Tag Archives: Geoscan RM85

End of week two, part 2

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

Just a quick update as week 3 will be starting in about eleven hours and I’d like some sleep!

The GPR crew on day 10 completed three areas of “sawtooth”.  Well done all for putting up with such an annoying, fiddly job, but it does look good along the edge of the survey.  It took a bit of setting-up, processing-wise, but all was well.  Sadly, not much showing (Figure 1).

Figure 1: the GPR survey in the northern area after day 10.

Starting from tomorrow, the crew will be working their way slowly southwards, back up the hill.  The downside is the hill, the upside is that they will be covering areas which clearly have buildings in them!

The earth resistance meter, operated by myself and Ellen, managed a modest two grids once we had set-up the other two machines.  The results were good, however, and clearly show many of the details of this building in the top-corner of the Theatre field.  The next three images show the mag, GPR and earth resistance results for this area.

Figure 2: mag data in the top corner. the building shows as white lines of low magnetism.

Figure 3: the GPR data showing this building very clearly as black lines of strong radar reflections.

Figure 4: the earth resistance data for the same building.

Although the GPR data appears very clear, the Earth Resistance and mag data appear to show more walls between the main range and the road.  There is a suggestion, also, that the “corridor” to the SW of the main range is in fact another phase.  It would be odd for a corridor to have subdivisions.  Plenty of room for debate over the details of this building.

Many thanks to all for your excellent work in the first two weeks.

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One hundred and fifty

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

We managed a full day today, and I’m just about keeping up! The mag team completed two grids yesterday, half of one in the aforementioned deluge. Today they completed 11 grids: three partials and eight complete ones. Way to go! Well done everyone. Figure 1 shows the survey so far.

Figure 1: the mag survey after day 9.

One really does wonder if that break in the mag data is an entrance.  It doesn’t seem like it on the ground.  I have downloaded the LiDAR data but haven’t had a chance to process it yet.

The GPR crew finished their 80x40m block, and then did some of the next “sawtooth” section, another 14m worth.  Figure 2 shows the time slices.

Figure 2: day 9, time slices 3 to 6.

Nothing jumps out at one, although there are some curious “light” lines in the fourth slice (top-right) which are parallel to the aqueduct.  Figure 3 shows that slice in context.

Figure 3: GPR survey after day 9, slice 4.

After all the rain I thought it would be worth trying the Earth Resistance survey (Fig. 4).  I spent the morning laying in grids for the mag, but managed some survey in the afternoon.

Figure 4: Earth Resistance survey in action.

Although the rain has softened the surface, it won’t have penetrated 50cm yet, and I was concerned that there would be no contrast at that depth.  I decided to survey a grid where we knew there was a building.  Fig. 5 shows the comparison between the GPR survey and the two squares of res I managed to complete (thanks Anne!).

Figure 5: Earth resistance survey compared to GPR results.

Given the drought, the results are pretty good.  It would be interesting to compare these to results from a normal English summer!

Tomorrow isn’t looking great.  We might get some work done in the morning.  Fingers crossed.

Many thanks to everyone who helped out today.  Especially big thanks to Mike, Ellen, Jim and Ruth who take on the responsibility of shipping the equipment back and forth.

By the way, this is the 150th blog post…

The final results from Alba Iulia (Apulum)

We managed the final ten squares on the last day completing the area I hoped we would cover. We managed 2.6ha in seven days of survey.  The final four grid squares had an old excavation trench in the middle of them and a sea of mud from the spoilheap being bulldozed back into the hole.

Fig. 1: Stefan and Wyatt surveying in the mud.

I have left the grid on the next image (Fig. 2) which gives you some sense of scale.  The grid squares are 20x20m, and site north is to the right.

Fig. 2: the surveyed area with grid for scale.

In the next image (Fig. 3) I have labelled a few of the features.  One can easily see many of the buildings.  Because of the large differences between areas of low readings and areas of high readings I have applied a log-transform to the data to make some of the features more visible.

Fig. 3: the 0.5m survey with log-transformed data.

An alternative (and very common) method of evening-out the differences is to apply a high-pass filter (Fig. 4).  The downside to this is that areas of very high readings will often end-up with a area of very low readings next to it which are purely an artefact of the filter.  It is essential, therefore, to have several images and compare them so that you do not create features where there are none.

Fig. 4: the data after the application of a high-pass filter.

As well as the main survey with a 0.5m probe spacing (thus ‘looking’ 0.5 to 0.7m below the surface, approximately), we were simultaneously collecting 1.0m probe spacing data (which ‘looks’ about 1.0m into the ground), but at only two readings per square meter rather than four.  This extra data did not add that much more to the survey (Fig. 5), but does make the north-south road show more clearly.

Fig. 5: the 1m probe spacing data, high pass filtered.

There is quite a bit to do in terms of interpretation and drawing-up maps of the results, but I am very happy with the new results which have greatly improved an already excellent set of results from 2002/3.

The journey home seemed to go on for ever, but at least I had some entertainingly-named beer on the way!

Fig. 6: Munich Hell.

Many thanks to the whole team, and especially to Stefan, Wyatt, Doru and Ian.

Somewhat tired

On Friday the snow kept on coming and we had to abandon the survey for the day. It gave us the chance to explore the citadel and visit the museum.

Fig. 1: Carol’s Gate, Alba Iulia.

The snow had stopped yesterday morning so we headed out to site once more. During the day the snow cover slowly melted and we managed an excellent eleven grids, although two of those were small partials.  Today, the weather improved…

Fig. 2: Stefan surveying.

I even ended-up with sun-burnt ears!

Having lost time through airline incompetence and snow, we were determined to try and catch-up and we managed twelve 20x20m grids at 0.5m intervals with just the two of us.  To say our feet ached would be an English understatement.  We did, however, get some lovely results.

Fig. 3: the half-meter probe spacing survey at the end of Sunday.

We believe that the thin line on the right hand side of the plot running diagonally is the line of the town wall.  We seem to have picked-up a set of three small buttresses on the inside of the wall.  We have some more buildings at the top of the plot.  Obviously, there is a great deal going on in this data and a detailed interpretation will take a little longer.

Tomorrow our target is the ten grids along the top edge of the survey to complete the block and join the survey up with the earlier excavations.  It is tempting, however, to try and survey a little more at the top to see what those two pairs of parallel lines are doing!

I’d like to thank the whole team but especially Stefan who has worked really hard, and Wyatt who has also helped a great deal, and also Ian Haynes and Doru Bogdan who made the whole trip possible.  The whole team have been great to work with and made what could have been a trying experience great fun.

Midwinter madness

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!

Fig. 1: “… and this bit goes here”. Building the mag at the start of the day, Photo by Mike Smith.

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.

Fig. 2: The Earth Resistance results over the “tumulus” as of December 2017.

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.

Fig. 3: the Earth Resistance results. Red lines mark the features mentioned in the text.

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.

Fig. 4: the magnetic data for the same area as Figs 1–2.

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.

Fig. 5: the mag data with the linear features from the Earth Resistance data superimposed.

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.

Nerdy bit…

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.

Fig. 6: plot showing the girds of Earth Resistance data before grid matching.

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.

Fig. 7: the crew having lunch accompanied by the dull roar of the A1(M). Photograph by Mike Smith.

I hope you all have a very Merry Christmas, and wish us all a successful year of surveys in 2018.

 

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.

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.