Tag Archives: geophysical survey

Forthcoming talk

On Friday 29th November 2019, Kris Lockyear will be giving a talk on the results of the survey work entitled “Verulamium: busy places and empty spaces.”  The meeting will start at 7.45pm, United Reformed Church hall, Church Road, Welwyn Garden City.  WAS members free, visitors £3.

One

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

One what, I hear you say?  Well, 1km2. What is 1km2? Well, that is the area covered by the mag in all the Verulamium-related surveys. Yup, one whole square kilometer. Impressive, eh? By the way, that is about 20,000,000 individual mag readings.  That doesn’t include, of course, squares that had to be re-done due to sensor freezes or areas blanked out where we were wheel spinning for partial grids.  Congratulations to all who have pushed that machine since the summer of 2013.

Figure 1: Jim West and the mag.

Today the mag team completed the far end of Church Meadow (Figure 2).  It is great to see such a huge proportion of the field done, and much of what is left is not worth doing as it is featureless alluvium.

Figure 2: the Church Meadow mag at the end of the 2019 summer season.

Figure 3 shows the details of the southern end of the survey.

Figure 3: Detail of the southern end of the Church Meadow mag survey.

Most of the new area today was either in the area impacted by the pipes, or featureless alluvium.  The little partial near the road, however, found a small feature which looks like a wall with something in the middle.  Given this is right next to the gate of the town, perhaps this is a mausoleum?  No real way of knowing without digging it, but certainly a possibility.

The Earth Resistance team of Debbie, Tim, Denley and Ellen were on form today and completed a super nine grid, thus satisfying my need for a tidy end to a season! Figure 4 shows the results from today.

Figure 4: the Earth Resistance data after day 3.

Figure 5: the Earth Resistance data high-pass filtered.

As can be seen, there are a number of wall showing clearly as dark (high resistance) lines.  The room which shows most clearly is the one which can be seen on the Google Earth image.  A high-pass filter shows the walls even more clearly (Figure 5).

The GPR crew, allowed down from the heights of the Theatre field, picked a 40×80 strip east-west across the middle of the buildings.  Figure 6 shows the first twelve time slices.

Figure 6: GPR time slices across the nunnery.

As can be seen, the building that shows well on Google Earth is visible right from the first time slice.  The stone work must be literally just under the surface.  Slices 7 and 8 shows the buildings in great detail as well as that pipeline running across the plot. Figure 7 shows slice 7 on the Google Earth image.

Figure 7: GPR data across the nunnery. Slice 7.

To close out the 2019 season posts, I asked Mike Smith to take a group photograph.  Not everyone who was involved this summer was there today, but Figure 8 shows a good number of us.

Figure 8: the gang isn’t quite all here. The CAGG team on the last day of the 2019 Gorhambury summer season. (Photo: © Mike Smith).

Many, many thanks to everyone who turned-out over the last four weeks, be it almost every day or for just an afternoon.  Without the CAGG team members, this project wouldn’t achieve anything!  Also, big thanks to Strutt and Parker and the Gorhambury Estate for facilitating access, and to Lord Verulam and his family for all their support.  Lastly, thanks to the AHRC for funding the original project back in 2013, the Institute of Archaeology, UCL for supporting the project and the loan of the GPS and the Earth Resistance meter, and to SEAHA for the loan of the GPR.

I’m off to Sligo tomorrow morning at about 4.30am and will be presenting some of our results to the International Conference on Archaeological Prospection on Wednesday afternoon.

 

 

Done

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

I think Mike Smith would argue that he is “done” in more ways than one!  I feel that a suitably sonorous 1950s newscaster voice ought to be saying “at 1pm this afternoon, members of…”  The reason?  Because at 1pm this afternoon the GPR survey finally surveyed the last part of the theatre field (Figure 1).

Figure 1: Mike Smith crosses the finish line.

That is some 27ha of GPR survey, mostly at 50cm transect spacing.  That works out to pushing the mag some 540km across the field.  Figure 2 shows the final coverage.  As always, the image is a mess because it has been created by different pieces of software at different times and even with slightly different conversions of OS coordinates to lat and long.  My big job now is to turn that pig’s ear (processing-wise) into a clear image.

Figure 2: Done. The complete GPR survey of the theatre field (but needs better processing).

Many members of CAGG have contributed to the GPR survey over the last five seasons.  This season Nigel Harper-Scott, John  Ridge and John Dent have contributed greatly (Figure 3).  The person who really deserves a rest, however, is Mike Smith who has not only led the GPR team over most of the last five seasons, but has also been the main GPR transporter during the season, and has been looking after it during the week.  Many thanks Mike, and well done on a great achievement.

Figure 3: Nigel (left), Mike (centre) and John (right), today’s GPR crew.

The area covered this year is best seen via the stripes in the grass (Figure 4).

Figure 4: Stripy grass left by the GPR.

Now, you would have thought that completing the survey would have persuaded the boss to let them go home early.  Nope.  None of that slacking.  One grid we did at 1m transect intervals had an interesting building in it.  So after lunch the team set to once again to resurvey it at 0.5m intervals.  In figure 5 I have made the high amplitude reflections white so that the building can be seen more easily and placed it on the mag survey for context.  It is a long, thin, building with a corridor just visible on the eastern side and a larger group of rooms to the south.

Figure 5: the re-done block on the mag data.

Tomorrow, Mike gets his pick of which two squares to survey in Church Meadow.

The mag team re-did all the dodgy squares from yesterday, and quite a few more (Figure 6)!

Figure 6: the mag data in Church Meadow after day 5.

The mag team have just three whole squares for tomorrow, and then a few partials at the ends.  For most of its length, it is not going to be worth surveying up to the edge of the field to the NE as it is clearly into alluvium as shown by the flat, featureless mag data.  Figure 7 shows a close-up of the southern end of the survey.

Figure 7: the Church Meadow mag data, southern end details.

The darker curvy line is possibly an old edge to the river.  There are quite a few small dark blobs (“positive magnetic anomalies”) some of which could be graves, and at least part of a ditch feature.  I am still puzzled by the long negative linear.  I guess I’ll have to talk to some other geophysicists!

The res team was joined by Denley Lane of the Arc and Arc,  Denley remembers some of the pipelines being built.  The team completed an excellent seven grid squares.  Figure 8 shows the results, and Figure 9 shows the same results high-pass filtered.

Figure 8: the res results in Church Meadow after day 2.

Figure 9: the res results in Church Meadow after day 2, high pass filtered.

As can be seen, the building visible in the Google Earth image shows very well indeed, but there does not seem to be many more buildings to the south.  Tomorrow we will fill-in the one last block in the eastern corner, and then do a strip of blocks along the northern edge.

Tomorrow is the last day of the 2019 season.  I’ll be speaking about the Project at the International Conference on Archaeological Prospection on Wednesday.  Not much time to fit in the new results!

 

 

The antepenultimate day (Part 5)

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

It was a very warm and sunny day today, and I think we all felt the heat.  We managed to complete quite a bit of work, though, and so congratulations to all the team.

The GPR team filled-in some of the missing bits on the west, north and east sides of the theatre field (Figure 1).  Just a few bits left now.

Figure 1: The almost-complete GPR survey.

One nice feature was a little detail found on the western side (Figure 2).

Figure 2: A detail of the GPR survey from today. The red arrow indicates a small building.

A nice little building is showing-up.  The block to the east may show some robbed buildings. Last year I noticed some curious white lines in that block which could be robbed walls.

The mag team completed an excellent eight grids taking their total area surveyed in four days to 5.09ha.  Frustratingly, however, I found that the sensors had frozen on the first three grid squares.  Horrible waste of time, but nothing to be done about it.  Figure 3 shows the whole survey.

Figure 3: the mag survey in Church Meadow after day 4.

Despite the frozen sensor, we can see some interesting details in the new area to the south (Figure 4).

Figure 4: the southern area of the mag survey in Church Meadow after day 4.

What I am finding curious is that the long linear features are white in the plots, i.e., below average magnetism.  The features look like ditches in form, but do not give the usually positive response one would expect from them.  My worry is land drains… but the features seem to connect with the edges of Watling Street.  How very curious.

Figure 5 shows the Earth Resistance survey after day 1.  The team completed an excellent eight grids.

Figure 5: the res survey in Church Meadow after day 1.

The broad dark line across the western corner is Watling Street.  Further east the various thinner dark lines are the walls of buildings.  Clearly we have parts of a number of structures showing clearly.  Great stuff.  We don’t have a cruciform-shaped building face east-west yet, but give us time.  Hopefully, tomorrow, we will cover the building which shows so clearly in the Google Earth image. Comparing the mag and the res shows how much the pipes obscure, and how they went right through the middle of this complex (Figure 6).

Figure 6: the res survey in Church Meadow after day 1, with the surrounding mag data.

Well it is now 1am, and I have to be up early in the morning for our penultimate day on site.  Many thanks to everyone who worked so hard in the heat today.

 

Being completist

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

The team are very patient with my need to be neat and tidy and do the silly little bits around the edges.  Today was perhaps an extreme example.  Due to yesterday’s little hiccup, I set-up the res kit and, with Graham’s help, surveyed about a sixth of a grid square, then packed it all up again.  Now I can sleep easy.  On a more ambitious note, I have finally put all the res grid squares into one large composite.  The survey now consists of nine hectares, which is about 360,000 individual readings.  Most Earth Resistance surveys use what is known as a twin-probe configuration.  That means that there are two mobile probes on the frame, and two stationary probes on the end of a long cable, normally about half a meter apart.  One mobile probe, and one remote probe set-up an electric circuit part of which is the soil.  The other mobile probe and the other remote probe measure the resistance.  The problem with this “standard” set-up is that when you have to move the remote probes, the readings for the same spot will change.  This leads to endless struggles to “grid-match” each set of squares.  Since 2016 I have gone over to using a pole-pole configuration.  This is basically the same except the remote probes are a long way away (I aim for about 30m or more) and a long way apart from each other (I aim for more than 20m). This helps enormously with the grid matching.  Grids completed on different days of the survey will match quite nicely, usually.  Where this is not true is when (a) there is a lot of rain in the middle of a survey and (b) when the survey is split over multiple years.  In the case of our 9ha block, this has been completed over four seasons.  Unsurprisingly, one can see the edges.  TerraSurveyor has a function called “periphery match” which will, sometimes, do an excellent job of grid matching.  In this case, it was pretty good.  Figure 1 shows the survey with a periphery match applied.

Figure 1: the res survey as of the end of Thursday.

If you click on the image and see it full size you can see the detail of many buildings.  Unfortunately, the range of values makes seeing some buildings quite hard.  A high-pass filter is a background trend removing tool that makes some buildings show more clearly (Figure 2).

Figure 2: the res survey, high-pass filtered.

There are still many features, many buildings, that one can only see when looking at smaller blocks.  With such a big area getting everything to look clear is going to be impossible, I fear.

From tomorrow the res will be working in Church Meadow where we hope it will map the remains of St Mary du Pré.

The GPR team is getting really close to finishing the theatre field (Figure 3).

Figure 3: the GPR survey so far.

The team have some fiddly bits around the edges to complete, one missed block, and one block we want to re-survey at 50cm intervals as there is a building and road in it.  Fingers crossed, two more days should do it.  Then, for a bit of last day fun, the GPR will also have a look at St Mary du Pré.

The mag team completed another eight blocks today.  Since moving to Church Meadow, they have managed to survey four hectares in three days which is very impressive, especially given that one block had to be repeated due to a sensor freeze.  Having lots of whole grids and no partials makes such a difference.  The team are now just 2.5 ha away from completing a square kilometer of mag at Verulamium.  Figure 4 shows the mag results in Church Meadow.

Figure 4: Church Meadow mag survey after day 3.

Three things are of note.  Firstly, earlier today I wondered which of the two raised areas in the field was Watling Street.  Looking at the survey results, it looks like the road splits in two near the edge of our survey.  Perhaps it is two phases?  I’m not convinced.  Secondly, we have some marked linear features showing that almost look like enclosures.  These are, however, low magnetism suggesting and might be yet more pipes, but not of metal this time?  Again, I’m not convinced.  They might well be archaeological features.  I will have to survey all the pipes I can see in the field.  Lastly, as we get closer to the town to the south, there are many more little black blobs.  Seasoned readers of this blog will know that usually little black blobs in mag data are often pits.  In this case, I wonder if we are starting to pick up the edge of the cemetery which is likely to have lined Watling Street?  In the Roman world, the richer you were, the closer you wanted to be buried to the road and the town.

Tomorrow is the antepenultimate day of the 2019 Gorhambury survey season.  The weather looks good so fingers crossed all goes well.

Pipe dreams

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

Saturday night I said the weather was predicted to be “unsettled”.  Well on Sunday at 10am it was raining cats and dogs.  (I wonder why cats and dogs?  Why not ducks and pigeons?, or frogs and mice?)  I was determined to set-out grids in Church Meadow and so I soldiered-on.  Up until now, I have stuck to the 40m grid based on the OS for all the fields we have surveyed in Verulamium Park and in Gorhambury.  Church Meadow, however, is a long thin field at approximately 45º to the OS grid.  Additionally, the fence along Gorhambury drive is very straight for much of its length.  I decided, therefore, to use a floating grid to minimise partial grid squares and wheel spinning.  The lack of an “end line” function is the Foerster’s Achilles’ heel.  We must have wasted hundreds of hours spinning the wheel due to that one simple omission.  Although the data processing involves some extra steps and jiggery-pokery to get the plot in the right place, it seems worth it in this case.  I think my decision was vindicated when the team completed nine complete 40x40m grids despite the wet start to the day, and the lunchtime deluge. Figure 1 shows the team in action.

Figure 1: the mag team in action in Church Meadow. Image © Mike Smith.

Figure 2 shows the location of Church Meadow.

Figure 2: the location of Church Meadow, outlined in red.

Looking closely at the Google Earth image reveals some features in the field (Figure 3).

Figure 3: Features visible on Google Earth.

With these features we had high hopes.  Figure 4 shows the results of the mag survey from the first day.

Figure 4: Mag survey results.

Sadly, the plot is dominated by the two pipelines running through the field (Figure 5).

Figure 5: Heavy metal. Image © Mike Smith.

One of the pipes clearly runs straight through the building seen in the GE image. There are, however, some archaeological features to be seen (Figure 6).

Figure 6: mag survey in Church Meadow with some labels.

It is very frustrating that we can see the walls in the mag data to the SW and between the pipelines, but the image is so dominated by them that it is hard to make sense of anything.  Hopefully the res or the GPR will show the details better.  The ditch is interesting, however.  Could this be the vallum monasterii? It could, perhaps, be related to Watling Street, or it could simply be the remains of the earlier route of Gorhambury drive.  It will be fascinating to see where it goes.

The Earth Resistance team completed an excellent six blocks of data.  Figure 7 shows the whole res survey.

Figure 7: the whole Earth Resistance survey after Sunday 18th.

With good luck and a fair wind we should reach the hedge line on the next survey day.  Figure 8 shows the grids completed on Sunday.

Figure 8: The grids completed on Sunday.

Not a great deal is showing in those grids apart from the faint line across the top corner.  Let’s look at the mag data from that area (Figure 9).

Figure 9: the mag data from the same area as Figure 8.

The light line in the res data is matched by the dark line of “the sinuous ditch”, which is exactly what we would expect.  The sinuous ditch is, we think, the town’s aqueduct.  We should pick-up much more of this on Wednesday.

The GPR team have been working down the western edge of the town with the end in sight.  Soon, soon, they hope, they can escape the theatre field and its rugged terrain (Figure 10).

Figure 10: GPR and the rugged terrain.

The GPR team have been doing a lots of sawtooth edges as well as extreme hill-climb GPR.  Figure 11 shows recent results.

Figure 11: the western edge completed between Thursday and Sunday (colour section).

GPR, perhaps even more than Earth Resistance, is affected by the weather and ground conditions.  It seems very difficult to get different days to match-up.  I tried three methods with this data collected over four days, and none were perfect.  This image was created by just treating everything as one big data set. It doesn’t help that each tweak to see what works takes half an hour to process!

Looking back over the first three weeks, we have managed to achieve quite a bit despite dry weather, wet weather and endless partials.  Many thanks to everyone who has been involved, especially those stalwarts who come most days (you know who you are!).

Nigel wonders what next week will bring…

Figure 12: The Thinker. Image © Mike Smith.

 

Saving the best ’til last?

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

Unfortunately the weather forecast was accurate yesterday.  Far too much rain for us to go out and play.  Today, however, was very pleasant and at least the pegs went in very easily for a change! I’m afraid that a new toy (Figure 1) resulted in me getting a late start processing data this evening, and so I only have the mag and res to report on.  The GPR team, however, did a sterling job and completed a multitude to sawtooth-edged grids along the western side of the theatre field.  I promise I’ll process all that tomorrow!

Figure 1: Kris’ new multiformat pinhole camera being put through its paces.

The Earth Resistance meter was operated by John and Grahame. We have moved north of the hedge line and are close to the cross-roads around which both mag and GPR have shown multiple buildings.  Figure 2 shows the entire res survey, and Figure 3 a zoomed-in view of today’s area.

Figure 2: the Earth Resistance survey so far.

Figure 3: the area surveyed today and some context.

We have just clipped the edge of Street 11 (in the top-right corner of the new block) and have revealed some wonderfully clear images of the buildings to the south of that street.  These are clearer than the GPR survey of the same area, so it is really pleasing to see.  Excellent stuff!

Over in Prae Wood field, the mag team completed two more strips of grid squares.  Figure 4 shows the survey so far.

Figure 4: the Prae Wood Field survey, so far.

Two more days should, weather and equipment willing, see the team finish the field.  After Prae Wood we are heading north into Church Meadow, so-called because St Mary du Pré lies within that field.  Watling Street also runs through it so we should get some exciting results.

But, back to Prae Wood.  Let us look in more detail at the western end of the survey (Figure 5).

Figure 5: the western side of the mag survey of Prae Wood field.

The dark lines in the new area represent ditches, and it looks like we have some enclosures running across this western end of the field.  Given that the major Iron Age settlement lies in the wood just to the SW, these could be of that date, but they could be Roman or medieval as well.  Once more, we need to check out the historic map data.  Still, after many blank grid we now have features in both the western and eastern extensions of this field.  Saving the best for last?

Many thanks to everyone who turned out today.  Tomorrow looks like we might have to pack-up early due to yet more rain. This season is just flying by.

Fighting the wind

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

The weather forecast was for a windy day, and it was correct! At one point, the wind was making the flag on the cathedral look as stiff as a board, but the sun made the the yellow cross glow. Even from the far side of the Roman town it was quite striking.

Logistics led to an unusual format for the day.  Jim, Ellen, Pauline and Dave continued to expand the mag survey in Prae Wood Field.  Meanwhile, Kris, Mike, Anne and Julia firstly finished yesterday’s block of GPR data, curtailed because of rain, and the went on to complete six Earth Resistance survey blocks.

Figure 1 shows the whole of the mag survey in Prae Wood field, and Figure 2 a detail of the eastern end.

Figure 1: Mag survey after day 9.

Figure 2: Eastern end of the mag survey after day 2.

You may well ask why we have a funny diagonal edge to the survey at the eastern end.  This is because there is an electric fence creating a paddock for horses.  As we have gone a little further than we intended, we will just take what we can get.  The big new find is a ditch running across the end of the field.  I have marked this in Figure 3 with red arrows.

Figure 3: Mag survey with arrows (see text).

We have no way of knowing what date this feature is.  The first thing I will have to do is check the historic maps.  It does, however, look like much more than a field boundary.  It is 2 to 3.5m wide.  The blue arrow in Figure 3 indicates a much slighter feature than runs at a right-angle to the big ditch.  Just to the west of the ditch is a strong magnetic feature that I have marked with a yellow arrow.  The form of the feature (a bigger blob next to a smaller blob) is reminiscent of the pottery kilns we have found on the south side of the town.  The magnetic values (c. -8nT to +130nT) is also in the right sort of range for pottery kilns.  Some work I have published previously shows kilns with a maximum range of about -27nT to +180nT.  It is typical that the area where we have started finding potentially interesting features is also where there is the most modern interference from services and so on.

The GPR survey just completed the block left over from yesterday.  A quick look at the data showed no surprises.  The good news, however, is we are just one day’s worth of 1m transects from joining-up with the survey to the north.  The survey is shown in Figure 4.

Figure 4: the GPR survey after day 9. The red box marks the block completed on days 8 and 9.

The bare strip to the west of the GPR data in Figure 4 is what is left to be surveyed. That to the east has been done (I just haven’t loaded them onto the GE image).  The eagle-eyed amongst you may have noticed that the GPR blocks do not quite match-up as well as they used to.  This is a bit complicated but is basically because the OS have up-dated their guidelines for converting OS National Grid coordinates to lat and long.  I work in the National Grid, but Google Earth works in lat/long.  I’ve changed to using the OS’s official conversion webpages, partly because I can up-load them in bulk saving me a great deal of cutting-and-pasting.  My crude use of GE to display the results, however, involves dozens, if not hundreds of image over-lays.  As a result, I have a great deal of work to do to update all the slices from all the GPR blocks to the new coordinate conversions.  Arrgghhh.  The joys of doing a survey over five seasons.

The entire Earth Resistance survey is shown in Figure 5.

Figure 5: The entire Earth Resistance survey after day 9.

As can be seen, we have covered an impressive area now, about 9ha in total.  The edges in the image are the different years which have been processed slightly differently and crudely put together in Google Earth.  I will be joining them all together soon and trying to make a more seamless image.  Figure 6 shows the western 2019 block.

Figure 6: the 2019 western block of res data (the lighter strip to the west).

Most of what we managed today is the open dry valley between the buildings on on Street 23 (seen in the latest data) and Street 25 which we have yet to survey.  Tomorrow, however, we will start hitting the buildings on the latter street, and especially the cluster of buildings which lie on the junction of Streets 25 and 11 on the western corner of Insula XXXI.  In today’s data (the upper six blocks on the 2019 strip), we picked up the western half of a large building which lies back from the road with a wing running up to the road with a sequence of smaller rooms.  Fascinating stuff.

Tomorrow we are going to start off with GPR and Earth Resistance giving me time to lay-in more grids for the ever-efficient mag team.

Many thanks to everyone who braved the gales today.  We got some great results despite the weather.

 

Kennel Farm, Little Missenden

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

As a result of the hot, dry weather (sound familiar?) last year, the Chess Valley Archaeological and Historical Society noticed some parch marks in the fields along the River Chess near Little Missenden.  They undertook some Earth Resistance survey in those fields.  Earlier this year they asked if CAGG would be willing to come and undertake further surveys at the site.  At the end of May we undertook three days of survey completing an area of mag survey, two blocks of GPR survey, and extending the Earth Resistance survey undertaken by CVAHS.  Peter Alley also undertook a topographical survey using his UAV.

To take the topography first, we have two sources of information: the LiDAR data which can be downloaded from the data.gov.uk website and the UAV aerial photogrammetry data collected by Peter.  Each method has its strengths and weaknesses.  I have posted the LiDAR image created by John Glover and Peter’s imagery below.

Figure 1: LiDAR image. The field we were working in is to the west of the buildings in the centre of the plot. Image courtesy of John Glover.

Figure 2: Geo-rectified composite image of the area. Image by Peter Alley.

Figure 3: Elevation map created using Structure from Motion (photogrammetry). Image courtesy of Peter Alley.

Figure 4: hillshade representation of the photogrammetry data. Image courtesy of Peter Alley.

From these various images we can see that the site consists of a flat area either side of the river (essentially the flood plain) and then a steeper slope up to the houses and the main road. The flat area is alluvium, the slopes are “New Pit Chalk”.  Although there are some undulations, most clearly shown in Figure 4, nothing much suggests archaeology.  Some of the features, especially in the field to the east, are probably alluvial or periglacial.

The mag, working as quickly as ever, covered two blocks either side of the river (Figures 5 and 6).

Figure 5: Yvonne Edwards and Rhian Morgan running the magnetometer. Image © Mike Smith.

Figure 6: the magnetometry results.

There are a few features showing in the mag plot, especially the modern ferrous features to the south of the river.  I have put arrows on the plot to make the discussion easier (Figure 7).

Figure 7: the mag results with arrows.

South of the river is a long line of strong magnetic features (red arrows).  I’m not entirely sure what this was, perhaps an old fence line?  Maybe an obsolete pipe which has been partially removed? Whatever they are, they have largely obliterated any other magnetic features in that area, as well as causing problems with the destriping routine!

North of the river there are two very strong features marked with dark blue arrows.  These are easy to interpret (see Figure 8)!

Figure 8: the explanation for the strong magnetic features indicated with the dark blue arrows.

The only interesting thing about these is that the services connected to them do not show on the mag data at all.  The most enigmatic of the features is the linear one indicated with the light blue arrows.  This feature could be seen in the parch marks from 2018. To the west, the straight line looks like it might be a service of some sort, but then it curves up towards the pond, and then continues towards the east.  I suspect that the linear feature indicated by the green arrows might be part of the same thing.  It is very curious, and I am very unsure what it might be.  The low and high magnetic readings keep swapping sides, which suggests it isn’t just the result of soil with a high magnetic susceptibility filling a linear feature.  Sadly, I think the odds are on it being a service again, but it needs to be tested.  The pond, by the way, is a relatively new feature in the landscape (Figure 9).

Figure 9: the pond.

The linear feature indicated by the white arrows in Figure 7 is the build-up of topsoil at the change of slope (cf. Fig. 3). The last remaining feature is a square of magnetic noise shown by the yellow arrow.  I suspect that this might be the location of something like a wooden building, or parhaps a flat platform for something.

As well as the survey which had already been undertaken by CVAHS, we did a small block of Earth Resistance survey on the lower slopes. Thanks to Ruth Halliwell and Peter Alley for helping me with the survey (Figure 10).

Figure 10: the Earth Resistance survey underway. Image © Mike Smith.

As usual, we used the pole-pole configuration for the cables (i.e., the remote probes were at least 30m away and at least 20m apart), and took one 1m mobile probe separation reading and two 0.5m mobile probe separation readings at each survey point.  On this occasion, the 1m readings didn’t add much to the results so I will just discuss the 0.5m survey.  Figure 11 shows the results.  Very unusually, the readings went from very low (6.1 Ω) to very high (250 Ω).  The high readings were so high that I had to change the settings on the res meter, and made my ever patient helpers re-do a couple of grid squares.

Figure 11: the earth Resistance survey results.

A further problem can be seen.  The long tail of high readings which can be seen in the inset graph in Figure 11 means that most of the survey is shown as very light low resistance readings and it is hard to see much of a pattern.  If we simply clip the values (i.e., make everything above a certain value black), the high resistance areas become one nasty blob.  The statistician in me is used to seeing skewed data sets like this in all sorts of cases.  The answer is some form of data transformation, and TerraSurveyor provides this via its “Compress” function.  This can apply a log-transform to pull in that tail and make the data look more normal (in the statistical sense of more normal). Figure 12 is the result.

Figure 12: the Earth Resistance results subjected to a log-transform.

In Figure 12 the transformed data values are shown as the inset graph once more.  Hopefully, you’ll agree it allows us to see patterns in both the high and low values.  There does seem to be a linear feature running across the plot.  I suspect, however, that this is, again, part of the geology.  I doubt that anything we can see in this plot is archaeology.  The high readings might be a deposit of flints.  The blobby mag results in the same area suggest depressions in the surface of the subsoil which have filled with slightly more magnetic topsoil.

The final technique we used was GPR.  We completed two blocks, one to the east of the area, north of the river and the second to the west, again north of the river (Fig. 13).

Figure 13: the GPR being operated by John Glover of CVAHS. Image © Mike Smith.

Figure 14 shows the time-slices from the first block.  Just to confuse you all, north is to the bottom of these plots. They have been created with a 40% overlap between the slices.  Ignore the depths, they are incorrect.

Figure 14: time-slices from the first block of the GPR data. North to the bottom.

Slice 1 is simply the impact of the surface layers on the GPR, probably variations in vegetation and water retention.  Often we can see things like mushroom rings in this slice.  Slices 3 to, probably, about slice 8 are the actual deposits under the surface.  Slices beyond that are probably “echoes” of the upper slices boosted by the software into visible images.  We seem to have three things happening.  Some linear features which are undulating and strong reflections shown in red.  A broad curving low reflection area shown in dark blue, and a linear feature shown in cyan.  Figure 15 shows these on the Google Earth image.

Figure 15: time slice 6 from the GPR survey of Block 1.

As can be seen from the figure, the dark blue swathe comes out from the current course of the river and returns to it.  I suspect we have picked-up an earlier meander of the river.  Similarly, the lines of red might be banks of river gravels from previous meanders.  In Figure 16 I have indicated the outline of the noisy area of mag data shown by the yellow arrow in Figure 7.

Figure 16: the GPR slice with the magnetic feature from Figure 7 indicated by the white line.

As can be seen from Figure 16, the southern edge of the magnetic feature aligns nicely with the northern edge of the dark blue “channel”.  Perhaps we have a building platform on the banks of the river?

The linear feature in light blue could well be another service trench.

The second GPR block was to the east, again on the northern bank.  It was sited to cover one of the parch marks seen in the previous year.  Figure 17 shows the first 12 time-slices.  Again, north is to the bottom and ignore the depths.

Figure 17: First twelve time-slices from the second GPR block. North is to the bottom, depths are incorrect.

The most interesting thing about this block are the linear features which show clearly from about slice 7 onwards.  We have two clear parallel lines running alongside the river, and one which runs at a right angle to the southern (upper) line into the river bank.  Figure 18 shows the eighth time-slice on Google Earth.

Figure 18: time-slice 8 from the second GPR block.

As can be seen, the northern linear feature follows the one seen in the mag data.  This feature is, therefore, something that (a) is more magnetic than the background; (b) a good radar reflector and (c) in places remains green when other areas are parched (see Figure 19).

Figure 19: photo taken using a UAV during the drought in 2018. Courtesy of CVAHS.

Comparing Figures 18 and 19, we can see the southernmost linear feature shows well as a green mark in the field and as a strong reflector.  This feature, however, does not show in the magnetic data.  We can look at the radargram and the time slice at the same time using GPR Slice (Figure 20).

Figure 20: a radargram and time slice seen in pseudo-3D.

Looking at the radargram we can see that the broad reflection layer at the top is continuous over the northern linear feature, but is interrupted over the southern one.  This suggests to me that the southern feature might be relatively recent.

So what have we discovered?  Largely, I feel, most of the features are to do with geology and the river.  The two main mystery features for me at the long linear feature and the shorter one with right-angles.  Both show clearly on the aerial and the GPR.  The longer linear one also shows on the mag.  I’m not sure what either of these are, and only “ground truthing” might give us an answer.  My instinct, however, is that neither are all that old, but this is only an instinct.

As always, many thanks to everyone who helped both during the survey and with moving the gear. Also, many thanks to the Institute of Archaeology, UCL and SEAHA for the loan of the equipment.

For those awaiting the first results from Verulamium 2019, they will be posted in the next day or so.

 

 

Where have we been? What have we done?

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

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

Figure 1: sites surveyed in Hertfordshire and immediate environs.

Figure 2: sites surveyed by CAGG.