Tag Archives: GPR

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!

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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.

 

 

“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)

 

The end is nigh?

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

In this case, two ends: we have just started the final week of the 2018 survey season and the mag team are within two partials of completing as much as we can of Mobbs Hole and moving into the field to the south.  First to the mag.

After the annoying plethora of frozen sensors, the mag team spent a good proportion of their day re-doing duff squares.  It was worth it, however, as today’s data looks fine (Figure 1).

Figure 1: the mag survey in Mobbs Hole at the end of Day 15.

Although we can be pleased with the area we have covered, surprisingly little apart from the Fosse itself and related features show.  We must keep in mind, as Isobel Thompson reminded me this morning, that “even such negative evidence is information”.  Negative information may be important, but at the end of a long day’s survey some tasty looking buildings would be nice.  Figure 2 shows one possibility, although we may be grasping at straws!

Figure 2: a possible building in Mobbs Hole?

The Earth Resistance survey takes fourth place in priority after surveying in pegs, mag and GPR.  Anne and I did, however, manage to extend the main block of res data by another three grids.  Figure 3 shows the results.

Figure 3: the Earth Resistance survey after day 15.

As you can see, we have picked-up some more of the building to the east, but also part of Street 25 running SW–NE.  There is quite a break in the line of the street which is curious.  Figure 4 shows the GPR data in this area.

Figure 4: the GPR data in the area of the res survey. The red box marks the outline of the 2018 survey after day 15.

It is useful to note that some parts of the buildings show more clearly in the res data, and some in the GPR thus making the extra effort of doing res as well worth while.  The GPR data also shows a break in the road.  Figure 5 shows the mag data.

Figure 5: the mag data. The red box shows the 2018 res survey area after day 15, and the blue line the course of the aqueduct.

Note how the buildings that show clearly in the res/GPR barely show in the mag data, but how the “burnt building” (assuming my interpretation is correct) only shows in the mag data.  Multiple techniques rule, OK?  I have roughly marked the line of the aqueduct in Figure 5.  Let’s now look at how that maps back onto the res data (Figure 6).

Figure 6: the Earth Resistance data with the line of the aqueduct indicated.

Not only does the aqueduct kink around the two buildings as we noted in an earlier post, but it goes through the break in the road.  I guess there could be a wooden bridge (which we would not detect) or maybe a culvert where the roof has collapsed or has been robbed. Fascinating stuff.

The GPR crew in their machine-like fashion completed yet another 80x40m block.  Figure 7 shows six time slices.

Figure 7: GPR survey, day 15, six time slices.

Most of the action, so to speak, is in the NE corner.  There is a particularly clear corner in the fourth time slice indicated with a red arrow (Figure 7, top-right slice).  This might be a surviving floor. There also appears to be a long linear negative feature, as shown in the fifth time slice by three red arrows.  Figures 8 and 9 show slices 4 and 5 in context with the day 14 data.

Figure 8: GPR data from days 14 and 15, slice 4.

Figure 9: GPR data from days 14 and 15, slice 5.

Three things caught my eye.  The squarish “floor” which crossed over the boundary between the two days data, the sub-circular white “blob” which also lies across the boundary, and the long linear low-reflection feature (shown in white) which runs diagonally SW–NE across the lower half. I traced the square and the blob and had a look at the mag data (Figure 10, click on it to see full-sized).

Figure 10: the mag data with the “square” and the “blob” outlined.

The white blob corresponds with a faint “blob” of higher readings in the mag data.  On its own, I would have been tempted to ignore this, but it does look like a feature about 6m across.  The square is harder to assess.  There are magnetic features parallel to it and close by.  We are probably looking at parts of a building.  I had a quick look at the radargrams and the square high-reflectance feature in the GPR data looks like a solid layer, probably a floor.  I also noticed the long linear ditch-like feature running across the mag data, so I traced that and went back to the GPR data (Figure 11).

Figure 11: GPR data with the linear feature seen in the mag data highlighted.

The linear feature in the mag data fits the linear feature in the GPR data perfectly.  Lovely result.

It was a busy day surveying today, and so I didn’t have time to goof off and take photos of people or the views.  Maybe tomorrow!

Thanks to everyone who helped today.

 

 

One hundred and fifty

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

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

Figure 1: the mag survey after day 9.

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

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

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

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

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

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

Figure 4: Earth Resistance survey in action.

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

Figure 5: Earth resistance survey compared to GPR results.

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

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

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

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

A light wind swept over the corn, and all nature laughed in the sunshine.

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

Not much sign of corn, but the wind was blowing over the parched grass.  It was a welcome relief to be working in cooler conditions, although the weather was still beautiful.

Figure 1: St Albans Abbey.

The mag team completed an excellent eight grids in Mobbs Hole (Fig. 2).

Figure 2: the mag survey after day 6.

As before I have overlain the survey on the 2006 imagery in Google Earth which shows the Fosse most clearly.  The hints of a line along the inner edge.  Maybe this is the remains of a palisade trench?  Disappointingly little otherwise.  One thing to note is that the “noise” from random ferrous trash is more prevalent to the south of the old fence line than to the north.  I wonder if the NW corner of this field was pasture previously?

The GPR crew completed their two grids.  Figure 3 shows some time slices.

Figure 3: Day 6 GPR results.

Not a great deal showing apart from in slice 4 (in the top-right corner of Figure 3) which clearly shows the aqueduct.  This is about as clear as I have ever seen it in GPR data.  Awkwardly, the direction of the transects is close to the direction of the aqueduct.  Figure 4 shows the slice in context with the others we have surveyed this year.

Figure 4: GPR day 6, slice 4.

The edges do not match because I keep playing with the settings in the software.  One day, I’ll slowly process the whole lot so that we get a nice final result.  One day (more like several months…).

Barney and Becca came and helped with the GPR in the morning.  After lunch, we blew the dust off our Bartington and did a couple of squares.  I rather liked this image of Barney.

Figure 5: Barney and a Bartington.

The survey moves on and we cover more ground.  Many thanks to everyone who comes and helps expand the area we have covered.  Luckily, most of the area you can see in the last inage has already been done!

Figure 6: Ellen and Mike operate the GPR.

 

Sawtooth Saturday and other tales

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

The lack of posts doesn’t mean we haven’t been out working…

Enough of that!  The lack of posts is simply that we had a friend staying and a BBQ and beer won over working on the blog.  Sorry…  Well, not very sorry.

The mag team have been working southwards across the “Fosse field”, the proper name for which is Mobbs Hole.  The area we have covered in around Verulamium is getting pretty large (Fig. 1)!

Figure 1: total area surveyed to date.

The survey has now started to clip the edge of the Fosse itself. In Figure 2 I have used the 2006 imagery in which one can see the Fosse clearly as a soil mark.

Figure 2: the mag data on the 2006 imagery in Google Earth.

Progress is excellent for four and a half days of survey.  Figure 3 shows the survey in more detail.

Figure 3: the mag data after day 5.

The edges of the Fosse can be seen in the mag data, mainly as a lighter line.  This is because the topsoil will be thinner over the lip of the ditch as soil has eroded down into the fill.  There is a line of large dark “blobs” along the lip of the Fosse.  Although these might be something interesting, I suspect they are tree-throws (i.e., the hole made by a tree being blown over).

The field system which shows in the upper half of the survey is interesting.  Jon Mein has kindly shown me the 1799 map of the parish and these boundaries do not match those mapped then.  There is a line of woodland following the line of the Fosse a little to the south of the area we have reached.  The track which runs along the northern edge of Mobbs Hole was a much more important road at that date.

Wheeler thought that the Fosse represented the “first Roman city” at Verulamium and cut several sections across it (Figures 4, 5 and 6).

Figure 4: Wheeler and Wheeler 1936, plate 109, detail showing the location of the sections excavated by the Wheelers.

Figure 5: Wheeler and Wheeler 1936, plate 18. Sections across the Fosse.

Figure 6: Wheeler and Wheeler 1936, plate 78. Sections across the Fosse.

We now know that the early Roman town was based down towards the forum where the Museum now is.  The Fosse, however, does appear from Wheeler’s finds to be first century.  But what was it for?  Hopefully, the geophysics within the line of the Fosse may give us a clue.

One aspect of the landscape I had not appreciated was that the dry valley which the aqueduct has to dog-leg across as shown in our survey (the V-shaped long linear feature within the town walls shown in Figure 2), becomes quite a major feature to the west of the walls.  The northern arm of the Fosse lies on the crest between the dry valley and the valley of the Ver, and then when it turns to form the southern arm it has to cross that valley.  Figures 7 and 8 compares the plate published by the Wheelers showing the view from the crest to a panorama I took the other day.  The hedge line shown in the Wheelers’ plate is no longer there.

Figure 7: view southward across the Fosse as published by the Wheelers in 1936.

Figure 8: Panorama of Mobbs Hole (click to see full sized).

Back inside the town walls the GPR crew completed an awkward couple of blocks on “sawtooth” Saturday and another 80 x 40m block on Sunday.  Figures 9 and 10 show some of the time slices.

Figure 9: the GPR from day 4.

Figure 10: GPR time slices from day 5.

On neither day do we have some of the beautifully clear buildings we have seen previously.  There does seem to be a small square structure showing on day 5: the NW edges showing in slice 8 (the sixth image in Figure 10) and the bottom edge in slice 10.  Figures 11 and 12 show slices 5 and 6 from days 2 to 5 in Google Earth.

Figure 11: GPR days 2 to 5, slice 5.

Figure 12: GPR days 2 to 5, slice 6.

Although we have not got lovely clear buildings, the “blobby” bits do have a generally SW — NE orientation which matches the town grid.  It is, however, very difficult to interpret.  Looking carefully, however, there is more than immediately catches the eye.  Lets look at the day 5 data more closely.

Figure 13: GPR day 5, slice 6.

It all seems rather dull.  Now plot that on top of the mag data.  Look at the point the red arrow is indicating in Figure 14.  Figure 15 just shows the mag data.

Figure 14: Day 5 GPR overlain on the mag data.

Figure 15: the mag data from the same area as Figure 14.

The feature which shows quite clearly in the mag data does show quite faintly in the GPR.  The moral: lovely clear buildings are wonderful, but sometimes there is more there than you think.

We have been enjoying our two days off, and will be back at it on Wednesday.  The forecast is for it to be a bit cooler.  Luckily, our guard dog has been keeping a close eye on the flags…

Figure 16: Guard Dog.

Many thanks to everyone involved.  The heat has made it quite hard work, but the survey looks ever more amazing with each new grid square!