Tag Archives: community archaeology

The smallest grid ever?

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

Well, possibly not. The mag team’s first grid this season was a 1.5m x 40m grid. Why? Well, in the penultimate grid of the very last day last year we had one single frozen sensor for one line of data. For the last 11 months this has annoyed me every time I saw it. Finally, I have been able to fix that grid! Yay.  Having completed that line, the team went on to complete another seven grids of data.  Well done team (Figure 1)!

Figure 1: Jim West and the mag.

Figure 2 shows the whole of Prae Wood Field and the survey completed so far (but without the dodgy line!), and Figure 3 zooms into the area completed on Sunday.

Figure 2: the mag survey of Prae Wood Field.

Figure 3: detail of the mag survey of Prae Wood field.

The overall impression one gets from both Figures 2 and 3 is a whole lot of nothing.  There are the occasional strong magnetic responses from iron objects, and on Sunday we picked-up two pipelines (shown in Figure 3 with yellow arrows).  The larger area of noisy magnetic data near the southern edge of the survey might be an historic structure.  In the new area, there is a very faint line (as indicated by the blue arrows) which might be an old fence line, or might be my imagination.  There are some “monopolar positive” features (i.e., ones which are mainly positive but with a slight negative response on the north side) which could well be pits.

What makes this all fascinating is that the Urban Archaeological Database (the UAD), suggests that the field is within a “rectilinear  enclosure”, Monument Number M27.  At the moment, I’m not sure where this idea comes from, but at the moment it seems as though it is an enclosure around not very much! This field, and part of Prae Wood itself, are within the area mapped by the Environment Agency using LiDAR (Figure 4).

Figure 4: LiDAR image of Verulamium. Data from the Environment Agency, image courtesy Mike Smith.

The Fosse, running through the woodland along the NE edge of the field shows nicely (Figure 4, right-hand red arrow).  The little fragment of Prae Wood itself shows a mass of features in the woodland, some of which are parts of the Iron Age settlement (blue arrow). Our field shows the faint hint of the ploughed-in Fosse (left-hand and central red arrow), and a whole lot of not-much-else.  How very curious!

Despite the very dry ground surface and the small team, we did manage a further three res squares on Sunday.  Many thanks to Pauline for putting-up with my cursing as we did the work. Figure 5 shows the results.  The edge between the earlier survey to the east and the current block of eight squares is due to my processing differing between the two seasons.

Figure 5: the Earth Resistance survey after day 4.

As can be seen, we have a line of buildings along the SW–NE road.  This road, Street 23 in Niblett and Thompson’s Alban’s Buried Towns, shows very poorly on all three survey techniques.  In the res survey, it almost looks like an eroded channel, and I have often been a bit confused as to the where the dry undulation (seems a bit grand to call it a valley) lies.  It lies, however, behind these buildings and those that face onto Street 25 to the west.  I have indicated the valley in question with a yellow arrow in Figure 4.

A further source of data is the work undertaken by the Oxford Archaeological Unit in January of 2000.  They excavated 379 1.6m x 1m test pits using a mechanical excavator to strip the topsoil.  As they were investigating plough damage, they did not excavate the features revealed.  The distribution of test pits is shown in Figure 6.

Figure 6: test pits excavated in 2000 by the OAU.

If we zoom into the area we have surveyed using res over the weekend we can see which test pits are relevant (Figure 7).

Figure 7: test pits and the res survey (click on the image to see a larger version).

Test-pit 268 is described as showing a possible floor and a wall foundation of chalk. Judging by its position in the middle of a small building that seems appropriate. Test pit 258 is described as having “?floors” and “?Fill of wall trench”, whereas pit 257 which lies either on, or more probably just outside the wall of the long building, just has a “layer”. In all three test pits the topsoil was between 27 and 32cm deep.  I have yet to see the plans of these trenches, but clearly combining the evidence of the trenches with the geophysics data is going to be very informative.

The GPR team jumped a few grids to complete a block next to the one that they  completed on the last day last season.  We intend to swap to 1m transects soon, and I wanted to catch the details of the buildings that clearly intruded into this block.  Figure 8 shows the location (the bit sticking out to the west).

Figure 8: the GPR survey showing the location of the block surveyed on Sunday.

As can been seen in Figure 8, we have managed to complete the building which lies over the grid edges, but there doesn’t appear to be much more.  Lets look at the first 12 slices (Figure 9).

Figure 9: the first 12 time slices from Sunday’s survey.

Slices 1 and 2 are basically showing the top surface and the topsoil.  In slice 3 we can start to see the building and a long, wide, linear feature.  These show in slices 4 and 5 too.  By slice 6, we are already in the natural and/or where the signal has started to attenuate.  Slices 7  to 12 are basically a few deeper things and echoes / attenuated signal.  The only thing of technical interest is the semicircle which shows on the eastern edge of slices 7 to 12, and also shows in slices 1 and 2.  This is an “airwave” caused by the radar signal bouncing off the underside of the tree canopy.

GPR slice comes with a plethora of palettes for display time slices.  Figures 10 and 11 show slices 4 and 5 in the first 12 palettes.

Figure 10: slice 4 shown in 12 different colour palettes.

Figure 11: slice 5 shown in 12 different colour palettes.

The building along the eastern edge shows well in palette four, and the big linear thing running across the plot shows well in slice 4, palette 11. So what is the big linear thing?  Figure 12 has a clue…

Figure 12: the GPR data with the line of the 1955 ditch indicated.

Yes, you’ve guessed it (or at least read the caption), the “long linear thing” is our old friend, the 1955 ditch.

Well I think that is enough for now.  We will be back on site again tomorrow, hopefully running all three machines if the long promised rain actually happens this evening.  Many thanks to everyone who has contributed this week.  It was a great beginning to the 2019 season!

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Verulamium 2019, days 1 to 4

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

After some wet weather just before the season was due to start, the first four days have proved to be warm and sunny. So much so, the ground is already drying out so much as to make the Earth Resistance survey a little annoying.  So far, we have been concentrating on Earth Resistance and Ground Penetrating Radar (GPR) in the main “theatre field”, simply due to person-power and logistics.  We are planning to start the mag survey tomorrow with the aim of completing Prae Wood field and then moving to Church Meadow. Mobbs Hole, the field through which the Fosse passes, will be completed at another time of year when we are less likely to disturb the pheasants.

The res survey (Figure 1), firstly concentrated on filling-in the triangle to the east side of the survey area, and then moved to add another strip of grid squares along the western edge.  Figure 2 shows the entire res survey so far.

Figure 1: Jim West and Rhian Morgan running the Earth Resistance meter on Day 3.

Figure 2: the entire Earth Resistance survey after day 4 of the 2019 season.

At the moment, the res surveys have been processed separately and crudely put-together in Google Earth. As a result, you can see the edges between blocks clearly, especially, for example, the triangle to the east. At some point, I need to combine all the grids into one master survey and process them properly!

Figure 3 Looks in more detail at the eastern triangle.

Figure 3: the completed eastern triangle of res data.

The very strong line near the top of the new area is a road partially excavated by Frere.  The lack of clear buildings either side of it near to the modern road is due to the excavations undertaken by Frere.  We have, however, picked up the last bits of the buildings which run parallel to the side of the Insula XVI temple, as well as some new buildings  alongside the modern road.  The building at the south of the new triangle was partially surveyed in 2017, and is not one previously known.

Today we completed five 20x20m blocks on the western edge of the survey area.  The results are shown in Figure 4.

Figure 4: the western survey area.

The buildings lie along a road, although the road is not at all clear.  The diagonal empty area appears to be a eroded channel cutting across the site visible in the modern topography.  The large corridor house to the east was surveyed in 2017 using the multi-depth Earth Resistance survey (aka “the beast”).

The GPR crew have been working south starting near the area which had such exciting results at the end of 2018.  The GPR leaves entertaining stripes in the grass (Figure 5)!

Figure 5: Stripy GPR grass.

Figure 6 shows the entire GPR survey up to the end of today.  This season’s block of GPR is in colour in the SW corner.

Figure 6: the complete GPR survey after day 4 of the 2019 survey.

Even more than the Earth Resistance survey, the crude use of images in GE is visible.  The data has been collected over five seasons, and processed with different software packages.  I am in the process of putting all the data together into one more consistent analysis.  It might take a while.  Figure 7 shows the four blocks completed thus far.

Figure 7: GPR results after day 4. Slice 7.

Despite the mass of buildings just 20m or so to the east of the block, and on the northern edge, very little seems to lie within the block surveyed.  Comparison to the magnetic data from 2016 (Figure 8) makes it possible to see the so-called ‘1955 ditch’, and one of the GPR blobs is matched by the magnetic blob (it is probably a filled-in gravel / chalk pit).

Figure 8: the magnetic data from the same area as Figure 7.

We are going to complete a couple more 40x80m blocks at 0.5m transect spacing, but then swap to using a cruder 1m transect spacing to ensure we complete this field this season.  Should we pick-up further structures, we can always re-survey targeted areas for more detailed survey.

Many thanks to all our volunteers.  The survey would be impossible without you!  Also thanks to UCL Archaeology and SEAHA CDT for the loan of the equipment. Also, big thanks to Lord Verulam, the Gorhambury Estate and the estate managers for enabling the survey to continue.

Where have we been? What have we done?

The project is now six years and a bit old. We started (officially) on Feb 1st 2013, and the funding ended on Feb 1st 2014. I thought people might like to see how many sites we have worked on.  I’m a little vague as do we count Verulamium as one site, or do we split it up (the Park, Gorhambury, Abbey Orchard etc.)?  I make it 30 sites altogether.  We have expanded from just magnetometry to regularly using mag, Earth Resistance and GPR, with occasional resistivity pseudosections and a little bit of magnetic susceptibility. I would like to thank everyone who comes and helps on the surveys, especially those who transport the gear around for me.  I would also like to thank the AHRC for funding the original project, the Institute of Archaeology, UCL, for allowing us to use their Earth Resistance meter and the dGPS, and SEAHA for the use of the GPR.  The Institute also pays the annual fee for the GPR software I use.

The two maps below show the sites in Hertfordshire that we have worked on, and all the sites that we have examined.  Here is to the next six years!

Figure 1: sites surveyed in Hertfordshire and immediate environs.

Figure 2: sites surveyed by CAGG.

Hogshaw Redux

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

Archaeologists often have skeletons in their cupboards. Sometimes they are real skeletons. Sometimes, as here, they are unfinished jobs that they haven’t quite got around to completing.  There are a few surveys we have undertaken that never quite got finished, and for which there are no blog posts (shock! horror!). Way back when we got together with the Buckinghamshire Archaeological Society’s Active Archaeology Group and CVAHS to undertake some surveys at Hogshaw in Buckinghamshire.  The AAG had undertaken an interesting research project on this site including topographic survey.  We managed some mag (even though the mag was down to three probes) and some Earth Resistance survey (using our old system).  The results were posted at the time.

The following year, in 2016, we returned and expanded the mag survey and undertook some Ground Penetrating Radar survey.  We had only just started using GPR and I was still learning how to process the data.  The following year, Mike and I returned with the GPR to survey another two areas.  Due to problems with that data (we were distracted by lunch), that I couldn’t solve at the time, the results were put on the back burner.  Fast forward two years and I am now a little more confident and have a better handle on the software.  Having finished processing the awkward survey at Bovenay, I thought I would have a go at re-processing the Hogshaw data.  As you might guess from the fact you are reading this post, I had some luck and so, two years late, here are the results! (See the older post for the previous results and the background to the site.)

The magnetometry survey was mainly aimed at finishing the awkward bits around the edges, and an area to the south where the landowner kindly took down his fence so we could survey across it.  The results are shown in Figure 1.

Figure 1: the magnetometry data after the 2016 survey.

At first sight the magnetic survey is rather busy and hard to interpret.  This is not unusual in historic period sites where iron artefacts and fired bricks are relatively common.  I have labelled the plot with some basic interpretative points (Fig. 2).

Figure 2: magnetic data interpretation.

The fence line is where the farmer kindly removed the fence so we could survey.  It is fascinating to see that even when the fence has gone, we still detect the line of it.  Iron rust etc. washes down and permeates the soil, I guess.  The platform is a large flat area in the NW corner of the site.  We do not know what it is for, and the mag does not help a great deal (neither did the Earth Resistance last time).

Perhaps the most interesting feature that we detected last time is the four squares inside a square.  This was quite a surprise.  It looks very much like a formal garden.  If it is a garden, there appears to be a line heading out westwards to an area of magnetic noise.  I rather ignored that last time, but now I wonder if that is where the remains of the manor house were?  It was abandoned in the 18th century.

There are two lines of very noisy magnetic readings, one along the current road and one along the northern edge.  I’d like to see how these relate to the topographic features.  I think they line-up with the banks, and could be lines of brick rubble.  Unfortunately, the LiDAR data for this area does not cover the site, ending just under half a mile to the north (Fig. 3).  Typical!

Figure 3: the LiDAR data overlain on a Google Earth satellite image.

Three blocks of radar data were collected.  We used SEAHA’s Mala GPR, and we thank them for the loan.  The location of the three blocks are shown in Figure 4.

Figure 4: Location of GPR blocks.

The southern block was surveyed in 2016 because an excavation had found a couple of stone walls in this area, and it was suggested this might be the location of the lost chapel. Figure 5 shows the top nine time slices (note that north is downwards in these images).

Figure 5: time slices from the southern block.

The first time slice shows the road nicely.  Also helps build confidence when the method detects the absolutely obvious! By about the fourth slice (second row, leftmost image) the road is largely gone but there are two parallel lines running north south.  Could these be our missing walls?  Perhaps, but I suspect they are compacted earth either side of the fence which the farmer took down for us.  The area of high amplitude reflections in the bottom-right corner (north-west) is the area of wet mud around the various temporary structures that were moved.  All in all, a rather disappointing result.

The platform block was an attempt to see if we could work out the function of the platform in the NW corner of the site.  Figure 6 shows nine time slices.

Figure 6: time slices from the platform block.

Again, note north!  There is a vague hint of something in slice 7 (third row, first image) that might be rectangular, but it is quite low down in the sequence, and a bit amorphous.  Looking at the radargrams (the original vertical slices), I cannot see anything particularly wall-like.  I suspect that what little radar energy has been reflected has been greatly emphasised in these plots creating the illusion of something.  Figure 7 shows slice 7 in context.

Figure 7: platform block, slice 7, in context.

Last, but not least, is the “garden” block (Fig. 8).

Figure 8: six GPR time slices over the “garden” block.

In slice 1 (top left), the results just reflect the uneven surface. In slice 2 we can start to see something, but it is in slices 3 and 4 that we can see the “garden” feature quite clearly.  The whole feature is about 36m across with the internal square about 12m by 12m.  To the south there appears another strong linear reflection.  Maybe a road to the house?

Figure 9 shows slice 3 in context.  I’m glad to say that the mag and GPR data match very closely.  The edging around the features must be something both magnetic and that reflects radar data.  Brick is one possibility, and some form of igneous rock is another.

Figure 9: the “garden” block, slice 6 in context.

One might ask why I am so keen on it being a garden feature.  Looking at another much grander garden, we can see many similar features (Fig. 10).  The part I have outlined in red is approximately the same size as ours.  The inner squares of that garden at Hatfield are 11m across, the enclosing hedge 28m by 42m, the outer edges 37m by 57m.  As always, the only real way to tell is to dig a hole…

Figure 10: the gardens at Hatfield House.

Many thanks to everyone who helped on the four days of survey, especially to the very helpful landowner.  Also many thanks to Anne Rowe for commenting on the “garden” feature and sending me some very useful information. Hogshaw still has some secrets to give up!

St Mary Magdelene, Bovenay

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

The small chapel of St Mary Magdalene lies in the south of Buckinghamshire, not far from Windsor and Eton.  Now only used for occasional services, it is looked after by the Friends of Friendless Churches.  It is a lovely little chapel, and well worth a visit (Fig. 1).

Figure 1: the chapel of St Mary Magdelene.

We were contacted via the Buckinghamshire Archaeological Society’s Active Archaeology Group to see if we would undertake a geophysical survey around the chapel.  The question was deceptively simple: are there graves around the church?  The reason for the question is partly because, usually, ‘chapels of ease’ were not used for burial, and partly because the nearness of the water table makes digging deep holes problematic.  Although the site is a long way out of our usual area, we agreed to try and see what we could find.

Three things make the job difficult:

  1. burials are notoriously  difficult to detect at the best of times.  They aren’t very big, and usually the same soil that came out of the hole goes back in again pretty quickly.  There is, therefore, relatively little contrast between the grave fill and the surrounding soil.
  2. Small areas are difficult to interpret. The whole churchyard is only 0.07ha (less than two 20x20m grid squares), and you have to subtract the footprint of the church itself and the path.
  3. GPR surveys near standing buildings suffer from airwaves.  Although the antennae are shielded, some of the radar signal will ‘leak’ and will bounce off nearby buildings etc.  Airwaves can be seen in the data as having hyperbolas with a much flatter profile than the usual point sources in the ground such as rocks or walls.

Given the very small size of the area, and the surrounding metal fence, magnetometry survey was going to be pointless.  Often, the best method for finding graves is GPR, and so that was our primary method (Fig. 2).  Due to the building, and the odd shape of the church yard, we had to do the survey in six small blocks at 25cm intervals.  Although it would have been easier to do the survey east-west, if we are trying to find graves working north-south would be more effective allowing the transects to cut across the grave rather than along it.  Lastly, we decided to try the multi-depth Earth Resistance survey, aka ‘the beast‘ (Fig. 3).

Figure 2: the GPR in action at Bovenay. Photo: © Mike Smith.

Figure 3: the ‘beast’ in action at Bovenay.

We all headed off to Bucks on a cold and slightly damp Sunday at the end of January (yes, I know this posting is late!) and we were assisted by members of the Bucks ASAAG.  Both GPR and resistance surveys were awkward due to the small space we had available.  The site is also very busy with walkers, cyclists and people enjoying their Sunday.

The idea of “the beast” is that the depth to which an Earth Resistance survey will measure is proportional to the distance between the mobile probes on the frame.  The two remote probes have to be at least 45m away!  Each time the machine is moved, it takes seven readings: one between two probes 25cm apart, one at 50cm, 75cm, 100cm, 125cm and 150cm.  Yes, that makes six.  Just for comparison, the seventh measurement is taken using a “Wenner array”.  This simply means that instead of using the two remote probes at the end of the cable, it uses the two outer probes on the frame to pass the current, and the two inner probes to take the reading.  This is an older method for laying out probes that has generally been abandoned in archaeology, although it can be useful in circumstances when having remote probes at the end of the long cable is impossible.  The results for all seven readings are shown in Figure 4.

Figure 4: results from the multi-depth Earth Resistance survey. (Click for larger image.)

Figure 4 looks a little odd because the satellite that took that image was clearly passing overhead a little to the south-east.  It is, however, the best one available on Google Earth Pro.  Comparing the various surveys at different depths, there is very little difference between them.  Unsurprisingly, near the walls are areas of high resistance, possibly due to the foundations.  The path was a pain.  One problem was a number of ‘spikes’ in the data.  These were probably caused by the rabbit holes: a hole with air is going to be high resistance (in fact the current passes through the soil around the hole).  I worry slightly that processing those out may also have processed out the graves, but somehow I doubt it.  It doesn’t look like we detected any graves with this method.

The GPR survey was processed using the package GPR Slice.  Figure 5 shows all the slices from the survey.  These were processed using 2.92ns slices with a slight over-lap between them.

Figure 5: the time slices from Bovenay.

The bright red line in the first slice is the path to the north entrance of the chapel. If you look at the image on Google Earth, one can see the south entrance too as a lighter blue line (Fig. 6).  Not an Earth-shattering observation, but it is always encouraging when one can see the obvious!

Figure 6: GPR time slice 1.

What seems interesting, at first, at the strong reflections (shown in red) in the lower slices.  They are at a slight angle to the hole in the survey where the church is.  Looking at slice 11 in more detail (Fig. 7) we can see they are parallel to the wall… it is my grid that is at a slight angle. (The grid was set-up along the southern fence line.)

Figure 7: GPR time slice 11.

If we look at the radar data in 3D, we can see these strong reflections low down curving-up towards the edge of the survey (Figure 8). You can see those strong reflections intersecting with the red line in the time slice (labelled ‘air waves’).

Figure 8: 3D image of the radar data.

To understand what is happening, we need to go back to GPR basics.  What is happening when we do a survey?

  1. The transmitting antenna sends out a radar pulse.  Due to the shielding, most of this goes down into the ground, but some will leak out and bounce around like echos.
  2. The receiving antenna measures the returning radar waves.  It records two things: the strength of the signal (amplitude) and the time since the pulse was transmitted.
  3. The software plots the strength of the return signal in shades of grey.  Strong returns are plotted in black and white, and weaker returns in mid-greys. These are plotted as a single vertical band below the centre point of the antenna. The radargram one looks at on the screen are all these vertical bands added together to give the overall image.
  4. Because the longer the time between the pulse and the return, the weaker the signal will be, we apply a ‘gain curve’ to the data.  This is just a multiplication factor so that the deep returns are visible compared to the shallow ones.
  5. As we push the GPR towards a wall, some of the signal will bounce off that wall.  At first, the distance between us and the wall is relatively large, so the reflection will be plotted by the software low down the profile.  As we get closer, the time taken for the signal to bounce off the wall gets less, and so the reflection is plotted higher up the profile.  As a result, the signals bouncing off the wall will show as a gently rising curve.
  6. Radar waves travelling through the air travel at the speed of light.  Radar travelling through the soil is much slower.  As a result, curves in our data which are quite steep are the result of a reflection from something in the soil.  Very gentle curves are “air waves” and are the result of the radar bouncing off buildings, or even the underside of tree canopies.

To cut a long story short, the strong, deep reflections are airwaves caused by the radar bouncing off the walls of the chapel.  Figure 9 shows the southern radargram from Figure 8 with some of the relevant reflections indicated.

Figure 9: Radargram 7028 at 12m east with some reflections marked (see text).

In Figure 9 the airwave has been marked in red.  See how it is a gentle curve across the radargram.  A more normal hyperbola from a point source is indicated in purple.  The green line near the surface represents the compacted soil outside the south door.  Notice how there are bands of strong reflections below it.  These are like echos.  There is another, deeper, surface marked in yellow.  I’m not sure what this it, perhaps an earlier entrance path?  If we look at time slice 5, we can see this area of high reflections outside the south entrance to the chapel (Fig. 10).

Figure 10: GPR time slice 5.

Archaeologists, (me included!), dislike looking at radargrams as they find them confusing (they are).  Often one will see reports with only the time slices presented.  One thing I have learnt from Larry Conyers, however, is that it is vital to look at both the time slices and the radargrams if one wants to understand in detail what is happening.  Although in general I am not a fan of pseudo-3D representations of things (don’t get me started on the invention of the Devil, the 3D pie-chart!), the 3D plots in GPR Slice do help work out what is going on.

So the million dollar question is: have we found any graves?  The short answer is: none that I can see.  The long answer is, sadly, that that does not mean there are no graves.  Geophysics does not detect everything, as much as we would like it to.  Also, I need to spend some more time going through the radargrams and trying to see if there are graves which show in the vertical radargrams but do not show in the horizontal time slices.  Later this year I plan to spend some time with a friend in the US who does this sort of thing all the time, having a look at the data from this site and a couple of others in the hopes I have missed something vital. Watch this space!

Flying the Flag

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

Before I start this post, I should apologise to those waiting for other surveys…  There has been some fiddly data processing (including software training over Skype from Hawaii!), as well as problems with coordinate transforms and other boring technical stuff.  I promise I’m working on them!

The group were asked to provide a day’s training on geophysical survey at Flag Fen for the volunteers from Vivacity who help run the site and others in the Peterborough area.  Many thanks to Peter Alley for setting this up.  We planned to survey two areas: a small strip in the public area of the site across the known line of the Roman road, and a second larger area to the west through which is known to pass the Bronze Age wooden causeway.  We managed to survey the small strip using all three techniques: mag, Earth resistance and GPR, but the howling gale resulted in only the mag being able to survey a little in the second area.

Flag Fen is rightly famous for its prehistoric, mainly Bronze Age, archaeology (Fig. 1) which includes a great deal of preserved timber including a causeway (Fig 2).  It was extremely unlikely we would detect any of those features because wet wood in wet mud has little contrast in any of the three techniques we use (Fig 3).  There is, however, a Roman road known to run across the site as well as a medieval road with a toll building, and those we hope we would find. (Note that the site photos were taken in 2018 on a Welwyn Archaeological Society coach outing to Flag Fen.  The weather was not so kind last weekend. Also note that if you click on the image you can see them in higher resolution.)

Figure 1: Reconstructed round house at Flag Fen.

Figure 2: preserved timbers in situ at Flag Fen.

Figure 3: Flag Fen.

Area A was a small strip just 14m wide and 57m long.  It does, however, cut across the known line of the Roman road.  We managed to complete all three techniques in this area.

For the Earth Resistance survey we were using the “1 plus 2” method.  When we do this, the machine takes three readings when we stick the mobile probes into the ground (Fig. 4).  The first reading uses the outer two probes which are 1m apart.  This means we are looking roughly 1m below the ground surface.  It then takes two further readings, the second using the left-hand probe and the centre probe, and the third using the centre probe and the right-hand one.  These pairs are 0.5m apart and we are therefore looking about 0.5m below the ground surface.  The deeper survey is at a lower resolution of two readings per meter square than the shallower survey which is at four readings per meter square.

Figure 4: the Earth Resistance meter in action.

The results were very encouraging.  Figure 5 shows the 0.5m mobile probe spacing survey.

Figure 5: the Earth Resistance survey (0.5m probe spacing).

In Figure 5 high resistance readings are dark, and low resistance readings are light.  High represents solid things like paths and roads.  In this image we can see one large high resistance feature in the middle, a thin linear one to the south, and a wider north-south high resistance feature in the northern third of the survey area.  I have labelled these in Figure 6.

Figure 6: labelled version of Figure 5.

The big high resistance feature matches the known location of the Roman road.  The thin linear feature matches the current path (Figure 7).

Figure 7: the path at Flag Fen shown clearly by the moss. Photo ©Peter Alley.

This leaves us with Mystery Features A and B, and the hole in the road.  The latter could be (a) robbing of the road for building materials (b) an old archaeological excavation or (c) levelling of a spot for the toll house.  My money is on (a) or (b)!  Mystery feature B can be solved by looking at the historical imagery available on Google Earth.  I have put the geophysics images over the photos from 11th September 2006, which I have reproduced below (Figure 8).

Figure 8: imagery from September 2006.

One nice thing about this image is that it shows an excavation in progress.  It also solves Mystery feature B: there is clearly a path or walkway running across the site at that spot (remember that the Google Earth images are not very accurately georeferenced).  There is also a curious bright square in the photo which I am guessing is a roof to protect where the section of the Roman Road could be seen in the edge of the dyke.

Mystery feature A, however, remains just that,  mystery.  Maybe it is one wall of the toll building, but it is very wide and so would just be a spread of rubble.  There is clearly a high resistance feature here, but exactly what it is I do not know.

Figure 9 shows the deeper Earth Resistance survey.

Figure 9: the Earth Resistance survey (1m mobile probe spacing).

This survey does not show much new but three things should be noted.  Firstly, the hole in the Roman road has gone and is, therefore, not right through all the layers of the road.  Secondly, mystery feature A is even clearer suggesting that this feature is more substantial at depth and is unlikely to be merely ground compaction.  Lastly, the modern path has vanished.  This is because the wider probe spacing means that we are now looking below the level of the superficial path.

The Ground Penetrating Radar survey was very popular (Fig. 10), partially because one can see things one screen!  (I should note that everyone did try the other techniques too!)

Figure 10: the GPR survey in action.

With GPR we collect data in vertical “radargrams” which are difficult to read until one has had some practice (I’m still learning!).  One then takes those vertical slices and stacks them together in the software, and then slice them horizontally to give map-like images at different depths called time-slices (or amplitude maps if one wants to be posh).  In Figure 11 I have plotted the top 8 slices with the shallowest slice in the top-left corner and the deepest in the bottom right.  In these slices blue represents weak reflections, i.e., little or no radar waves are bouncing back to the antenna. Red represents strong reflections, i.e., a great deal of the signal is bouncing back.

 

Figure 11: time slices from the GPR survey.

In the first slice, the modern path shows very clearly right on the surface.  Note that the tracks from their little cart also show along the western edge.  This is simply from soil compaction. By the third slice, the Roman Road,  and mystery features A and B are starting to show nicely.  By slice 6, pretty all that one can see is the Roman Road.  Note that one should take the depths with a pinch of salt.  Firstly, I should have built-in to the processing the topography, which I haven’t.  Secondly, the speed of the signal has not been calculated so this is just a rough guess. Figure 12 shows the fifth slice in place so that you can see how it matches up to the res and the site.

Figure 12: GPR slice 5.

As I mentioned before, understanding the radargrams (the original vertical slices) can be difficult. This is because what we are dealing with are reflections.  High amplitude reflections can show down the profile like echoes in a empty room. Also, a single point, like a stone, will show as a hyperbola (a curve with the middle at the top).  This is because when you are off to one side, some of the radar signal will bounce off that point but the distance is the diagonal.  As you move closer, than diagonal gets shorter until you are over the top and the reflection is directly below you.  As you move past, the distance increases once more.  The software I use allows one to create 3D images with both the time slices and the radargrams combined which I find helpful in understanding the latter.  Figures 13 to 15 show three examples.

Figure 13: 3D representation of the GPR data.

Figure 14: 3D representation of the GPR data.

Figure 15: 3D representation of the GPR data.

In Figure 13 I have picked a time-slice which shows the Roman Road well. Notice the strong reflections in the radargram (showing as dark black and bright white) matching up with the red in the time slice showing the strong reflections.

In Figure 14 I have used a radargram that cuts across mystery feature A.  Notice the very jagged and noise area of the radargram over the line of the mystery feature.  We are not getting very strong reflections like the road, but we have also gone along the feature rather than across it.  When surveying something linear, it pays to try and cut across it rather than go along it.  Our transects are 50cm apart, but we take a set of readings every three centimeters.  That is why we always survey Christian cemeteries north-south not east west.

In Figure 15 I have two time slices.  the top one shows the modern path clearly.  Note how the strong reflections start from the very top. The bottom slice show the reflections from the Roman Road, just clipping the edge of the hole.

Hopefully, these images will help to explain how to work with GPR data!

Last, but not least, the magnetometry survey.  Magnetometry is the mainstay of geophysical survey because (a) it is quick and (b) is often finds stuff!  It’s main weakness is in areas with lots of ferrous material around, like a public spot with paths, fences, old excavations and a yurt…  As a technique, it likes the wide open spaces (Figure 16)!

Figure 16: Mag survey at Flag Fen (Area 2).

Figure 17 shows the results from Area A.

Figure 17: Area A magnetometry survey.

The modern path shows very clearly with a strong magnetic response.  Remember that magnets have a negative and a positive pole, and these are plotted as white and black in the image.  Mid-grey is the background reading.  The path probably used a magnetic rock, like granite or basalt, in the gravel. Mystery feature B also shows very strongly.  I’m not sure what they used for that path.  Along the western edge to the south we have picked-up the modern fence line. Mystery feature A does not show in the data.  There are various other bits of old iron about. In the northern half of the site are some features which are less strong than the others, and have a very weak negative pole (whitish) and the stronger positive pole.  Some of these may be pits full of organics or burnt materials.

Area B was much bigger.  I had, ambitiously, laid out four 40x40m blocks but the howling gale (I don’t exaggerate), we could only manage two of those.  (Tapes stretched out across 40m don’t stay straight for long in a gale!)

Figure 18: the Area 2 mag survey.

The first thing to note is the stripyness running diagonally across the grids.  This is the remnants of ploughing showing in the data.  This is very common in mag surveys.  I have labelled some of the other features in Figure 19.

Figure 19: the Area 2 mag surveyed labelled.

The black blobs like the one labelled A are not that likely to be exciting.  They are probably spreads of organics on the surface, or possibly burnt areas from getting rid of the stubble.  There is a moderate amount of old iron in the field, as is always the case in agricultural fields.  I have labelled just one as B.  There are a few features which might be something more interesting.  For example, at C, is a strong magnetic feature where the negative pole is very weak compared to the positive.  That might be a pit with organics or burnt material in it.  Perhaps the most exciting thing is the hardest one to see.  At D there is a faint curving line.  Almost a complete semicircle can be seen in the data which is about 7.5m in diameter.  Have we found a round house?  It is definitely a possibility!

Hopefully, everyone who attended the training day enjoyed it and got something from it.  Many thanks to Peter Alley, Jim West, Mike Smith, Pauline Hey and Nigel Harper-Scott for helping with the day, and to Gill Benedikz and the rest of the staff at Flag Fen for making this possible.

 

 

A chilly day at Little Hadham

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 those waiting for the results of the GPR survey in the churchyard at Ashwell undertaken just before Christmas.  I need to do some more data processing to see if we have some graves or not.

Meanwhile, the site at Little Hadham has been one to which we have returned on-and-off almost since the beginning of the project.  (Use the drop-down box on the right of this page to see previous posts on this site.)  The site clearly extended beyond the area we had surveyed so far, and so CAGG once more teamed-up with members of the Braughing Archaeological Group (itself part of the East Herts Archaeological Society).  We could only access one of the fields, the one we first surveyed in April 2014.  Our aim was to extend the survey to the edges of the field, and to examine one group of features we had detected with the mag previously with an earth resistance survey.

The morning was cold and crisp with a hard frost.  The thermometer in Jim’s car registered minus 3 Celsius.  Brrrrrr…  Unfortunately, the frost melted quite quickly and muddy wheels on the mag became a problem (Fig. 1)!

Figure 1: Nigel wheels the mag across the field.

The mag team completed six grids, five of which were partials.  Given the amount of time spent trowelling the mud off the wheels, this was a good haul.  We have four partial grids left to complete the field.  The results are given in Figure 2.

Figure 2: the mag survey.

The areas completed yesterday were the block to the far east of the survey area, and the incomplete strip of partials on the southern edge.  In the new area we can see the ditch previously detected (shown by the red arrow) carrying on across the site.  It is fainter in Millfield to the west of the road, but is still evident.  Near the hedge is another clear line of a ditch (marked with the yellow arrow).  This one is worryingly straight and almost parallel to the  field boundary.  It maybe more modern than some of the other features.  The curvy, more irregular ditches (shown with the blue arrow) may be something like a farmstead with boundary ditches.  Picking apart the phasing of all these features is going to be difficult and would require some targeted excavation.

As we had a good sized team we also undertook some Earth Resistance survey (Figure 3).  We targeted one of the possible farmstead enclosures.

Figure 3: Katie Burgess and Peter Baigent (BAG) using the RM85 Earth Resistance meter.

The team completed five 20x20m grids at a 0.5m reading spacing.  The results are shown in Fig. 4, and the underlying mag in Fig. 5.

Figure 4: the Earth Resistance survey results.

Figure 5: the mag results in the area of the Earth Resistance survey.

I had been hoping — rather optimistically — that the resistance survey might pick-up some structures.  Sadly, it did not.  There are, however, correspondences between the mag and res results.  The ditch with the right-angle corner in the mag survey shows well, if slightly more diffuse, in the resistance data.  Similarly, the long curving ditch also shows well. I have indicated one end of it with a blue arrow in Figure 6.

Figure 6: Resistance survey results with arrows.

More curious, however, is the change from low to high resistance along a straight line indicated with a red arrow in Figure 6.  This corresponds exactly with the diagonal line in the mag data which cuts east-nor-east west-sou-west across the D-shaped enclosure. I’m at a loss to know what this represents.  It maybe a reflection of the various cut features in the underlying geology.

At the end of the day we were treated to a beautiful moonrise and sunset.  Not quite the blood wolf moon seen some 12 hours later (when I was tucked-up and asleep in bed!).

Figure 7: Moonrise. Shame about the electric cables!

Many thanks to everyone who turned-out on a freezing but beautiful day.  This site continues to repay our attention, and it worth the effort.  We should try and survey some of it in the summer, however!