Tag Archives: Verulamium Park

Multichannel GPR

As many of you know, when we do a “normal” GPR survey we collect lines of data 50cm apart. The closer the lines, the more detailed the image. Mala, the manufacturer of SEAHA‘s GPR, make a “Mira” system which has multiple antennae in one box.  By attaching one of these multiple antenna system to a suitable vehicle like a quadbike, and using a high-accuracy GPS, it is possible to collect data for large areas quickly, and at an optimal line spacing of 8cm…  The “mini-Mira”, which has featured on Time Team, has eight antennae in the one box.

The mini-Mira attached to the back of a quadbike with a dGPS.

Mike Langton of Mala Geoscience, kindly offered to come and do some survey at Verulamium for us if we could find a quadbike to tow the system. That took some doing, but AA Sherriff and Son saved the day.  We spent a day and a half in Verulamium park with Mike driving the bike, and Mike Smith, Julia Merrick and myself providing support.

Mike Langton with the MiniMira at Verulamium.

The proper data processing is going to have to wait until Mike returns from holiday but he couldn’t resist sending me a quick snap of his laptop screen with some very preliminary results.

Very preliminary view of the results of part of the Mira survey.

This is the part of the town partly surveyed by Larry Conyers in 2013.  The road in the middle has been robbed.  The building above it is the one partly surveyed by Larry, the one below is a new result for GPR (although it shows partly in the mag data).

We are very excited by the new results and await the full survey data!

Many thanks to everyone who helped, especially Mike Langton, David Thorold and AA Sherriff Ltd.


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.

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?


The sun was shining, we have a GPR and a Park to play in, so why not? Mike, Peter and I headed out to Verulamium Park to do a little more GPR, although with a difference. We used a 25cm spacing between lines instead of 50cm. We managed a 40m x 60m block.  Without taking into account turning around, or getting to the spot and back again, that is 9.6km of walking, or just under six miles in old money. The results were great, however.  Later, I will do a posting comparing the 0.5m spacing with 0.25m to see if the extra 4.8km was worth it.

First things first.  Where in the Park were we?  I am especially interested in Insula XXIV simply because the Wheeler’s did not dig anything there and we have very good mag results.

Location of the mag grids in the northern half of Insula XXIV.

Location of the mag grids in the northern half of Insula XXIV.

Here is a more detailed view of the mag results.

Detail of the magnetometry results from the northern half of Insula XXIV.

Detail of the magnetometry results from the northern half of Insula XXIV.

The stark black-and-white feature in the NE corner is a practice cricket wicket made of concrete and green carpeting. It overlies the Roman road.  The diagonal feature in the NW corner is the other Roman road, and at least two buildings show clearly as white lines against a dark background.  The western 40m square was surveyed by Ralph Potter at 0.5m intervals.  The middle square and the western 20m of the eastern square were surveyed yesterday.

I used Larry Conyer’s program to slice the data we collected.  The slices start at 8ns and go down in 4ns thick pieces.  Ground surface in the Mala radargrams is usually at about 7ns and so the first slice is very much the top surface of the field.

Slice 1: 8 to 12ns.

Slice 1: 8 to 12ns.

The cricket wicket can be very clearly seen in the top-right corner.  The change across the plot at about 27m is where we went for lunch.  But what on Earth is the big circle?  Here is a clue…

GPR in the Park.

GPR in the Park.

Yup, it is the centre circle of the soccer pitch.  The second slice reveals the striping we have often seen in the Park.

Slice 2: 12 to 16ns.

Slice 2: 12 to 16ns.

I am guessing these are cultivation marks before the land was turned into the Park in the 1930s.  How old they are I am unsure, but could they be the residue of cultivation ‘strips’ as seen at Ashwell?

In the third slice, we begin to see the Roman archaeology.

Slice 3: 16 to 20ns.

Slice 3: 16 to 20ns.

Notice how there is a white line running through the lower building cutting the black lines representing internal walls.  This is a wall which has been partly robbed.  We also appear to have some surviving floors as shown by the larger black areas in the building.

Slice 4: 20 to 24ns.

Slice 4: 20 to 24ns.

In the fourth slice down we can now see the road running across the top-right hand corner in a similar, but not identical position to the cricket wicket.  I wonder if we might be seeing evidence of a hypocaust in the middle of the lower building?

Slice 5: 24 to 28ns.

Slice 5: 24 to 28ns.

In slice five we can see the outer walls of the building are much fainter suggesting shallower foundations, but the main wall which was only a light line previous is now showing more clearly.  This is presumably what is left at the bottom of the robbed wall.  There is also a building along the road which was not visible in the higher time slices.

Slice 6: 28 to 32ns.

Slice 6: 28 to 32ns.

Slice six is very black-and-white.  This is because the radar signal is now very weak and the slices are either showing something or nothing.

All in all, a very nice set of results.  As always, these are my rather rough-and-ready time slices.  With some experimentation and practice they could be better.

Here are Ralph’s grid square (processed by Mike Langton of Mala) and our new one roughly plotted together in Google Earth.

Ralph's and our blocks of GPR data plotted together using Google Earth.

Ralph’s and our blocks of GPR data plotted together using Google Earth.

I think it would be worth surveying the whole of Insula XXIV if we can!


A hot and sunny day in the Park

After the cold and rain of the previous day’s survey, today was a scorcher!  Burcu Urundul (Welwyn Archaeological Society), John Dent (St Albans and Herts Archaeological and Architectural Society) and Ralph Potter (West Essex Archaeological Group) were out today continuing our survey in Verulamium Park.  There was a minor hiccup at the start of the day when a toddler wandered off with the pin flag we were using as a backsight for the Total Station!

Survey in the Park

Burcu and John surveying in the park.

We did, however, manage to survey another five 40 x 40m grids.  The results are posted below and are very interesting.  The ‘1955’ ditch clearly shows across the plot, and there are a number of ditches and linear features.  One feature that catches my eye is the rectangular one indicated by the white arrow.  It is about 4.5m long and 2.75m wide and is quite strongly magnetic at about 12 to 20 nT, but not so much so as to be metallic.  Very curious indeed.  It is also interesting to see that there appears to be more activity inside the ‘1955’ ditch (to the NE of the plot) than outside it (to the SW).


The results of the survey after the third day.

Tomorrow sees us out for the last time before next weeks course.  Wish us luck!