Tag Archives: earth resistance survey

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!

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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!

Waffles in Ashwell?

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

Gil Burleigh asked us if we could survey a field in Ashwell. The field is quite small at just 1ha. Some nice finds had come from this field including a nice scabbard chape (Fig. 1) in the 1970s. The field was ploughed for a short period in the 1980s, and North Herts Museums undertook a fieldwalking survey in 1986 which retrieved pottery from many periods with concentrations of Roman, Medieval and post-Medieval material and a thinner scatter of Iron Age and Anglo-Saxon pottery.

Figure 1: scabbard chape © North Herts Museums.

Members of North Herts Archaeological Society excavated three small test trenches over the weekend of the 16/17th March and found archaeological features in two of them. Soil test trenches has also revealed archaeological features in test pits SA01 and SA02 (although they were misinterpreted as just deeper topsoil! Figure 2).

Figure 2: Location of the NHAS test pits (in red) and the soil test pits (SA01 to SA05). © CAGG.

As the site is pasture over chalk, it seemed likely we would get a good result.  Magnetometry was the obvious technique to try (Fig. 3).

Figure 3: Nigel “compensates” the magnetometer. Photo © Mike Smith.

We managed to complete the whole field in a day with the mag, which was excellent progress.  I was exciting to see what we had detected and processed the data that evening (Fig. 4).

Figure 4: the mag survey © CAGG.

My initial reaction was “ugh”.  Why was the data so horribly noisy, like some sort of giant waffle maker? Although we can see some features, especially on the western edge, we cannot see all that much going on, and nothing around where the trenches had revealed archaeological features. Can we explain this?

Peter Alley flew the site with his UAV.  The photos from his flight can be used in two ways: they can be stitched together to produce an orthomosaic (basically an aerial photograph corrected for distortions) or to map the topography.  The orthomosaic is fun, but doesn’t help us solve our problem (Fig. 5).

Figure 5: Orthomosaic from the aerial survey © Peter Alley.

Mapping the topography shows the basic form of the site, basically a flat field on a slope (Fig. 6)!

Figure 6: the topographic survey derived from the UAV images. © Peter Alley.

If we use a hill-shade, however, we can see the micro-topography of the field.  Lighting the field from the SW creates Figure 7.

Figure 7: topography lit from the south west. © Peter Alley.

We can see from Figure 7 that even though the field has not been ploughed since the late 1980s, the plough scars remain running up and down the slope.  If we move the light to shine from the NW we get Figure 8.

Figure 8: topography lit from the NW. © Peter Alley.

Now we can see plough scars running across the slope!  This is, in part, the origin of the noisy data: the cross cutting ploughing has left a surprisingly uneven surface, like the proverbial giant waffle iron.  We should assume this is mirrored by scars in the surface of the underlying chalk and archaeology. One further possible cause of the problem are thirty years worth of ant hills (Fig. 9).

Figure 9: ant hills at Ashwell. © Kris Lockyear.

These aspects of the site, in part, explain the noisy looking data but do not completely explain the lack of contrast between the fills of the features and the chalk.  Gil reported that the metal dectectorists were also having problems with large numbers of responses in this field, so the mystery isn’t completely solved.  A plane did crash in this field during World War Two, but I cannot see how that has created this problem unless the whole field went up in flames.  Ideally, we would take some samples and test the magnetic susceptibility of them.

There are, however, some features in the data, despite the noise.  I have outlined the ones I can see in Figure 10.

Figure 10: magnetometry survey results with interpretation shown in yellow (cf. Fig. 4). © CAGG.

Two of the possible features run down and across the slope in a manner similar to the plough scars, and we must be cautious in their interpretation as a result.  The corner that is protruding from the western field boundary is much more interesting.  It looks like we may have clipped one side of an enclosure. (We seem to specialise in enclosures that don’t enclose things.)  It looks like it probably runs under Ashwell House next door.  In the middle is a “dark blob”.  Initially, I thought this was likely to be metal as it is so close to the edge of the field where the fencing creates a strong response.  Checking the values in TerraSurveyor, the range of values towards the east of the blob is from -2nT to +12nT.  If one goes right to the edge where one can see the impact of the fence on either side, the values jump to about -4nT to +26nT. Contrary to my initial thought, this might well be an archaeological feature in the enclosure.

On the Saturday we completed a 40x40m block of Earth Resistance survey in the NE corner of the field.  Not a great deal showed!  We decided to persist on the Sunday so that we could include the area of the “enclosure”.  We had some unusual help (Fig. 11).

Figure 11: Unusual help with the Earth Resistance survey. © Kris Lockyear.

As has become our standard method, we used the Earth Resistance meter on the 1+2 setting.  In other words, every time we stick the mobile probes in the ground the machine takes three readings.  One uses the two outer probes which are a meter apart to measure down to about a meter or so.  The other two readings use the inner probe to take two side-by-side readings with a 0.5m probe separation, looking about 50–70cm into the ground (Fig. 12). Given the topsoil is only about 30cm deep, this should be fine.

Figure 12: the Earth Resistance survey underway. Photo © Gil Burleigh.

The results, were, underwhelming… (Figs 13 and 14).

Figure 13: Earth Resistance survey with 0.5m mobile probe spacing.

Figure 14: the Earth Resistance survey, 1.0m mobile probe spacing.

I can only conclude that the difference in moisture retention between the features and the chalk was minimal.  Peter Alley made the excellent suggestion that the banding we can see is possibly related to the layers within the chalk geology.

We have one last source of data to examine.  In 1986 Keith Fitzpatrick-Matthews undertook a field walking survey here for North Herts Museum Service.  Obviously, he didn’t have the luxury of a GPS to log his finds or lay in his grid.  We can, however, roughly position the results on the map with a bit of careful editing in a drawing package.  Figures 15–17 show the three most common types of pottery: Romano-British, Medieval and post-Medieval.

Figure 15: Romano-British pottery distribution © North Herts Museum Service.

Figure 16: Medieval pottery distribution © North Herts Museum Service.

Figure 17: Post-Medieval pottery distribution © North Herts Museum Service.

The post-Med distribution doesn’t seem to fit our survey results.  Both the Roman and Medieval ones, however, are temptingly close to the feature on the western side.  By making the distributions transparent we can see how these match (Figs. 18 and 19).

Figure 18: Romano-British pottery overlain on the mag data © North Herts Museum Service and CAGG.

Figure 19: Medieval pottery overlain on the mag data © North Herts Museum Service and CAGG.

Both the Romano-British and Medieval pottery distributions lie just to the north of the “enclosure”, down slope from it. This is what one would expect if the material collected on the surfaces derives from the fills of the features. We can only be sure of the date of this feature by further excavation.

Although this survey has not produced the beautifully clear results we have had from other sites such as Kelshall, it has shown some interesting features.  It is also a good example of the value of combining different data sets, in this case field walking, aerial photogrammetry and magnetometry.

Many thanks to everyone who helped with the survey and fetched and carried equipment.  Also, thanks for Gil for suggesting we have a look at this site and David Short for allowing us access to the site.  As 19 images seems an odd number to finish on, I thought I would sign off with another photograph of one of David’s magnificent sheep (Figure 20)!

Figure 20: A sheep. Photo © Kris Lockyear.

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!

Neat and tidy

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

Due to being rained off on our last day, a small team of us decided to go out and finish off some things on Bank Holiday Monday. Many thanks to Pauline, Judith, Ruth, Dave and Jim for turning out to do “just one more grid.”  I think it must be a geophysicists ailment that we always would like to be able to just a little bit more…

The mag team completed an impressive ten grids including two awkward partials.  Figure 1 shows the entire survey at the end of the 2018 season.

Figure 1: the mag survey after day 19.

The team have managed to add 19 ha to the survey in the last month.  Figure 2 shows the southern area that we have been surveying this week.  (This field is, confusingly, called “Prae Wood”.)

Figure 2: the southern area (Prae Wood) after day 19.

The team have picked-up an area of intense ferrous noise.  This looks like a small historic period site.  We will have to check out some old maps to see if we can work out what that might be.  The one hiccup in a brilliant last day of work is a single line of data where the sensor froze.  It is very annoying and I’ll have to find some way of fudging that until next summer!

The Earth Resistance survey had one last little block left to make the plot look all neat and tidy.  Many thanks to Pauline and Judith for helping me fill that in (Figures 3 and 4)!

Figure 3: Kris, Judith and Pauline (out of shot) extended the resistance survey. Image © Mike Smith.

Figure 4: the main block of Earth Resistance data collected 2016–2018.

The data collected shows some faint indications of buildings in that corner (Figure 5).

Figure 5: the northern area of the res survey. The NW corner was completed on day 19.

Although my trick of spreading the remote probes wide apart has worked on the whole, this year there is a bit of an edge.  This is because we started with a block in the SW corner, worked eastwards, and then when we had got to the corner, worked back along the hedge line westwards.  Between when we started this block and yesterday we have had in excess of 100mm of rain (or about 4 inches in old money) so it isn’t surprising this shows in the results.

We have now cleared away all the pegs and flags, packed-up the machines and left Gorhambury for another year.  It is a beautiful place to work and we are very grateful to Lord and Lady Verulam and their family for allowing us to extend the survey, to those who work the estate and put up with us getting in the way, and to the estate managers, especially Stuart Gray. Thanks to the Institute of Archaeology, UCL, for lending us the dGPS and the res meter, and SEAHA for the loan of the GPR.  I hope everyone involved thinks the results are worth all the effort. Most of all I would also like to thank all the volunteers who came this year, whether you only managed a day or two, or you came for the whole season.  You are what makes this project so much fun!

 

“The way I see it, if you want the rainbow, you gotta put up with the rain.”

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

As I start this entry of the blog, the rain is splashing against my windows as was predicted by the Met Office. Although we might question Dolly Parton’s grammar, the sentiment seems true enough.  Yesterday, however, was a superb day with all three techniques collecting data across the site.

After yesterday’s excellent results, the GPR crew had great expectations.  The only problem was a tree in the way under which the shepherdess had put hay when the grass in the field was dead from lack of rain.  Unfortunately, sheep mean sheep droppings (Figures 1 and 2).

Figure 1: Mike on sheep poo removal duty.

Figure 2: Check out those wheels!

Luckily for everyone concerned, I think the effort was worth it (see Figure 3)!

Figure 3: GPR time slices from Day 18.

I could misquote Dolly along the lines of putting-up with sheep poo if you want excellent GPR results but I might be pushing my luck…  The many buildings are quite obvious in this data set.

Figure 4 shows this grid in context of the other GPR grids in this area.

Figure 4: GPR results including the day 18 data (SW corner).

We have added a very large number of new buildings to the map of Verulamium.  As I was only just starting with GPR data when we started collecting it in 2015, the processing keeps changing a bit from block to block. One of my jobs is to start from scratch and reprocess the whole thing so that the maps are consistent.  Should keep me busy for a while.  Figure 5 is a crude mosaic of images just showing the entire area surveyed so far.

Figure 5: crude mosaic of GPR time slices at the end of the 2018 season.

This represents 19ha of GPR data collected at 0.5m transect intervals.  Just pushing the machine along the lines, not including getting to the block, setting-up, moving strings etc. is 380km.  It also means 380km of radargrams!  No wonder the data takes-up 33gb of my hard disk and consists of over 70,000 files.

The mag team completed nine 40x40m grid squares which is 1.44 hectares.  Excellent progress!

Figure 6: the mag team in the southern field.

Figure 7 shows the whole of the 2018 survey (along with a big chunk of Verulamium).

Figure 7: the mag survey after day 18.

Even though we have been using the machine for some years now, and it does have its frustrations, when all is going well we can really cover some ground.  The season was planned for 20 days: we lost 3 days to rain, and most of a day to testing the mag at the start.  Despite this, the team have managed to collect 17.7 hectares of mag data.  Without actually getting to the grids and back (which is quite a bit of walking in itself), the team have pushed the cart 88.5km over the past four weeks.

Figure 8 shows the southern area in more detail.

Figure 8: the southern area of mag data after day 18.

The blue arrows in Figure 8 indicate the lines of old field boundaries.  These can be seen on old maps such as the 1699 parish map.  The yellow arrows mark ferrous objects.  Some are very big, but there are a scatter of smaller ones too.  Last, but definitely not least, there are a few magnetic features which may be archaeological, such as pits.  I have picked a few out with red arrows.  Although they look small at this scale, they are probably 1m to 2m across, a quite respectable size for a pit.

Although large mainly  blank areas are disappointing to collect, they are important nonetheless. The immediate environs of Verulamium are extremely rich, archaeologically. The field lies:

  • 360m W of the busy area of buildings recorded by the GPR discussed above;
  • 600m NE of the major Iron Age settlement at Prae Wood;
  • 600m N of the fields at Windridge Farm where metal detecting rallies have taken place;
  • 500m NW of the major cemetery at King Harry Lane;
  • 1,100m SE of Gorhambury Roman villa;
  • 1,000m NE of the new villa found at Windridge Farm.

Also, the Fosse, which is preserved in the woodland along the NE edge of the field, is a really very impressive earthwork.   We just seem to have hit an empty bit of landscape between all these sites!

The res survey now covers some 6.58ha, that is about 263,200 earth resistance readings.  Not into the millions like the mag and GPR, but this is res after all!  Figure 9 shows the entire survey.

Figure 10: the entire Earth Resistance survey after day 18.

At this scale the roads show very nicely as do some of the more substantial buildings.  Figure 11 is the area surveyed in 2018.

Figure 11: Res survey after day 18.

Given that the fields were baked hard and the grass was dead at the start of the season, I am pleased we managed any Earth Resistance survey at all this season.  The team yesterday put-up with my geophysics OCD and completed right into the corner by the theatre. We then doubled-back and started filling-in between the top of the survey block and the drive.  We have picked-up some parts of buildings seen in grids to the south, but in general along the edge the deep colluvium, as shown by the sunken nature of the drive, is to some extent masking the archaeology.

Many thanks to everyone on the team who made the 2018 season such a success.  A especially big thanks to those who helped move the equipment about including Ellen, Mike, Jim and Ruth.

For those who haven’t been involved but would like to join future surveys, do get in touch.  We are a friendly group, and provide on-the-job training.

And finally… (as they used to say on the news)