Tag Archives: magnetometer

Upgrading our mag cart

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I believe Jim enjoys mountain biking!


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.

Another day, another town

You would think I would have had enough. But no… hot on the heels of the end of the Gorhambury season, we headed off to the mysterious east side of the county. The Greenwich meridian seems to exert a powerful influence in Hertfordshire with its citizens seemingly afraid to cross the invisible line.

Back at the start of the project, we planned to do some survey in and around Braughing.  We managed just one site. The area is extremely important with multiple late Iron Age and Roman sites including the Roman “small town” on Wickham Hill.  We had an opportunity to work on the small town along with members of the Braughing Archaeological Group for a couple of days, mainly to see if magnetometry would show something useful.  The field was, however, rather rough and caused the odometer on the cart to over-run by about a meter, and the nuts and bolts needed constant tightening. On the second day I adjusted the odometer settings which improved matters a bit.

The survey underway on Wickham Hi

The survey underway on Wickham Hill.

We managed to complete 13 grid squares which was pretty good going, especially as the data logger crashed three lines before the end of the fourteenth square and we lost the rest of the grid.  The results, after a bit of work in TerraSurveyor, were very interesting.

The survey results.

The survey results.

The broad line running east-west towards the south of the surveyed area is the road.  It can be seen in the Google Earth image in the background.  Towards the west, the very dark band must be where the road becomes a sunken way as it goes up the slope.  What is very obvious is the difference to the planned public town at Verulamium.  This site was clearly a very different type of settlement.  What we have clearly shown is that it is worth expanding the magnetometry survey to cover as much of the settlement as possible.  Hopefully, the field surface will be a little more benign when we return!  One thing won’t change, however, and that is the slope…

On top of the hill.

On top of the hill.

Many thanks to Jim West for coming all the way from Chorley Wood to run the mag on the first day while I lay-in the grid, and also many thanks to all the members of BAG who joined in. Looks like we’ll be back!

A picture is worth a 1000 words

The Verulamium magnetometry survey.

The Verulamium magnetometry survey.

I suppose I cannot really get away with that simple a post.

First of all, CONGRATULATIONS to everyone, it is a fantastic achievement and I am so proud of all of you.  Secondly, a big thanks to everyone who turned out for an extra day on Bank Holiday Monday to complete the Macellum field.

How about some numbers?  Well, Verulamium is the third biggest Roman town in Britain, after London and Cirencester.  It is, however, the largest Roman town in Britain which doesn’t have a modern settlement built over most of it.  We have surveyed 64.5ha of the total area of 81ha.  It has taken us 83 working days starting in the summer of 2013, but we didn’t do much at Verulamium in 2014.  It took 12,900,400 readings to cover those 64.5 ha.   That, of course, doesn’t include the grids we did twice because of frozen sensors or other problems. People pushing the cart walked about 322km, not including having to go back to the start for partials, getting to the squares in the first place, or laying in the tapes and strings.

Let us look in more detail at the last bit surveyed in the Macellum Field.

The area surveyed during day 37) high contrast).

The area surveyed during day 37) high contrast).

Several things come to mind.  Firstly, there is very little there!  Towards the NE and along the western side there may be a ditch feature, although it is quite faint.  Other than that, the main (and annoying) thing are the strong magnetic anomalies along the edge of the field.  Some of you may remember the 12″ gas main which runs across the Park… well here it is again.  What I do not entirely understand why there are differences between the negative and positive readings along our grid lines.  Jim and I spent some time making sure I put the composite together correctly, and we are sure it isn’t a survey error.

This end of the field is know to contain two Romano-Celtic temples.  These are known from aerial photographs taken in the hot summer of 1976.  I wonder if this area of the town was kept clear of encroaching buildings, pits, ditches and the like deliberately?  If we turn the contrast down (i.e., clip the image at +/- 40nT instead of +/- 7.5nT, we can see one of the temples close to the hedge as a faint white line.

Low contrast version of the area surveyed on day 37.

Low contrast version of the area surveyed on day 37.

Yet another target for the GPR next year!

Some of the team (many thanks Ellen, Mike and Jim!) helped re-do a number of areas of the res survey, plus one extra bonus square.  The biblical deluge of Sunday night (Lamer Lane was flooded once more) was not ideal.  This is the final area completed in 2016:

The 2016 resistance survey.

The 2016 resistance survey.

It is a pretty good result.  There is almost no use of the “edge match” feature of the software to get the various grids to join neatly.  It could be improved.  The very high contrast of the temple rather makes the buildings faint, but either the creation of selective composites (i.e., processing bit of the survey separately), or use of a high pass filter, would improve that.  The survey is quite big for a res survey: 2.5739ha according to TS (or 2.6ha to sane people who round numbers), which equals about 103,000 resistance readings.  That, of course, doesn’t include the large numbers of squares we re-did due to the dry conditions.

There is a great deal more to do in terms of data processing and interpretation, but I think we all deserve a well-earned rest.  Well, at least until Thursday…!

Record breaking

It was an odd day, weather wise. Largely dry with just one quick, light shower, windy at times, sunny spells… Luckily nothing interfered with the fieldwork!

The res team consisted of myself (when I wasn’t putting in grids for people), Ellen, Tim and Pauline.  They pulled out the stops and managed a record-breaking eight grid squares.  Area-wise, that is what the GPR covers in an average day, but for resistance survey at 0.5m intervals, that is very good going.  Well done everyone.

The resistance survey at the end of day 32.

The resistance survey at the end of day 32.

Today’s grids behaved themselves and make the four odd ones from yesterday stand-out even more.  I did make sure that some of the connectors were off the ground today.  How annoying. We may have to re-do those four grids.  The survey did, however, show the buildings along the road in the SE corner beautifully.  The big question… where now?  North to the sinuous ditch?  South for more shops?  West to cover the cross roads?  Only four days surveying left, and we have to assume that we won’t cover eight squares every day.

The mag team also had a very successful day in the Macellum field.

Detail of the mag survey showing the Macellum field.

Detail of the mag survey showing the Macellum field.

We can just see a hint of the cross-roads running NE-SW across Watling Street.  The ‘1955 ditch’ barely shows.  With the eye of faith one might see it in the high readings along the edge of the cross-road, but very much with the eye of faith.  Is the ditch just so built over we cannot see it?  Or was it never built here?

With just four survey days left to go, the team is getting close to finishing the field, but I think we are a day or two short of being able to do that.

The entire mag survey to date.

The entire mag survey to date.

Way down across the field, the GPR team tackled another fiddly staggered bit along the hedge line.  In the next three images I have made the previous days’ surveys partially transparent.

The day 32 GPR, slice 3 (12.5 to 15.5ns).

The day 32 GPR, slice 3 (12.5 to 15.5ns).

The day 32 GPR, slice 4 (15.5 to 18.5ns).

The day 32 GPR, slice 4 (15.5 to 18.5ns).

The day 32 GPR, slice 35 (18.5 to 21.5ns).

The day 32 GPR, slice 5 (18.5 to 21.5ns).

The curious shallow valley to the west of the surveyed area (‘valley’ seems a strong word for it!) that runs down the hill towards the temple is just as devoid of buildings or other recognizable archaeological features as the mag data.  In all three time slices not a great deal shows.  Was this valley always empty?  Or has the archaeology been eroded away, or even buried?  Difficult to say,  There is, however, a long narrow building just to the right of the middle of the surveyed area almost parallel with the hedgerow.  It seems fairly ephemeral, but it definitely there and one corner was picked-up in last year’s grid to the south.

Although the GPR hasn’t covered as much as the mag, we have still collected a mass of data.

Montage showing the area surveyed with the GPR to date.

Montage showing the area surveyed with the GPR to date.

It certainly takes-up a large chunk of my hard disk.

Many thanks to everyone who came out today and worked so hard.  A very successful day all round.  Our next survey day is on Thursday.

Done it!

The mag team have completed the Theatre Field. Yay!

The mag survey of the theatre field, complete.

The mag survey of the theatre field, complete.

Congratulations everybody.  It is quite an achievement.

A detail of the area surveyed on day 29.

A detail of the area surveyed on day 29.

The detail above shows that the very north end of this field is quite busy.  This is unsurprising being right next to the Chester Gate.  The sinuous ditch, or should we just call it the aqueduct?, leaves the town about 50m away from the gate.  Unfortunately, it goes straight into woodland. Hopefully, we may get a chance one day to look for it the other side of the wall.  Rosalind Niblett traced what she thinks is the aqueduct from aerial photographs all the way the valley to Redbourne.

The mag team also started in the next field, but I’ll report on that when there is a little more completed.  The res team struggled with the hard, dry ground conditions and managed another two grid squares.  We will have to stop doing res until we have had some rain.  There is some forecast tomorrow.  The GPR team also completed two areas.  I wanted to post the completion of the field, so I’ll report on everything else this evening.

Rain stops play

Yesterday we had warning that it was going to rain in the early afternoon, but we went out anyway.  We managed a 40x40m block of GPR data and all the annoying partials around the end of the hedgerow with the magnetometer.  Then, in the early afternoon, it started to drizzle.  Luckily we decided to head home as later it really started to rain.  Today has also been washed out.  We’ve had 31mm of rain in the last 24 hours.

The break has, however, enabled me to catch up on some data processing, especially with the GPR.  Firstly, we can see how much we have done so far.

The magnetometry survey after day 13.

The magnetometry survey after day 13.

As can be seen, we have already covered a good sized area of the theatre field.  Still quite a bit to do, however, and the weather forecast is looking a little grim.

Zooming into the area we are working on at the moment…

Mag survey near the hedgerow.

Mag survey near the hedgerow.

We have a part of the SW-NE road (street 25 in Niblett and Thompson 2005) and buildings alongside it.  It looks like we have buildings around the outside edges of Insula XXX and a quite open area in the middle.

The rain has given me a bit of time to work on the GPR results.  Here is the overall plot. Please remember this is a simple mash-up in Google Earth and that I should be able to get much better plots eventually.

All the GPR results up to day 13 plotted together in Google Earth.

All the GPR results up to day 13 plotted together in Google Earth.

The same area as before without the GPR data.

The same area as before without the GPR data.

If you compare the mag and GPR data you can see that some features that look like walls in the mag data show clearly in the GPR data, but some show as faint negative (light) areas. This is probably the difference between buildings which have had their footings robbed and those where they survive.  The temple does not show very clearly in either data set.  It seems likely it was badly robbed.

The area surveyed on days 12 and 13 is especially interesting.

The block of GPR data collected on days 12 and 13.

The block of GPR data collected on days 12 and 13.

The same area with just the mag image.

The same area with just the mag image.

The corridor building shows very well, but the sinuous ditch is conspicuous by its absence.

Hopefully the rain will let up long enough for us to manage a little more survey tomorrow.