Tag Archives: magnetometry survey

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.

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I think we found a building…

Today we managed to run all three instruments despite the very blustery weather and the occasional shower. Many thanks to everyone who put up with the bad weather.

Firstly, the magnetometer survey went very well today with another nine grid squares completed, albeit there were many partials!  Here are the results:

The mag after day 31.

The mag after day 31.

As can be seen, the nice straight line of Watling Street continues running from the SE to the NW. Curiously, there appears to be something cutting across the street and running down either side (shown as white lines).  How strange.  The star find, however, is the very nice building parallel to Watling Street.  The wall foundations show as clear white lines against the mid-grey, i.e., non-magnetic wall foundations (or foundation trenches) cutting through the more magnetic background.  Just in case anyone is wondering where the building is:

Today's mag showing the building.

Today’s mag showing the building.

OK, I’m not being entirely serious.  It is perhaps one of the clearest buildings I have seen in our mag data despite the rather noisy background.

The GPR crew were working quite some distance away.  Here is a location plan:

Location of the three surveys.

Location of the three surveys.

It is 550m from the magnetometer survey to the GPR survey as the swallow flies, slightly further as the archaeologist trundles.

The GPR team were not to be outdone by the mag team.  Here are four time slices from the area they did today. Remember each slice is going a bit deeper into the ground.

Day 31, GPR slice 2 (9.5 to 12.5ns).

Day 31, GPR slice 2 (9.5 to 12.5ns).

Day 31, GPR slice 3 (12.5 to 15.5ns).

Day 31, GPR slice 3 (12.5 to 15.5ns).

Day 31, GPR slice 4 (15.5 to 18.5ns).

Day 31, GPR slice 4 (15.5 to 18.5ns).

Day 31, GPR slice 5 (18.5 to 21.5ns).

Day 31, GPR slice 5 (18.5 to 21.5ns).

The top image (time slice two) is just showing noise in the ploughsoil.  The second image is showing a nice Roman building very clearly.  There might be more than one phase, with the upper phase being somewhat damaged.  The third image shows the building very clearly.  In the last image some of the internal walls show a little more clearly suggesting that they may have been robbed or replaced at some point.  All in all, a rather nice building in a prime position on the road across the town.

The GPR team have now covered quite a large area of the town!  Remember that they have to walk four times the distance to cover the same area as the mag, but have the advantage of being able to create images at different depths.

The extent of the GPR survey after day 31.

The extent of the GPR survey after day 31.

The Earth Resistance survey managed four grid squares today after the rain made it possible to take readings at all!

The earth resistance survey after day 31.

The earth resistance survey after day 31.

The res has picked up some more details of the Insula XVI temple.  Unfortunately, the trick of spreading the remote probes widely apart to avoid problems with grid matching has finally failed.  We did not have enough rain to change the moisture levels at 50–75cm down, so I am not entirely sure why this is the case.  Perhaps some current leaks through the grass?  I’ll have to investigate.

The weather forecast is better for tomorrow and so we will be out again in force.

Watling Street

Firstly, apologies to those who turned-up and found themselves unemployed. With ground conditions difficult for the resistance meter, we just couldn’t run all three machines. Thank you for being so understanding.  Looks like it will rain most of tomorrow, so hopefully we can press-on with the resistance survey on Saturday.

The mag survey in the second field, which I am going to call the macellum field for convenience (it isn’t the field’s real name) is going well.  Firstly, an image of the whole town:

The mag survey of Verulamium.

The mag survey of Verulamium.

My long-held dream of having a complete survey of Verulamium is getting close to becoming a reality!  Thank you everyone.

Looking at the new area in more detail:

The area surveyed on days 29 and 30 in the macellum field.

The area surveyed on days 29 and 30 in the macellum field.

Hopefully, everyone can see the clear linear feature running from the SE to the NW.  This is Watling Street, the main road from London to Chester.  As one might expect just near to the gate (which was robbed and then excavated, and lies in the trees), there appear to be lots of little buildings along the road which show as white lines against the mid-grey background. What is more curious is the rather different look of that background: much less even and more noisy. I’m not sure why, yet.  The field certainly feels different: flatter, slightly different vegetation, and is obviously closer to the river.  Perhaps we have moved from chalk to the river gravels?  I must check the geology map…

The GPR has been doing lots of bitty blocks around the edge of last year’s survey area.  Rather than hold-up the posting of the mag data, I thought I would do the GPR later when I have dropped all the different areas on to Google Earth.

Thanks to everyone who helped today, especially Graeme for acting as my driver and go-fer as I limped around the field  (don’t ask!), and Ruth for helping transport all the equipment.

 

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.

The mystery deepens

We managed to run all three machines today, although we were short handed on the resistance meter so many thanks to Peter and Ellen for soldiering-on. The unfortunate side-effect of the lovely weather is the top surface of the field is now like concrete!

First of all, the resistance results.

Day 27 resistance results.

Day 27 resistance results.

Day 27 resistance results, high pass filtered.

Day 27 resistance results, high pass filtered.

Not much new showing today, but we are getting a little more of the temple.  Tomorrow’s grids should show quite a bit more, if we can get the probes in the ground!

The GPR has, for the last few days, been working its way along the northern edge of the field in a series of stepped blocks.  With GPR it is not so easy to have a ragged edge.  Mike Smith, our expert GPR cart wrangler, felt sure they had found some stuff today.  Here are the last three days (the three blocks with jagged top edges) with today’s being the easternmost one.

GPR survey, days 25--27 (slices vary).

GPR survey, days 25–27 (slices vary).

Timeslice 3 (12.5 to 15.5ns) is usually a good starting place at this site.  Not much showing in that bottom block.  How about the next slice down?

GPR survey, days 25 to 27. Day 27 (the easternmost block with a jagged north edge), slice 4 (15.5 to 18.5 ns).

GPR survey, days 25 to 27. Day 27 (the easternmost block with a jagged north edge), slice 4 (15.5 to 18.5 ns).

I think Mike is right!  Excellent stuff.  Let us look a little closer at today’s block:

Detail showing day 27 results, 4th time slice (15.5 to 18.5 ns).

Detail showing day 27 results, 4th time slice (15.5 to 18.5 ns).

What is really interesting is not only do we have evidence of yet another building in the western side of today’s block, but the white lines surrounding darker areas on the eastern side suggest robbed walls of a large building.  Tomorrow’s results are going to be very interesting!

And to the topic of the TLA from the post earlier this evening.  Let us recap the story.  Last year we started picking-up a long ditch which ran right across the site.  It had been found in the two transects of mag undertaken by English Heritage in 2000, but at the time they were not linked. Last year we speculated that the ditch ran up the dry valley that the mag team have been laboring up and down the last few days.  On the 19th century maps there is a well marked on the other side of the Fosse field.  In the next image I have marked the line of the ditch.

The sinuous ditch.

The sinuous ditch.

Much to my surprise, the ditch curves around on itself at the western end, perhaps following the contours of the sides/bottom of the valley.  I will have to check that with the dGPS in the near future.  But where is it going?  Is it still an aqueduct as we thought, or something else entirely?

Looking more closely, we can see there are lots of other, smaller linear features in the area, some quite straight, and possibly even another building.  This top-corner of the field is proving very interesting and we will have to overlay the results on the topography to see what is happening.  This bit of the field is anything but flat!

Detail of the western end of the sinuous ditch.

Detail of the western end of the sinuous ditch.

Tomorrow we see the mag team edging closer to completing the Theatre Field, the GPR working along the northern edge towards the theatre and the the resistance meter covering more of the Insula XVI temple.

Almost round the bend

Today saw a slightly smaller team than we have had, but we still managed a good area of magnetometry survey and GPR, and even one small square of Earth Resistance survey.

First, the mag survey.  The team are starting to work their way north along the western edge of our survey area filling in between what we have already surveyed and the third century town wall which is hidden in the trees in the Google Earth image.

The magnetometry survey up to the end of day 24.

The magnetometry survey up to the end of day 24.

Looking at the area surveyed today in more detail, we can see the beginnings of the corner of the “1955 ditch”.

The area surveyed on day 24.

The area surveyed on day 24.

There is surprisingly little of anything much showing inside or outside the ditch in this corner.  The ditch is, however, slightly narrowing and bowing.  How curious!  Tomorrow should, fingers crossed, see us pick up the rest of the corner.

The GPR team, way down the hill near the drive, completed another 40x80m block.  Here are four 3ns thick time-slices.

Day 24 GPR, time slice 2 (9.5 to 12.5ns).

Day 24 GPR, time slice 2 (9.5 to 12.5ns).

Day 24 GPR, time slice 3 (12.5 to 15.5ns).

Day 24 GPR, time slice 3 (12.5 to 15.5ns).

Day 24 GPR, time slice 4 (15.5 to 18.5ns).

Day 24 GPR, time slice 4 (15.5 to 18.5ns).

Day 24 GPR, time slice 5 (18.5 to 21.5ns).

Day 24 GPR, time slice 5 (18.5 to 21.5ns).

The top image (time slice 2) just shows the noise in the ploughsoil.  The second image, however, shows a lovely little building 16.5m by 10m in size, aligned with the “1955 ditch”.  I’m not certain what this building is, it seems an unusual plan for a domestic structure.  The third image (time slice 4) shows this building, but also a very strong reflection from a wall on the eastern side.  Presumably this is part of a building which has been robbed out more thoroughly.  In the last time slice the signal has “attenuated” and we are only getting the strongest reflections showing.

The overall image gives some idea of how much we have now covered.

The area covered by the GPR at the end of day 24.

The area covered by the GPR at the end of day 24.

This image is a bit of a mismatched mishmash as the data was collected at different times and the time slices are somewhat variable as I have learnt to process the data over the last year.  At some point all the data will need to be reworked systematically, but that is beyond me while we are out collecting yet more data every day!

Many thanks to everyone who helped, and welcome to the people who have recently joined the team.  Your efforts are producing spectacular results.

 

Apse mad

We had a very successful day. It was grey and a little drizzly first thing, but true to the forecast it warmed up and turned into a gloriously sunny, if a little windy, day.  The mag team got right into the deep south and have made the mag plot look very tidy.

The magnetometry survey at the end of day 23.

The magnetometry survey at the end of day 23.

The team were determined to finish off the last few partial grid squares.  I think they were fed up walking the 650m from the cars!  The next swathe of grid squares will head north completing the Theatre Field at last.  Let’s look at the area surveyed today.

The area surveyed on day 23.

The area surveyed on day 23.

The band of buildings running from the NE seems to meet the 1955 ditch at a point where the magnetic response from the ditch changes markedly.  Perhaps the ditch was more deliberately filled-in here?  To the SW of the 1955 ditch there are fewer clear indications of stone buildings, but there are many linear features (probably ditches), and “blobs”, probably mainly pits but some of them are very large.  Of particular note is the small square enclosure right up against the town wall towards the south (seen as a black square).  One wonders what this was so close to the third century town wall.

At the other end of the field, the GPR crew managed another 80x40m block of data.  Apparently, nothing much was showing.

The day 23 GPR block (fourth slice, 15.5 to 18.5 ns).

The day 23 GPR block (fourth slice, 15.5 to 18.5 ns).

I beg to disagree!  Another small apsidal building is showing in the data, overlying the line of pits.  There is a large, rectangular, building to the west of it.  Are apsidal buildings the geophysicist’s version of busses?  One interesting aspect of this building is that it does not show in the mag data at all.

The same area as the previous image showing the magnetometry data.

The same area as the previous image showing the magnetometry data.

This is a great example of why undertaking both the GPR and the magnetometry surveys is so useful in giving a fuller picture of the town.  The GPR surveys are getting quite extensive!

All the areas surveyed with the GPR on the Gorhambury side of the town.

All the areas surveyed with the GPR on the Gorhambury side of the town.

The earth resistance team completed another excellent five 20x20m squares.

The earth resistance survey after day 23.

The earth resistance survey after day 23.

We can see some more buildings and bits of road, although there are some very high resistance areas.  One weakness of resistance survey is that there can be underlying variations in the data related to factors such as slope or geology which mask the archaeological patterning.  We can, however, apply a “high pass filter” which attempts to remove the underlying background trend and show the sharper differences more clearly.

The earth resistance survey, high pass filtered.

The earth resistance survey, high pass filtered.

The technique can create some artefacts in the data, but is very useful for bringing out some of the buildings.

Tomorrow and Tuesday are our “weekend”.  Many thanks for everyone who joined in during the first week, and here is to the next three!