Tag Archives: GPR

Pipe dreams

Yes, I know, blog posts are like busses…

Some of you may have seen a recent BBC News item about the holes for the gas pipe in Verulamium Park.

Fig. 1: BBC News item about Verulamium Park.

I must admit to being a little peeved that a few small roughly dug holes to look at a gas pipe got more media attention than CAGG’s extensive surveys in Verulamium!  The gas pipe has always been, however, the bane of our lives as it’s magnetic field is so strong it masks a 30m wide strip of the archaeology in our magnetometer survey.  That field, however, has no impact on the GPR and I have been intending to extend the GPR survey to cover the areas affected by the pipe. This, and the forthcoming meeting of the Roman Finds Group in Verulamium this Friday, prompted me to see if anyone could come out at short notice and survey around one of the pipe holes.

Fig. 2: Surveying the pipeline (photo: Mike Smith).

So, Mike Smith and I spent a day surveying around the site of the hole in which the traces of a Roman building were discovered.  It got a little fiddly at times, especially as the pipe had to be dragged along 11m every 30 minutes as they joined the lengths together.  The location of our new survey is given in Fig. 3.

Fig. 3: the location of the latest GPR survey shown in red.

As usual, we surveyed in 40m lengths (where possible) at 0.5m transect spacing.  The data were sliced using GPR Process and the images produced using Surfer v. 8, and imported into Google Earth via Adobe Photoshop.  The last step is needed to make the missing bits transparent. Unusually, the deeper slices (nos. 8 and 9) still showed useful information.

One annoying thing is that Google Earth have recently updated the satellite image for Verulamium.  It is a bit dark to my eyes, but more seriously it is in the wrong place.  In the time slices I could see the relationship between the path and the image was clearly wrong.  I spent some time checking my notes, but when I checked the historical imagery, I found the 2009 images matched the plots much better.  I have, therefore, used the older imagery for the figures below.

Fig 4: the magnetometer survey in the area of the new GPR survey.

In Fig. 4 we can see the magnetometer survey for the area in question.  The pipe line, running under the path and then diagonally across the football pitch, dominates the plot.  We can however, see the road running clearly SW–NE across the map (Niblett and Thompson’s Streets 16 and 17 crossing Street 12), and bit of buildings showing as the white lines against the grey and black.  As well as the gas main, there are three pipelines running from the cafe in the NE corner.

Fig. 5: third time slice, 10.5 to 13.5ns.

In Fig. 5, the topmost time slice in the sequence, we can already see the road surface running SW–NE, and we can also see signs of the pipe running across the plot although this might be a marker in the upper levels as the pipe itself shows more clearly lower down.

Fig. 6: fourth time slice, 13.5 to 16.5ns.

The next slice (Fig. 6) shows the road more clearly, and some hints of buildings.  I am unsure what the area of high reflections is to the south-east and parallel to the road is.  It is almost as if the road line was changed a little at some point.

Fig. 7: fifth time slice, 16.5 to 19.5ns.

Fig. 7 is starting to show the buildings up near the path much more clearly.  These are buildings that front onto Street 12 to the north.  Street 12 eventually leads into the forum and is probably part of Watling Street.  At the southern edge of the survey there are some white areas which may be robbed walls, with perhaps a surviving floor surface.

Fig. 8: sixth time slice, 19.5 to 22.5ns.

In Fig 8. we can now see two very clear lines either side of the road.  These maybe stone conduits or roadside drains?  The buildings near the path are starting to show more clearly, and now we can see the gas pipe quite clearer.  The fact that is disappears to the west is probably due to topography.  Ideally, time slices would be topographically corrected.

Fig. 9: seventh time slice, 22.5 to 25.5ns.

Fig. 9 shows the gas main beautifully, and more of the buildings near the path.  Those to the east of Street 16 and south of Street 12 are labelled “baths” by Niblett and Thompson.  The GPR data would make that interpretation a little more tentative, I think.

Fig. 10: eighth time slice, 25.5 to 28.5ns.

Fig. 11: ninth time slice, 28.5 to 31.5ns.

Figs 10 and 11 show some of the deeper features.  Some hints of more buildings are showing, especially in the NW corner of the plot.

Personally, I think this is much more interesting and informative than the gas pipe trench!

Fig. 12: the gas pipe.

The one thing that will be useful, is that by knowing how deep the gas pipe is below the surface, we can calibrate the speed of the GPR signal and convert nanoseconds into depth.

I am hoping to continue to expand the GPR survey when I can over the coming months.

Many thanks to Mike for coming out for the day in the cold sunshine.

CAGG on tour: Chisbury, Wiltshire

At the request of Andrew Reynolds and Stuart Brookes, medieval archaeologists at the Institute of Archaeology, UCL, members of CAGG headed out on tour to Wiltshire.  The site we were asked to survey was the Iron Age hillfort of Chisbury, near Great Bedwyn, Wiltshire.  One may ask why Chisbury is of interest to early medievalists?  An early 10th century document called The Burghal Hidage records a site called Cissanbyrig (Baker and Brookes 2013, p. 228) which may be Chisbury.  The Burghal Hidage records the defended settlements (burhs) of the Kingdom of Wessex set-up after the defeat of the Vikings in the late 9th century.  The Historic England Scheduled Monument listing notes that:

Although no formal excavations have been carried out within the hillfort, observation of 20th century disturbances has produced evidence of urns, bronze swords and of storage pits containing Late Iron Age and Romano- British pottery.

The site also has a well preserved 13th century chapel under HE Guardianship.

Fig. 1: the chapel at Chisbury.

The interior is quite plain but has some interesting details.

Fig. 2: the interior of the chapel at Chisbury.

One of those details is a surviving consecration cross painted on the back wall.  Apparently these were painted on the wall when the church was dedicated.

Fig. 3: The consecration cross.

Peter Alley’s high-level photograph from the UAV shows how the chapel lies right across the defenses of the hillfort.  Maybe this position was significant in somehow slighting the earthworks?

Fig. 4: high level photo showing the location of the chapel.

The defenses are very well preserved around most of the circuit but they are covered in trees so hard to see and photograph.  From the satellite image the site is an extended oval of trees.

Fig. 5: Chisbury from Google Earth.

Jim West took a good image showing part of the defences to the west.

Fig. 6: Chisbury defences from the west (photo: Jim West).

The weather was wonderful and we all got a bit sun burnt.  The bluebells and primroses were out in force.

Fig. 7: bluebells on the defences.

The theme of the week was, however, definitely “horses”!

Fig. 8: Nosey horsey.

Unfortunately, horses are quite magnetic due to their ferrous footwear.

Fig. 9: Horseshoe alert! (Photo: Mike Smith).

Those horses are also a bit careless with their shoes…

Fig. 10: Missing footwear.

The plan was to complete as much of the inside of the fort as we were able with the magnetometer, and to do some selected areas with the GPR and the Earth Resistance meter. Right from the beginning we were beset with problems. We arrived at lunchtime on the Thursday, and managed to complete quite a few squares in the first afternoon, but the odometer started to over-run, eventually by four or five meters.  I swapped a few emails with Pat Johnson from Foerster, and the next day we managed to cure the problem.  A couple of days later, one of the pins in the “spider” — the cable that joins the sensors to the control box — snapped so we were down to three sensors.  On the last full day, the odometer started slipping again…  We did manage to survey the whole available area of the fort, but only just and without much time to try the other methods.  Fig. 11 shows the overall survey.

Fig. 11: magnetometry survey.

Although some of the major features can be seen at this scale, I have created two images with the north and south parts of the survey and some annotations.  (You might like to look at these downloaded and at full size.)

Fig. 12: the mag survey of the northern part of the site.

In Fig. 12 we can see a series of parallel linear features which have been annotated in cyan. These look like field drains to me.  Very faintly, however, there are some circular features.  These may well be the “drip gulleys” of Iron Age circular round houses.  I have marked some of the possible ones in red.  The problem with these is that the more one stares, the more one invents!  I am sure you can spot a few more possibles if you look long enough.  There is a great deal of ferrous noise, especially around the edges from fences, gates and water tanks, but also in the field from old nuts and bolts, horse shoes and the like.  Looking carefully and the little blobs and measuring the minimum and maximum values in nanoteslas (the unit of magnetism), one can start differentiating between bits of old iron and possible pits.  I have marked a small number of the possible pits with green arrows.

Fig. 13: the mag survey of the southern part of the site.

In Fig. 13 the red line is the pipe which joins the main water tank in the middle of the field.  I have marked just one piece of ferrous rubbish with a red arrow, there are lots more.  The cyan lines mark the possible field drains.  The dark blue line is a negative-magnetism feature which runs from the edge of the water tank to the pond.  This can also be seen in the GPR data (below).  I am guessing this is some sort of drainage / outflow from the water tank to the pond. There are some areas with such high ferrous noise it is impossible to see anything, for example the north end of the eastern field.  There are, however, quite a few pits once more, and I have marked just a small selection.  What is curious, however, is how much of this area seems devoid of any features at all.

Although the mag results are not exciting in the sense of being able to clearly see a building, as we often do at Verulamium, there is quite a bit of detailed information buried in the data.

We managed just a couple of days of GPR survey thanks to some local help.

Fig. 14: the GPR survey underway. (Photo: Mike Smith.)

I used the GPR Process and Surfer programs and created 3ns time slices.

Fig. 15: GPR time slice 3 from 10.5 to 13.5ns.

Fig. 16: GPR time slice 4 from 13.5 to 16.5ns.

Fig. 17: GPR time slice 5 from 16.5 to 19.5ns.

Fig. 18: GPR time slice 6 from 19.5 to 22.5ns.

Fig. 19: GPR time slice 7 from 22.5 to 25.5ns.

Fig. 20: GPR time slice 8 from 25.5 to 28.5ns.

To be frank, not a great deal shows.  The last time slice shows the suggested pipe from the tank to the pond.  The southern area has more high reflections, especially in slice 5 (Fig. 17) which one could try to make into buildings, but I find myself suspicious that these features are close to the water tank and they may be something to do with the tank’s construction.

Giving the billing this site has, the results are not all that stunning.  There are, however, features which would be worth investigating further, and hopefully we will get to “ground truth” some of these features as part of Andrew Reynolds and Stuart Brookes’ wider project.

Many thanks to those CAGG members who came all this way to do the survey: Ellen Shlasko, Ruth Halliwell, Peter Alley, Jim West, Nigel Harper-Scott and Mike Smith.  Many thanks too to the members of the local group who came to help: Shaun Wilson, James Kay and Lynn Amadio. Lastly, but certainly not least, thanks to the stud and the farm for allowing us to play on their land.

The results from Darrowfield

First of all, apologies for the delay to this post. Life has been a bit busy, and the data processing got a bit complicated, but I think I have cracked it now.  Our principal aim was to complete the area using magnetometry.  This we did easily.  We also aimed to try and cover as much as possible using GPR and Earth Resistance.  We managed quite a bit of GPR, but not much resistance survey, partly due to manpower and time, and partly due to the stunningly uninteresting results.  Lastly, we did two resistivity pseudo-sections and two topographic surveys, one using the dGPS and one using a UAV and “structure from motion”.  So be prepared for a long post!


I am going to start by discussing the topography because this has an impact on the results and the interpretation.  We did two types of topographic surveying.  Firstly, I walked back and forth taking readings with the dGPS at six-pace intervals, or closer if there was a change of slope such as a bank.  Secondly, Peter Alley used his DJI Phantom UAV to take a series of aerial photographs which can be used to create a topographic model using a technique called “structure from motion”.

Fig. 1: Peter Alley landing his UAV after taking aerial images of the site.

The two techniques have their strengths and weaknesses.  The model using SfM can be very detailed, far more so than is easily possible with the dGPS, but the model is of the top of the vegetation.  The camera cannot see through the stinging nettles.  Mostly, this does not matter but there are areas where one has to be aware that the nettles are masking the topography.  I, however, could not easily get dGPS readings in the garden or the orchard because the trees blocked the satellite signals.  The UAV could, however, map these areas. Using the two methods side-by-side is thus a useful thing to do.  The other big advantage of the UAV is that the survey took about 20 minutes compared to almost a day using the dGPS!

Fig 2: Darrowfield paddock topographic model from the dGPS survey.

Fig. 3: Topographic map derived from the UAV survey.

Ignoring the areas outside the paddock / gardens in Fig. 3, we can see that the two surveys show pretty much the same thing.  The highest ground is the bank along the SW edge of the paddock with a steep slope along one edge.  The bank is less pronounced as it curves around through the garden and the orchard in the NW corner of the area.  I had initially thought this was a road line, but it doesn’t line up with the roads seen in the wider magnetometer surveys of the park.  It does seem to be mapped as a long thin enclosure on early OS maps. There is a second, much less pronounced bank along the NE side of the paddock, which fades out towards the east corner.  This bank is assumed to be the line of the Roman wall. Between the two higher areas is a lower lying area in the middle of the field.  In the east corner, the UAV seems to show a channel running west-east into the corner (Fig. 3, the dark blue line running through a lighter blue area).  This is a footpath running through an area of high stinging nettles.  On the dGPS survey (Fig. 2) we can see a low channel running north-south in the same area — shown in red — which is masked by the nettles.

The magnetometer survey

The main area of the paddock was surveyed on the normal grid we have been using at Verulamium.  The two small extra areas, on the lower lawn of Darrowfield House and the orchard, were surveyed using small floating grids set-up to fit the awkward areas better.  In the paddock, nettles, rabbit holes and badger burrows all made surveying a bit awkward.  Very many thanks to Pat Johnson of Foerster for bringing the repaired magnetometer down to site from Tamworth.

Fig. 4: Ruth Halliwell and Jim West surveying the Lower Lawn of Darrowfield House.

Given where we are in the Roman town, i.e., just behind the basilica in the centre of the the city, the results were surprisingly unexciting!

Fig 5: the magnetometry results.

The majority of the area seems to be dominated by a faint striping running SW-NE across the middle of the field.  I am guessing these are cultivation marks, and this interpretation is supported, as we will see below, by the GPR results.

Fig. 6: annotated magnetometry results.

In Fig. 6, at point A, we can see a curving “linear magnetic anomaly”.  This matches the curving bank seen in both topo surveys (Figs. 2 and 3).  The clearest indications of buildings are along the SW edge of the paddock, at B.  The light lines mark walls, and the darker areas magnetically enhanced materials between the walls. It is difficult to know if these are Roman or buildings associated with the farm.  If, however, we project the line of the roads from the wider survey (Fig. 7), it seems likely these are Roman buildings lining the SE–NW running road.

Fig. 7: rough projection of the Roman roads into Darrowfield Paddock.

At Fig. 6, C there appears to be a linear feature running across the lawn.  At the west end is a black dot with a bright white halo.  This is something ferrous, it has a range of -104nT to 2371nT!  The linear feature next to it only has readings from -5nT to +15nT and is much more likely to be something like a ditch.  Just inside this linear feature are three black “blobs” which have similar ranges and may be pits.

The steep bank shown on the topographic maps which runs parallel to the SW hedge has a linear low magnetic feature (Fig. 6, D).  This may just an artefact of the bank, but I am beginning to wonder if there is something holding the bank up, especially in the light of the GPR results below.

Lastly, At Fig. 6, E there are two faint linear low magnetic features.  Is this a hint of the wall line?

The GPR survey

Between the animal burrows and the nettles, the GPR survey could be quite a struggle…

Fig. 8: Jim versus the nettles at Darrowfield (image: Mike Smith).

We did, however, get quite a bit of the paddock done.  As usual, I have used Larry Conyers’ GPR Process program and Surfer v.8 to produce the time slices.  I have created five slices 3ns in thickness.  I’ll discuss them in order from the top down.

Fig. 9: time slice 3, 10.5–13.5ns.

The topmost time slice shows the bank parallel to the SW hedge very clearly indeed.  The clarity of this feature makes me think that this bank must be artificially reinforced to create the flat space between it and the hedge. Perhaps this was part of the entrance to the house, or something to make a view from it?  The other feature is the very clear line to the east which matches where the bank curves in the topo data.  I’m unsure what would make a line like this.  It is very close to the surface being in the top slice.

Fig. 10: time slice 4, 13.5–16.5ns.

In the next time slice (Fig. 10), the bank to the SW has turned into two linear features.  The bank to the NW shows as less clear linear features as well.  There are a series of lines running SE-NE across the lower part of the field.  These become clearer in the lower slices.  Finally, in the middle of the NE edge, cut into by the “dog tooth” pattern of the survey edges, is a broader linear feature running NW-SE.  This may be a hint of the town wall.

Fig. 11: time slice 5, 16.5–19.5ns.

In the next slice (Fig. 11), the parallel lines in the lower area are now very clear.  These seem most likely to be some form system of land drains.  To the south are some other linear features which may be the tops of surviving walls.

Fig. 12: time slice 6, 19.5–22.5ns.

In the next slice (Fig. 12) we are getting below the drains, but the signal is starting to attenuate.  There are, however, some linear features to the south and the west which are parts of buildings lying quite low.  A number of the “wiggly” features, particularly to the north, are badger setts.

Fig. 13: time slice 7, 22.5–25.5ns.

The last time slice (Fig. 13) finally shows the buildings that can be seen along the SW edge in the mag data.  The GPR cannot really push into the undergrowth as well as the mag and so the evidence is a little more bitty.  We can see, however, the road which runs from the SW to the NE, almost parallel to SE hedgeline, pretty much where I suggested it should be in Figure 7.

It would appear, therefore, that the archaeology is more deeply buried in most of this field than elsewhere at Verulamium.  Perhaps soil eroding down the slope built-up at the back of the Roman wall?  The wall itself, however, is conspicuously absent.  A massive structure like that should show quite clearly in GPR data.  The one thing that I ought to do is build the topography into the GPR processing, but I cannot do that as yet.  Watch this space!

Earth Resistance survey

Earth resistance came a poor third in the techniques we used on the site.  We managed one block 40m by 60m using the Institute of Archaeology’s RM85 at 50cm intervals.  We have also recently had the Welwyn Archaeological Society’s machine repaired (thanks Bob!), and did just one 20x20m square at 50cm.

Fig 14: Earth Resistance surveys.

After the excellent results from Gorhambury last summer and Durobrivae last October, these are more the sort of thing I expect in Hertfordshire, i.e., nothing very useful!  In this case, I suspect that, based on the GPR data, the interesting archaeology is deeper than the method with a 50cm probe spacing is looking.

Resistance Pseudosections

The West Essex Archaeological Group has kindly lent us their Resistance Pseudosection kit.  The idea of this is that one takes a series of readings in a line with a 1m spacing between the four probes.  One then repeats the operation with a 2m spacing, then again with 3m, 4m, 5m and 6m.  By increasing the spacing between the probes each time one repeats the line, one is looking deeper into the surface.  The data can then be plotted using a program called res2Dinv which creates a “pseudosection”, an image of a vertical slice through the ground.  We decided to try two of these: one across the road and one across the bank where the wall may be.  The idea was that maybe being able to look deeper into the ground we might be able to see something the other techniques missed.

Professional versions of this system use a switching box and are very expensive.  Bob Randall, however, created a useful budget version using his TRCIA resistance meter.  With just a couple of people this can be hard work moving all the probes between pegs, but if you have five people it can be quite quick. It does look, however, quite amusing as four people bob up-and-down changing probes in unison!

Fig. 15: the team undertaking a resistance psuedosection.

Figure 16 shows the location of the two lines we did overlain on the GPR data.

Fig. 16: the location of section A (red) and B (blue).

For both of the sections, I also ran the GPR along the same line so that we would have a direct comparison.  Unfortunately, the free version of res2Dinv will not undertake corrections for topography and will not save images for sections 40 probes (39m) long, although I can get a screen grab and use that, which is good enough for this blog.  The commercial version is extremely expensive.  I hope I may be able to process the data with the topographic correction in the future.

The first thing I did was to compare each section with its companion radargram.  The radargrams have been corrected for topography.

Fig. 17: Pseudosection A and a radargram along the same line.

There isn’t a huge correlation between the two for section A (Fig. 17).  The broad curving layer shown low on the left of the radargram does not have a similar signature in the resistivity data unless the large grey-green area of slightly higher resistivity is reflecting that.  The very high areas of resistivity shown to the right of the section in red similarly do not clearly match anything much in the radargram (although see below about the ‘circular thing’).

Fig. 18: Section B and its radargram.

The second section was remarkably featureless.  There is a large area of lower resistivity to the right which may represent the robbed out wall, but this doesn’t really show in the radargram. The large curved response in the radargram on the left doesn’t really show in the resistivity section.  All rather frustrating.  I decided to put the images onto the Google Earth images to see if that helps.

Fig. 19: Resistivity pseudosection overlain on the mag data. Note that the red line marks the position of the pseudosection and north is to the left.

In Fig. 19 the red line indicates the position of the pseudosection.  The area of medium resistivity on the left shown in a greeny colour matches where the section cuts across the linear area of low magnetism shown in light grey / white.  These are probably, therefore, caused by the same feature which would appear to be something associated with agricultural activity judging by the wider pattern in the mag data.

Fig. 20: The radargram for the same line, marked in red. North is to the left.

Comparison of the radargram to the mag data doesn’t help much.  The large curving feature to the left of the radargram (N), doesn’t really show in the mag data.  The area of low magnetism mentioned above only really correlates with half of that feature, if at all.

Fig. 21: Radargram overlain on the lower time slice (cf. Fig. 13).

Fig. 21 does show, however, that the radargram goes right over one of three clear sub-circular features which can be seen in the bottom time slice (cf. Fig. 13).  This feature shows very clearly in the radargram.  In Fig. 22 I have annotated it radargrams take some use to ‘reading’.

Fig. 22: Radargram showing the position of the subcircular feature.

I don’t know what these three features are, but there are three of them in a line at right-angles to the suggested line of the road.  Very intriguing!

Fig. 23: Pseudosection B on the mag data. The blue line marks the position of the section.

In Fig. 23 the area of slightly higher resistivity to the west matches very well with the bank of lower magnetic responses that run parallel to the bank from NW to SE.  The low resistivity block on the right (east) doesn’t seem to match anything in the mag data.

Fig. 24: Radargram along section line B. The actual line of the GPR transect is the blue line.

Fig. 24 shows the radargram on the mag data.  The curved feature low in the profile on the western side only partially matches the low magnetism band.  I cannot see anything that clearly matches the low resistivity feature seen in the pseudosection.


As I have often said, using more than one technique can be very powerful as the different methods detect different properties of the below-ground stratigraphy, and therefore different aspects of the archaeology.  The downside is the huge increase in complexity in comparing, contrasting and trying to interpret the various results.  This small survey is an excellent example of this,  There is much more that could be done, especially going through the GPR results in more detail and comparing and contrasting them with the magnetometry data.

For now, we can say the following:

  1. There is no clear evidence for the town wall.  There is a good possibility that it has been robbed in the this area as suggested by the fact it only shows in one time slice, and there is a low resistivity feature in the pseudosection in about the position one would expect the wall.
  2. The clearest evidence for buildings is along the SW hedge line.  These show best in the mag data, and parts show in the GPR but difficulties with nettles and burrows prevented the GPR getting closer into the hedge line.
  3. The lower part of the field in the centre mainly shows evidence for agriculture in both the mag and GPR data, including probable land drains.
  4. The deepest time slice does show some evidence of the road and Roman buildings.  If my estimates of the speed of the radar signal are about right, the tops of these remains lie between 90cm and 125cm below the surface.  The extra depth of soil compared to elsewhere may be due to soil creep down the slope building up behind the bank to the NE.
  5. Part of these building remains include three sub-circular features which have strong radar reflections.

Many thanks to everyone who came out to help with the survey.  Many thanks also to Pete and Flora Letanka for allowing us to survey in their paddock and garden, and to Stuart Gray for putting us in touch.  Also, thanks to WEAG for the use of their pseudosection equipment, and SEAHA for the loan of the GPR.  Lastly, thanks to Pat Johnson for bringing us the mag at the start of the survey.

Filling in the gaps

Thanks to Stuart Gray of Strutt and Parker, I was able to contact Pete and Fiona Letanka and obtain permission for us to survey the paddock of Darrowfield House which lies immediately behind Verulamium Museum. It is the last large-ish area of Verulamium left that has not been totally surveyed using magnetometry. Members of CAGG are, therefore, spending this week filling in the gap.  Our mag was away being repaired, but Pat Johnson of Foerster kindly agreed to bring it down to us today.  We started, therefore, with Earth Resistance and GPR, but after lunch the poor old res meter was left abandoned on the grass while the team got started with the mag survey.

The paddock ought to be exciting.  It lies just behind the line of the Roman wall, and there is a road shown in publications running SW-NE across it.  The first mag results are shown in Fig. 1.

Fig. 1: the mag survey.

They are surprisingly unexciting.  The diagonal lines running SW-NE look like cultivation marks of some sort.  There is a quiet area behind the supposed line of the wall, running NW-SE, and then one high-magnetic curved feature which may match a bank we can see in the field.  No real sign of the wall, or of the road,

The GPR survey has been processed in 3ns thick slices.  As usual, I’ll work my way down from the top.

Fig. 2: GPR survey, time slice 3, from 10.5ns to 13.5ns.

Fig. 3: GPR survey, time slice 4, from 13.5ns to 16.5ns.

The third slice shown in Fig. 3 really shows surprisingly little. I’m guessing we are only looking into the topsoil here.  The fourth slice (Fig. 3) starts to show some diagonal striping similar to what we can see in the mag data.  Again, this is probably agricultural.

Fig. 4: GPR survey, time slice 5, from 16.5ns to 19.5ns.

Fig. 5: GPR survey, time slice 6, from 19.5ns to 22.5ns.

The fifth slice (Fig. 4) shows the striping very strongly, with two alignments at right angles.  I think these must be land drainage.  They look too straight for “ridge and furrow” and do not have a headland. What is also interesting — or is that worrying? — is that the wheel-ruts which show so clearly on the Google Earth image in the background, also show in the GPR data at this depth.

The sixth slice (Fig 5) faintly shows echoes of the drainage.  To the north are some black squiggly features.  Yes, folks, we have mapped the badger setts!  Nothing very archaeological, so far.

Fig. 6: GPR survey, time slice 7, from 22.5ns to 25.5ns.

In the final slice (Fig. 6), we still have the setts in the north, but we have a very strong linear feature to the south running SW-NE.  Comparing this to the proposed line of the road in Fig. 7, we can see that this would appear to be one edge of it.

Fig. 7: GPR survey, time slice 7, from 22.5ns to 25.5ns, with overlain map of Verulamium.

Right at the edge of the GPR’s signal depth where the returns are very attenuated, we are starting to see the archaeology. Unfortunately, I cannot yet easily convert the time (22.5ns–25.5ns) into physical depth.  It would seem, however, that the archaeology in the lower area may well be at the limit of what we can pick up.  If we look at the radargrams, however, we can see a big feature running across our vertical slices and I would guess this is the road surface.

Fig. 8: an example radargram showing the possible road.

Many thanks to everyone who turned out today and enjoyed the beautiful sunshine.  Also, big thanks to Pat for bringing the mag back for us, and Pete and Fiona for arranging for us to be able to survey the Paddock.

There probably won’t be a posting tomorrow, but I hope on Weds to catch-up.

February fun

Partly as a training exercise for UCL students, and partly just to extend our surveys at Verulamium, members of CAGG were out in Verulamium Park and Abbey Orchard last Thursday and Friday.

The magnetometer was out in Abbey Orchard on Thursday in the hopes of completing that small area.  Unfortunately, I think it is jinxed.  On Thursday we had battery problems, and on Friday the odometer refused to work properly.  The sum total of a day and a half’s work?  Two partial grid squares…  Oh well, just chalk that one up to experience.  Here is the image of the slightly extended area, such as it is.

The Abbey Orchard survey.

The Abbey Orchard survey.

Having given-up in the mag, Jim West, Pauline Hey and I decided to use the Earth Resistance meter on Friday afternoon.  I had singled out an area where there is a clear building in the mag data, but some ferrous noise masked the western end of the building.  The weather was glorious for a mid-February day.

The Earth Resistance survey underway.

The Earth Resistance survey underway.

We managed four grid squares at the usual 0.5m spacing, not too bad for one afternoon’s work. As you can see in the next image, the mag survey shows a lovely building as white lines representing low magnetism.

The mag results in the area of the Earth Resistance survey.

The mag results in the area of the Earth Resistance survey.

The wide dark line coming from the SW corner is the ‘1955 ditch’, the first century boundary of the town first excavated by Frere in 1955.  The two parallel lighter lines which run just to the north of the building are part of a road.

Unfortunately, the Earth Resistance survey showed nothing of the building at all.

The Earth Resistance survey results.

The Earth Resistance survey results.

It shows the edge of the road beautifully, and a high-resistance feature parallel to the 1955 ditch.  Even the ditch itself shows as a wide band of low resistance.  Of the building, however, nothing!  It may be simply that the soil is so wet at the moment there is no contrast between the building and the surrounding soil matrix.  Alternatively, the building may have been robbed out.  We will have to run the GPR over it one day.

Unlike last November, the GPR suffered no glitches, and Mike Smith, John Dent and Graeme Spurway completed an area 160m by 40.  Added to the same sized area completed in November, we now have a nice block of GPR data 160m by 80m to look at.

The data were sliced using Larry Conyer’s system in 3ns slices.  I’ll go through these from the top down.  There are three areas of GPR survey shown.  The top half of the large block is the latest survey, the bottom half that undertaken last November.  The detached block to the west was undertaken by Ralph Potter in 2014.  Remember that this is a rather crude “mash-up” in Google Earth so the edges do not match very well.  As always the GPR data are deserving of a much more detailed analysis.

GPR slice 3, 10.5 to 13.5ns.

GPR slice 3, 10.5 to 13.5ns.

Slice 3 (above) mainly shows modern features surviving in the topsoil, especially broad cultivation marks running NNW–SSE across this field.  There are some hints of the archaeology just starting to show through.

GPR slice 4, 13.5 to 16.5ns.

GPR slice 4, 13.5 to 16.5ns.

Slice 4 clearly shows the upper levels of the archaeology.  Watling Street, which is running roughly north-south in the eastern half of the main area, has a big hole in it.  It has been severely robbed for building stone.  There is a minor road running SW-NE with a square building alongside it to the north with a small room on the western wall and what looks like a courtyard on the eastern side.

GPR slice 5, 16.5 to 19.5ns.

GPR slice 5, 16.5 to 19.5ns.

The main addition in slice 5 is the complex of buildings in the NW corner of the main area. These are probably associated with the pottery kiln which we have just clipped (the feature that looks like Mickey Mouse’s ears in the underlying mag data).

GPR slice 6, 19.5 to 22.5ns.

GPR slice 6, 19.5 to 22.5ns.

In slice 6, the spur road is clearer, especially near to Watling Street.  It looks as though there is a shallow valley running parallel to Watling Street which is now filled with a greater depth of topsoil which means that the archaeology does not show until the deeper slices.  It is also noticeable that the centre of Watling Street has fewer reflections than in the upper slices.  I guess that we are getting below the surface of the road, and the reflections either side of the road may be the ditches that Wheeler found filled with rubble.  The building complex just to the north of the spur road has hints of two more small buildings.

GPR slice 7, 22.5 to 25.5ns.

GPR slice 7, 22.5 to 25.5ns.

In this final slice we can see the two small buildings north of the spur road in more detail. There is also a long linear feature running N-S between Watling Street and the modern path.  It looks like a modern utility to me, but there isn’t one indicated on the map I have been given.

The GPR results are excellent, and it will be worth continuing to expand this area.

I’ve had a busy time speaking to various groups about CAGG’s work recently.  One of the lectures was as part of the Society of Antiquaries public engagement lecture series held on a Tuesday lunchtime once a month.  They video the talks and put them online, so if you would like to hear me talking about Verulamium once more, here is the link.

As always, many thanks to Ruth Halliwell, Peter Alley, Jim West, Mike Smith, Pauline Hey, John Dent,  and Graeme Spurway, as well as my students from UCL, for turning out in mid-February, although we were extremely lucky with the weather,


Four days in the Park (part 1)

A combination of members of CAGG and students from the Institute of Archaeology, UCL, undertook four days of survey in Verulamium Park a couple of weeks ago.  It was supposed to be five days, but the Saturday was called off because of rain.  We aimed to:

  • extend the magnetometry survey into Abbey Orchard, the field between the Park and the Cathedral;
  • undertake some more GPR survey in the Park;
  • try the six-probe six-depth method of Earth Resistance survey over one of the buildings we have previously surveyed;
  • try a resistivity psuedo-section across the line of the town wall near St Germain’s Block.

It was definitely one of “those” survey periods.  Every single technique had some problem or other of varying degrees of seriousness.  We managed to sort most of them out in the end, but that and the Archaeology in Hertfordshire: Recent Research conference which was held on Saturday 26th have delayed this posting.

I am going to post the results in two parts, starting with the mag and GPR data.

The mag survey

The area being surveyed is not very large, but is quite awkward.  It lies on a steep slope, has many trees and a great number of people walking by.  It should, however, have evidence of the abbey, and possibly a late Roman cemetery.  It also may have an Iron Age enclosure which was seen on an aerial photograph, and maybe the early Saxon town.

Fig. 1: The survey team in Abbey Orchard.

Fig. 1: The survey team in Abbey Orchard.

Between teaching, equipment issues and rain, we didn’t get as much done as we hoped, but we have at least started on this, our first extramural area at Verulamium.

Fig. 2: the mag survey results.

Fig. 2: the mag survey results.

Very little can be seen in this plot.  It is very “noisy”.  There is one potential thin feature, and the vague hint of a larger, wider feature (marked with red arrows in Fig. 2).  Given where we are working, this is all very disappointing so far.  We do need to complete this area if we can, however, so we’ll be back at some point.

The GPR survey

We did have some problems with the GPR as well, but eventually we managed to resolve those and completed an area 160m by 40m on the southern side of the town near the London Gate.  The weather and obstacles could prove challenging (Fig. 3).

Fig. 3: GPR survey in the Park, November 2016.

Fig. 3: GPR survey in the Park, November 2016.

As usual, I used Larry Conyers and Jeff Lucius’ free software to “time slice” the GPR data.  I created 3ns thick slices which seem to work well at Verulamium.  The third slice (10.5–13.5ns; Fig. 4) shows features close to the ground surface.  The dark smear running NS is the topmost layers of Watling Street.  The diagonal lines, which also show in the magnetometry data, must be some sort of drainage.

GPR time slice 3 (10.5--13.5ns).

Fig 4: GPR time slice 3 (10.5–13.5ns).

The next slice (13.5–16.5ns, Fig. 5) shows the archaeology much more clearly.  Watling Street is the black north-south feature to the east of the plot.  There is, however, a hole in it!  I guess this is another example of the extensive robbing of the town for building materials.  Slightly to the west, a narrower minor road is running SW–NE across the plot.  This road lines up with the light linear feature in the mag data.  On the north side of this smaller road is a building with at least two rooms and what appears to be a paved area to the NE.

GPR time slice 4 (13.5--16.5ns).

Fig 5: GPR time slice 4 (13.5–16.5ns).

Slice 5 (16.5–19.5ns; Fig 6) still shows Watling Street and the building.  There is a hint of some linear features that are on the same alignment as the rectangular enclosure which shows in the mag data to the south, and a wall in the SE corner.

GPR time slice 5 (16.5--19.5ns).

Fig 6: GPR time slice 5 (16.5–19.5ns).

The next slice (Fig. 7; 19.5–22.5ns) as there are hints of robbed walls showing in the south side of the plot next to Watling Street.  These are showing as white lines of reflection free data and do seem to form structures of some sort.  These align with the strange enclosure seen in the mag data.

GPR time slice 6 (19.5--22.5ns).

Fig. 7: GPR time slice 6 (19.5–22.5ns).

The last slice (Fig. 8, 22.5–25.5ns) is interesting in that the main line of Watling Street is still visible, but does not have such strong reflections as before.  There are, however, two bands of strong reflections either side of the road.  Perhaps these are the roadside ditches known from excavation filled with rubble from the road surfaces and construction?  The signal at this depth has started to attenuate and I will not show the deeper slices.

GPR time slice 7 (22.5--25.5ns).

Fig. 8: GPR time slice 7 (22.5–25.5ns).

In Part 2 I will show the results of the two electrical techniques.

Ellen has designed a logo for CAGG.  What do you all think?


T-shirts anyone?


Just to the west of Peterborough lies the Roman town of Durobrivae. This town is one of the so-called ‘small towns’, i.e., not one of the public towns with an administrative function.  It is, however, somewhat larger than some of the public towns such as Caistor-by-Norwich, the civitas-capital of the Iceni.  Duriobrivae had a town wall, parch marks from which can be seen on the Google Earth image along with Ermine Street (the straight line through the town from SE to NW) and the irregular street plan (Fig. 1).

Fig. 1: Google Earth image of Durobrivae.

Fig. 1: Google Earth image of Durobrivae.

Oblique aerial photography over the years has revealed much about the interior of the town, as well as extensive suburbs, prehistoric features, villas and so on.  Fig. 2 shows an oblique image of the town.

Oblique aerial photograph of the town. Reproduced courtesy of Stephen Upex.

Fig 2: Oblique aerial photograph of the town. Reproduced courtesy of Stephen Upex.

I was particularly fascinated to see a large group of circular features to the south of the town, some of which appear to be the ditches around round-barrows, but others are far too large and are tentatively suggested to be some form of henge (Figs. 3 and 4).

Fig. 3: Google Earth image of the field to the south of the town showing circular prehistoric features.

Fig. 3: Google Earth image of the field to the south of the town showing circular prehistoric features.

Fig. 4: Oblique aerial photograph of the field to the south of the town showing the Roman suburbs and earlier prehistoric circular features. Photograph courtesy of Stephen Upex.

Fig. 4: Oblique aerial photograph of the field to the south of the town showing the Roman suburbs and earlier prehistoric circular features. Photograph courtesy of Stephen Upex.

Ruth Halliwell (WAS), who has worked with CAGG regularly, is working on the town for her dissertation and we arranged to go and team-up with local archaeologists to undertake some survey.  It was very much a “proof of concept” trip: which techniques would work best there? Would the surveys add to what could be seen from the air?  We undertook three days survey running all three main machines (magnetometry, resistance and GPR), and Peter Alley also used his UAV to take high-level photographs, partly with a view to creating topographic maps.

Despite early problems with the mag, we managed to survey an 80m wide, 360m long strip NS across the town.  The overall results can be seen in Fig. 5.

Fig. 5: the magnetometry survey.

Fig. 5: the magnetometry survey.

There is a great deal going on in the results.  Ermine Street shows clearly running across the NE corner of the survey transect and matches the parch mark beautifully.  Either side of Ermine Street are a series of buildings with their gable ends onto the road in the approved Romano-British manner.  Other streets can be seen, again matching the parch marks.  Not all the buildings are so clear, but there are clearly other walls that can be seen in the data.  Towards the south, the pattern is more complex.  The results could be cleaned-up a little more.  In places we were suffering from some stagger, partly as a result of the fine reddy-brown dust that settled over all the machines and their operators  (Fig. 6) which, combined with the lubricant we use on the cogs, turned to a sticky slurry.

Fig. 6: the red dust all over the GPR. Image courtesy of Mike Smith.

Fig. 6: the red dust all over the GPR. Image courtesy of Mike Smith.

We undertook a radar survey using the Mala GPR we have on loan from SEAHA.  Pushing the GPR was quite hard work in the long grass, especially as one goes over the agger on which Ermine Street appears to have been constructed (Fig. 7).  The team did, however, manage to complete an excellent six blocks of data.

Fig. 7: Pushing the GPR over Ermine Street.

Fig. 7: Pushing the GPR over Ermine Street.

The images on the screen of the GPR shows that we were getting reasonable depth penetration. I created amplitude maps in 3ns thick slices.  The third slice map (Fig. 8) clearly shows the surface of Ermine street, but for most of the area surveyed the radar signal is still in the ploughsoil.

Fig. 8: time slice 3, 10.5-13.5ns.

Fig. 8: time slice 3, 10.5-13.5ns.

In the fourth time slice (13.5–16.5ns, Fig. 9)  some of the other roads are starting to show, and odd bits of wall.  One very curious feature is the lighter coloured band across the middle of the southern area.  Although it would appear to be related to our grid, our survey was conducted NS across that band.  The aerial photograph (Fig. 2) does show a band across the field so perhaps this is related to some sort of cultivation pattern?

Fig. 9: time slice 4, 13.5-16.5ns.

Fig. 9: time slice 4, 13.5-16.5ns.

The fifth slice (16.5–19.5ns) shows more details in the buildings (Fig. 10).  In the centre of the lower block is a square feature.  This is the Romano-Celtic temple known from aerial photographs.  This type of temple, well-known from many sites across the north-western provinces of the Roman Empire and consists of two concentric squares, usually reconstructed as an inner sanctum and an outer ambulatory.  The two roads the the north and south of the temple appear to mark the edges of the temenos or sacred precinct.  There is a hint of a possibly paved area to the west of the temple, and a solid feature between the internal and external walls to the east.  In the northern block there are hints of the walls on either side of the road as seen in the magnetometry data.

Fig. 10: time slice 5, 16.5-19.5ns.

Fig. 10: time slice 5, 16.5-19.5ns.

In the sixth time slice (19.5–22.5ns) we can start to see some of the buildings along Ermine Street not, as I had expected, as black ‘high amplitude’ features shown in black i.e., stone walls, but as low amplitude features, i.e., areas which have fewer items that would reflect radar waves (Fig. 11).  At Verulamium, I have interpreted these as where the stone foundations have been robbed, but here we know less about the construction techniques used.  Part of the difficulty is that Ermine Street is on a quite marked bank which means the radar has a greater depth of deposits to penetrate.  I undertook a topographic survey of just the northern block area (Fig. 12) and in the future will be able to process the GPR data taking into account the topography.

Fig. 11: time slice 6, 19.5-22.5ns.

Fig. 11: time slice 6, 19.5-22.5ns.

Fig. 12: topographic survey of the northern block.

Fig. 12: topographic survey of the northern block.

In the seventh and eighth time slices (22.5–25.5ns, 25.5–28.5ns), the GPR radar waves are starting to attenuate and we are getting quite faint reflections, but some of the deeper foundations show in these lower time slices (Figs. 13–14),  For example, some of the buildings along Ermine Street start to show very well in Fig. 13, and the outer wall of the Romano-Celtic temple shows very well in Fig. 14.

Fig. 13: time slice 6, 22.5-25.5ns.

Fig. 13: time slice 6, 22.5-25.5ns.

Fig. 14: time slice 6, 25.5-28.5ns.

Fig. 14: time slice 6, 25.5-28.5ns.

There is a great deal more which can be extracted from the GPR data, especially by looking at the radargrams (the vertical slices) and comparing them to the time slices.  The results are less “black and white” than at Verulamium and quite complex, but there is a great deal going on in this data which will take a bit of work to tease out all the details.

As well as the magnetometry and radar surveys, we undertook a resistance survey using UCL’s new RM85 meter (Fig. 15).  We took readings every 50cm.  We managed to survey an area 60x by 80m which had also been surveyed using the GPR and the magnetometer.

Fig. 15: Richard Cushing and Stephen Upex working on the resistance survey.

Fig. 15: Richard Cushing and Stephen Upex working on the resistance survey.

The result of this survey was quite surprising (Fig. 16).

Fig. 16: the earth resistance survey.

Fig. 16: the earth resistance survey.

The square within a square plan of the Romano-Celtic temple could not have been more obvious.  The small room on the eastern side, partially seen in the GPR survey, shows clearly.  The temenos is also quite clear.  A spectacular result, but one that raises a question. Why is the inside of the temple such low resistance?  Normally, low resistance like this is related to water retention.  Is the outer wall of the temple causing water to pool within the wall?  It is useful to compare the three surveys (Fig. 17).

Fig. 17: comparing the three survey techniques and the parch marks in the area of the temple.

Fig. 17: comparing the three survey techniques and the parch marks in the area of the temple.

It will take a bit of work to draw-up a composite interpretation plan.

Peter Alley also undertook some surveys with his UAV (Fig. 18).

Fig. 18: Peter Alley using his UAV to map the site.

Fig. 18: Peter Alley using his UAV to map the site.

As well as taking high-level images of sites, the UAVs pictures can be used to create topographic models using a technique called “Structure from motion”.  Fig. 19 shows a topo plan of part of the site derived from the photographs.  The actual heights vary from my plan because the UAV’s plan needs to be corrected against some control points, but the relative heights are great.  This technique is going to prove very useful in future.

Fig. 19: Topo map in QGIS derived from the UAV's aerial imagery.

Fig. 19: Topo map in QGIS derived from the UAV’s aerial imagery.

The aim of this three days of fieldwork was simply to see which survey techniques would provide useful information at this site.  The answer is: all of them!  We already have a huge amount of data to examine in more detail, and a great deal of thinking to do.  It certainly seems that a more extensive programme of geophysical survey would add to our knowledge of the town greatly, as well as other archaeological features such as the “henges” to the south.

As always, many thanks indeed to everyone who came to help, especially Mike Smith for transporting and running the GPR, and Jim West for helping to run the mag.  This was a great team effort between local group members and CAGG, and exactly what our group exists to do.

Fig. 20: the end of day.

Fig. 20: the end of day.