is happening in many places.  TerraSurveyor can apply a Two-dimensional Fast Fourier Transform (2D FFT) to the data to try and removing striping such as this.  Using a 2D FFT doesn’t always help, but in this case the results were excellent (Fig. 7).

Fig. 7: The Earth Resistance results after applying a 2D FFT.

To aid discussion I have labelled-up the plot as before (Fig. 8).

Fig. 8: Resistance results, labelled.

The two circular features (A and B) show clearly in the res data.  Feature B appears to have taken a “bite” out of the high resistance area F.  This area is where the flints were on the surface, and rather than being a building, it seems more likely we are dealing with a pocket of flints in the periglacial drift geology. The ditch which runs ENE–WSW just above the letter F shows very clearly where it has cut through these flints.  The ditch shown with green arrows in both Figs, 4 and 8 is quite clear.  What is intriguing is that ditch C, which appears only faintly to the west of the road in Fig. 4, and then peters out, clearly continues as shown by the pink arrows in Fig. 8.  We must never forget that ditches show well in mag data because they act as “traps” for more organic, magnetically enhanced soils.  If the fill is not magnetically enhanced, as may happen when one moves away from occupation sites, we may not be able to detect them.

The res survey is particularly pleasing from a teaching point of view.  Not only has the 2D FFT worked very well, but this is the exception to my usual statement that “res is less good at picking-up ditches and pits…”.

Fig. 9: Kai makes sure I am putting the grid in correctly.

The Windmill

When I first emailed the people helping on the survey and said I thought feature B was a windmill, Peter Alley immediately pointed out that the lane which runs through the site is “Millfield Lane.”  Well done Peter!  This is what Wilson (2000, p. 108) says:

Medieval post mills stood on crosstrees whose foundation-trenches formed a cross measuring about 10m wide overall.  The crosstrees were usually embedded in, or set in the top of, a low mound surrounded by a ditch.  The higher the mound, the broader its ditch, but the less likely that the timbers have penetrated the subsoil.  Crop-marks of windmill-mounds thus fall into two groups: those with proportionally broad ditches that usually display no central cross… and those with modest ditches (2 – 3m wide) and a cross within.  The ditch is ordinarily 25m in diameter; it may have two even three entrances.

Our feature is almost exactly 25m in diameter but the longest part of the surviving crosstree foundation is slightly less at about 8.5. The ditch is between 3 and 4m wide.  The “ditch”, however, does not look like a classic ditch feature in the mag data, and it may have other origins.

I checked the book by Howes (2016) but he only discusses the smock mill known from elsewhere in the parish which was burnt down in 1981 (pp. 132–4).

Jim West wrote:

I have been looking at windmill design to try to identify what would create the large circle (dia about 20m)  in the mag results.  Thoughts so far:

The windmill was probably a post mill, i.e., the whole of the upper structure rotated on a single post.  The post was often supported on a cruciform base rather than set in a hole. This design was in use for several hundred years until c.19 when the more powerful smock mills were introduced.

An example of a trestle base (this one is on brickwork which seems to be a modern improvement)
Post and trestle 18th May 1979from: http://www.norfolkmills.co.uk/Windmills/tottenhill-postmill.html

with the upper structure it looks like this (different mill)

from: https://en.wikipedia.org/wiki/Post_mill

These mills had to rotated to face the wind which was done by pushing on the long beam (threaded through the steps) on right in the image above.  This beam (or tailpole in miller speak) had to be long enough to get the leverage to move 15-35 tons of mill.  Most of the larger mills had a wheel on the end of the tailpole, usually with an iron rim.

Some later mills used wind power to rotate the mill.  The example below gives some idea of the length of the tailpole.

Fantail trolley June 1936
from: http://www.norfolkmills.co.uk/Windmills/tottenhill-postmill.html

My initial conclusion is that the large circle in the results is a record of the arc of the tailpole;  there may have been a surface laid to reduce rolling resistance because the width of the “path” is too great for a wheel rut and with a prevailing SW wind in the UK more wear would be expected in the NE sector.

The results also show what could be one of the beams of the trestle; the beam at right angles to it is less clear but I think supports the ides of a post mill on a trestle.

Having overlaid the res and mag data, the feature suggested by the res does seem a little larger than that suggested by the mag, although they do overlie to some extent.  The outer ring, therefore, may be a complex mix of the outer ditch (likely to be more irregular) and the sweep of the tail pole (which would be a perfect circle).

Ruth examined some of the historical evidence:

I have been looking at the old maps I have access to, to see if I could work out when the lane was named ‘Millfield Lane’, the house ‘Millfield Cottage’ and see if I could find any mills in the area. Working back from the modern OS maps, the house only became identified on the map as ‘Millfield Cottage’ between 1960–80. It was previously ‘Millfield Houses’. [NB: you can browse through old maps on the National Library of Scotland website]. I looked back through 20th century maps and back to 1870’s and the name of the lane was consistently Millfield Lane, but there was never a mention of a windmill at that site. On the 1880 Shire view map, there is a windmill (corn) just to the NW of Little Hadham.  It is beside Mill Common. The Bryant Map of 1822 has a windmill drawn at, what is probably, the same place as the 1880 map – but none in our field.

Dury and Andrews, 1766 [NB: available to view online here]  does record windmills as there is one at Hadham Lordship, but none in the area of Hadham Ford, Berry Green, Hadham on Ash and Green Street. Rowe and Williamson (2013, p. 261) mention that there was a mill in Little Hadham built 1786–7, which is likely to be the one in the Bryant Map and the 1880 OS map. The mill must post-date the map by Dury and Andrews (1766) [NB: available to view online here]. The construction of 1786–7 must have been a ‘new build’.

It may also be the one I found mentioned in Wikipedia entry – Hertfordshire Windmills, Little Hadham, which gets listing from Moore (1999). Only two are mentioned: the first a smock mill that was built in 1786 and burned down in 1981 that matches the location and description of the one at Mill Common mentioned by Rowe and Williamson.

The second entry is that listed by Moore (1999) and dates to before 1700.   Moore (p. 77) notes that ‘… just north of Bury Green there is a house today, which O.S. maps name Millfield Houses. Fields on both sides of the road in this position are names ‘Mill feelde’ on a map dated 1588 but it is possible that the field could have been names from a horse mill situated nearby in medieval times. There is no doubt that there was a windmill in medieval times and possibly two sites. The only miller’s name found was in the 1587 Muster Roll Richard Howell – myller.’  There are a couple of references to a 13th century mill, but Moore was unable to show that these refer to this site.

The Domesday Book does not list any sort of mill within Little Hadham.

There is clearly some more historical work to follow-up on.  It would be good to see the 1588 map mentioned by Moore which is at HALS, and I’d like to see the references in Holt’s 1988 book cited by Moore.

Hopefully, it won’t take us three-and-a-half years to return to this fascinating site.

Fig. 10: At the end of day 3.

References

Historic England (2011). Mills. Available online.

Holt, Richard (1988). The Mills of Medieval England.  Blackwell.

Howes, Hugh (2016). Wind, Water and Steam.  The story of Hertfordshire’s mills. Hatfield: University of Hertfordshire Press.

Moore, Cyril (1999). Hertfordshire Windmills and Windmillers. Bishops Stortford: Windsup Publishing.

Rowe, Anne and Tom Williamson (2013). Hertfordshire: a landscape history. Hatfield: University of Hertfordshire Press.

Wilson, D. R. (2000). Air photo Interpretation for Archaeologists, second edition.  Stroud: Tempus.

 

Day 54

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.

Buildings galore

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

Before looking at today’s results, I thought everyone might be interested in seeing the reaction to our survey at Verulamium at the Near Surface Geophysics Group conference last December. Not being someone who twits, I hadn’t realised that a webpage of postings from the conference had been put together at https://storify.com/girlwithtrowel/tweets-from-the-nsgg-recent-work-in-archaeological You have to scroll down a long way, but look out for the comment by Magnitude Surveys.

Today saw the GPR crew tackle an awkward block that has the hedgerow half way across it.  Here are nine time slices from it.

Fig. 1: Nine time slices from the survey on day 17.

There are some very clear buildings.  Look, for example, at slice 7 (right hand column, middle slice).  In proof of the only universal law, look at the first “sawtooth” on the south side of slice 8 (bottom left hand corner).  There is a lovely little apse just peeping out into the plot.  Typical… the building is under the hedge.

Let us see where this block fits in the overall GPR survey (Fig. 2).

Fig. 2: all areas surveyed using GPR at Gorhambury. Today’s block is in colour.

The absolute area we have now surveyed is quite impressive thanks to the efforts of members of CAGG.  Well done all.  Let us now have a look at a couple of time slices from today.

Fig. 3: GPR time slice 4 from the day 17 survey,

The fourth time slice (Fig. 3; roughly 0.4 to 0.5cm below surface), mainly shows the road running diagonally SW to NE.  The cross-roads just to the north of today’s block is a very busy locality with lots of buildings clustered around it. The block just to the south of today’s survey also has some pretty substantial buildings.

Fig. 4: time slice 8 from the GPR survey on day 17.

Time slice 8 (Fig. 4, roughly 0.8 to 1.0m below the surface) shows some of the buildings alongside the road to the south of the hedge line.  The road is squeezed in between the building found earlier in the block to the south and the new rooms found today.  We also have a little more of the building to the west.  The two lonely walls in the southern block would seem to be related to that building too.  All-in-all, some very nice results.

The Earth Resistance team of Ellen, Pauline and Graham headed north to tidy-up the top-edge of last year’s survey.  They managed a surperb eight 20x20m grid squares.  Here are the results.

 

Fig. 5: Earth Resistance data from day 17.  The pink line marks the 2016 survey.  The lone grid square on the northern edge was re-surveyed.

Fig. 6: the Earth Resistance data from day 17, high pass filtered.

As can be seen, especially from Fig. 5, we have picked-up some more details of the nice large building in the middle of the plot, as well as other buildings such as the small one at the western end of the strip of grid squares.  At the eastern end we have a large square high resistance feature.  A surviving floor, perhaps?

Fig. 7 shows the mag data from this area.

Fig. 7: the mag data from the same area as Figs. 5 and 6.

Not much sign of the small building to the west, or the “floor” to the east in the mag data, although the “floor” seems to be associated with quite a few walls.

The “sinuous ditch” cuts across the line of today’s plot (seen as the broad dark linear feature entering Fig. 7 top centre, and heading to the SE).  This is almost certainly the town aqueduct as it lies along the 300ft contour. Comparison to Fig, 8, however, shows we we do both mag and res…  no sign of the aqueduct in the res data at all.

Fig. 8: today’s Earth Resistance data overlain on the mag plot.

One last push tomorrow and we are “done” at Gorhambury for 2017.  Many thanks to everyone who has worked so hard, and also thanks to the Earl of Verulam for allowing us access.

 

Just two more days

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

I did consider using antepenultimate again, but I thought you might think me pretentious…

Everyone worked extremely hard today.  Mike and Jim on the GPR finished yesterday’s block and managed another 40x40m grid square.  No easy task over the long grass and thistles.  Ruth, Dave and Julia completed five earth resistance squares, including two that had to be done in two parts and joined together in the software later due to an inconvenient hedge!  Good job everybody, and many thanks.

Here is the Earth Resistance survey, both normal, high-pass filtered, and the magnetometry data from the same area.

Fig. 1: the Earth Resistance data at the end of day 16.

Fig. 2: the Earth Resistance data at the end of day 16, high pass filtered.

Fig. 3: the magnetometry data for the area shown in Figs. 1 and 2.

I hate to say it, but our five squares, including the two annoying partials, appear to lie between the buildings along the road to the north of the hedge line, and to the north of the buildings we found yesterday. We do, however, have a nice tidy area surveyed now.  We couldn’t have left quite such a silly hole in our survey data.  Tomorrow we head north to survey along the northern edge of the block we did last year.

The first block of GPR data from today was a continuation of yesterday’s

Fig. 4: the GPR time slices from the block completed on day 16.

Nothing very exciting jumps out from the plots, although there are some things to check out.  Slice 6 (second from top on left) has a strange upside-down M shaped feature (in black) and slice 8 (bottom-left) has something semi- or sub-circular near the northern edge.

How about the second block?

Fig. 5: the GPR time slices from the second block completed on Day 16.

Not a great deal there either.  Sorry guys!

Tomorrow sees the GPR crew filling-in an odd gap between last year’s survey and this years.  The plus side is that the mag shows lots of buildings, so tomorrow’s results ought to be much more interesting!

Many thanks to everyone who worked so hard in the sun today.  Just two more days.

Templeless

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

The rather odd title will become clearer! Today saw Graham, Dave and Ruth on the Earth Resistance meter, Jim, Mike and Robert on the GPR, and myself rushing around with the dGPS. I managed to find time to do a little topographic surveying, the purpose of which will be revealed in due course.

First, the resistance survey.  The team managed an excellent seven 20x20m grid squares.  Thankfully, all whole grids today, although we are back in partial-land tomorrow.  Here are the “normal” and the high-pass filtered versions of the data.

Fig. 1: Earth resistance survey at the end of day 15.

Fig. 2: Earth resistance survey at the end of day 15, high-pass filtered.

Two things can be seen from today’s survey.  Firstly, there are some nice buildings showing on the western edge of the survey area, especially in the ‘sticky-out’ bit (technical term that) which was the last grid square we did.  They show great.  The second thing that arises, is: what on earth is going on the the south-west corner?  The irregular lines of high resistance running down slope from SW to NE do not look like archaeology, so are either geology or erosion.  Very curious!  I need to drape the results onto a topographic map (probably the LiDAR data) to see what the relationship is.

The mag data (Fig. 3) also shows the buildings on the west, but not so clearly.

Fig. 3: Mag data in the same area as Fig 1 and 2. The 2017 survey area marked in red.

The strong linear things in the SW corner do not show at all.  In fact, it is rather bland!

Lastly, the GPR data (Fig. 4).  Please note that the image is made of a variety of time slices created by two separate systems. Eventually, I will process it all to make a nice clean image, but this will do for now.

Fig. 4: GPR data in the same area as Fig 1 and 2. The 2017 survey area marked in red.

Interestingly, the plans of some of the buildings look clearer in the GPR than the res, and for others it is the other way around!  The “muddier” looking walls in the western side of the GPR plot are because I am using inverse weighting to interpolate the results as the kriging in the new software is impossibly slow. The old software’s kriging routine was much faster and gives crisper looking walls.  I probably need a faster computer!

The GPR team finished one block they started yesterday, and started a second block today.  Here are the slices.

Fig. 5: GPR survey time slices. Yesterday’s second block and today’s first block.

Fig. 6: GPR survey time slices, today’s second block.

You can be forgiven for being underwhelmed by the results.  I am a bit puzzled.  The radargrams look quite busy in places, but the slices look very dull.

Now comes the explanation of the strange title of this post.  Let us first look at the slices in position (Fig. 6), and then at the mag data for the same area (Fig. 7).

Fig. 6: a crude mosaic of the time slices in Google Earth.

Fig. 7: the mag data in the same area as Fig. 6.

In Fig. 7 you can see a small square of in black on the lefthandside, probably representing a small square enclosure.  I had hoped this was another Romano-Celtic temple like the one we surveyed at Durobrivae.  Not much sign of anything is showing in the GPR data, however.  Shame!  I had hoped for a small temple overlooking the Insula XVI temple and looking across the valley to the one at Folly Lane.  Sadly, not to be.  The little square enclosure remains a mystery.

Many thanks to everyone who helped out today. Tomorrow is our antepenultimate day.  Fingers crossed for good weather.

One more week

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

As we start our final week of the 2017 season at Gorhambury, our minds turn to what we can finish this year. I have a master plan for the Earth Resistance meter which aims to complete a sensible looking block of data to join to last year’s survey.  Thanks to the amazing efforts of Ellen, Pauline and Julia we managed not only to complete two awkward blocks with trees, nettles and thistles in them, but another four blocks too.  This means we are two grids ahead of schedule compared to the master plan.  Worry not…  I have plenty I would like to get done to fill the time available!

The next three images show the Earth Resistance data, firstly as normal (Fig. 1), then high-pass filtered (Fig. 2) and finally the mag data from the same area.

Fig. 1: the Earth Resistance data at the end of day 14.

Fig. 2: the Earth Resistance data at the end of day 14, high-pass filtered.

Fig. 3: the mag data for the area covered by the Earth Resistance data shown in the previous two figures.

One interesting feature is the building which is to the south of the obvious east-west road (Street 11 in Niblett and Thompson, Alban’s Buried Towns) in between the two easternmost clumps of trees.  We did not know about this building because neither the mag or the GPR really like fighting with nettles and thistles.  A res meter wielded by someone with good boots and trousers can venture where other machines fear to tread!

The other interesting feature is the lack of anything much on the western edge.  The pattern looks like erosion and water-borne sediments.  This is one of the dry valleys which come down the slope from the deep south.  We are still unable to say whether there were no buildings in that area, or if they have been eroded away or buried.  My guess is the former, but it is really only a guess.

The GPR team in the deep south did an amazing job completing the saw-tooth section into the far corner, and starting the next line.  Well done Mike, Jim and Robert.  Excellent job.  They are getting tantalizingly close to my mystery enclosure which shows in the mag.  Here are two sets of time slices.

Fig. 4: All 12 time slices for the survey in the deep south.

Fig. 5: All 12 time slices for the second block of GPR survey.

In Fig. 4 one can see the impact of splitting a survey over several days (let alone years!).  The second slice shows that today’s data has very strong reflections early on.  That is the impact of the rain over our days off, and the generally damp conditions today.  It is interesting — well at least to me it is! — that the literature on the various survey techniques talk a great deal about the impact of rain and weather on Earth Resistance survey, but don’t really mention the problems of GPR surveys run over multiple days.  The lower slices (e.g., slice 9) do show the boundary discussed in the previous post continuing south to the Roman wall.

The second block (Fig. 5) seemed as though it was going to be exciting.  The radargrams are full of strong reflectors and we are starting to go over the enclosure seen in the mag data.  The slices are less than exciting, however. There are a few strong reflections, e.g., near the top of slice 7, but nothing too much to write home about.

I had hoped to get this posting done quickly tonight as I have a report to finish, but the new software refused to play ball for ages and now it is too late and I am too tired!  Only four days left for the 2017 survey season at Gorhambury.