Tag Archives: magnetometry

Back to the fire

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

Week three started well with all three machines collecting data.  The Earth Resistance survey was the poor cousin as regards person-power but Ellen and I, helped by Rhian, completed three grid squares after lunch.

Figure 1: Ellen and an earth resistance meter.

The grids are over the fascinating burnt building seen in the mag.  Figure 2 shows the mag data in this area.

Figure 2: the mag data in the area of the res survey.

The black line snaking across Figure 2 from top-left to bottom-right is the aqueduct.  The very bright black-and-white area in the NW corner of that figure is probably a burnt building which was never replaced in stone.  Figure 3 shows the Earth Resistance data.

Figure 2: the Earth Resistance data.

Figure 3 is a crude composite of the data collected in 2016, 2017 and 2018.  The three squares at the north edge are this year’s grids.  We have clearly picked-up a long wall running NW-SE, and some square areas of higher resistance (?floors, maybe).  This survey makes an interesting comparison to the GPR in this area (Figure 4).

Figure 4: the GPR survey in the same area as Figures 1 and 2.

There is a lot of work to do tracing off walls and features from the three surveys.

The GPR crew completed another 80x40m block, although the slope was quite a challenge.  Figure 5 shows the time slices.

Figure 5: day 11 GPR time slices.

Slices 4 and 5 (top-right and middle left) seem the most interesting.  No stunningly clear buildings but lots going on.  Figures 6 and 7 compare the fifth slice with the mag data.

Figure 6: Fifth time slice from day 11 (indicated by the purple line).

Figure 7: the mag data in the area of the day 11 GPR data (shown by the purple box).

Notice how the square of higher mag response shown in darker tones towards the bottom of the purple rectangle are an area of light “low reflections” in the GPR data.  It is possibly something like an earth floor?  Off the west corner of that square in the mag data is a lighter coloured line running to the SW which is matched by a black line of high reflections in the GPR data.  That is clearly a wall, probably made of flint. The very narrow section of the aqueduct which runs east-west across the plot shows very clearly in the GPR data whereas the broader sections do not.  Something odd happens with the aqueduct at the eastern edge of the GPR plot. A  great deal more to tease out.

One problem we have had this year is the sheep.  In general they keep away from us.  The main issue is that some of them think the flags are tasty… (Figure 8).

Figure 8: Tasty! A nibbled flag in Mobbs Hole with the mag crew in the background.

The mag crew consisting of Jim West, Peter Alley and Dave Minty had three annoying partials to do before marching eastwards across the field.  I’m afraid I have not finished processing those annoying squares but I have added in the complete ones to Figure 9 so you can see progress.

Figure 9: mag data in Mobbs Hole after day 11.

Unfortunately, today was a bust as it rained 8.5mm.  The forecast for tomorrow is looking good though.

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End of week two, part 1

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

Despite the unpromising weather forecast (and the unpromising weather at about 8.30am), we managed a whole day of survey including mag, GPR and a little bit of earth resistance. There was some drizzle around about lunchtime, but other than that is was an OK day, if a little windy at times.

The GPR team were working on “sawtooth” edges to the theatre field.  As we have a couple of days off, I will post the results of their efforts tomorrow.

The mag team completed twelve grids, including two partials and despite one case of “sensor freeze”; a super effort (Figure 1).

Figure 1: the mag crew, consisting of Ruth Halliwell, Jim West and Dave Minty, operate the Foerster in Mobbs Hole.

Figure 2 shows all the areas surveyed so far at Verulamium.

Figure 2: all the mag survey so far.

This really shows that the strength of the Foerster cart system is when one has large open fields.  So far, according to TerraSurveyor, we have completed 9.8ha of Mobbs Hole over ten days, but remember we only had part of an afternoon on day 1 and lost parts of two days this week to rain.  Compare that to surveying in Verulamium Park with all its trees, hedge lines, park benches and so forth.  There we surveyed just under 30ha in 45 days.

Figure 3 zooms into the area surveyed over the last few days.

Figure 3: mag survey after day 10.

So far the results are curious.  There are linear features associated with the line of the Fosse, plough scars running NW–SE down the slope, a few large “blobby” anomalies (pits, maybe?) and some bits of old iron.  Very little which could be interpreted as structures has been found.  Why build an enormous ditch and bank around nothing?  Ploughing may have removed some superficial features, but there is very little that could be seen as pits or ditches either.  The Fosse remains a mystery.

Figure 4 shows where the Fosse enters the wood.  The dip in the fence marks the ditch.  Once inside the wood the Fosse does a sharp left turn and heads southwards.

Figure 4: the dip in the line of the fence shows where the Fosse enters the woodland.

Tomorrow I will report on the earth resistance and GPR surveys in the theatre field.

Many thanks to everyone who has helped this season.  We are already half way through.  Doesn’t time fly?

 

7.2mm

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

That is the quantity of rain which fell on Lamer Park today up to when I started writing this posting.  It is now at 10.3mm! No wonder we gave-up at lunch time all feeling a bit cold and damp.

The mag team did manage three grid squares before lunch (Fig. 1).

Figure 1: mag survey after day 7.

There is a interesting possibility that we have detected a break in the Fosse, maybe some form of entrance.  I have noticed that some of the maps of the Fosse show a break, and some do not.  Perhaps we may be able to resolve this.

The GPR crew managed their usual 40x40m block before the rain drove us away at lunch time.  Figure 2 shows four time slices.

Figure 2: GPR data from day 7.

There is a quite substantial feature showing in slices 3 and 4 (top row) to the west.  This “big blob” is on the edge of the aqueduct, but I am not sure what it is.  Finally, however, we have some clearer looking buildings, best seen on the east side of slice 4 (top-right of Figure 2).  Figures 3 and 4 show this slice in context.

Figure 3: GPR from day 7 in context, slice 4.

Figure 4: the mag data for the same area as Figure 3.

As we have seen before, the buildings in this part of town show in the GPR data but not in the mag data.

On an unrelated note, this week’s Herts Advertiser (dated August 9th) carries a short letter from me about the project.  It also carries the news that the school at Batford has been given the go-ahead (see https://wp.me/p3AAk9-gy for our survey there).

Getting from the Theatre Field to Mobbs Hole is a bit of a problem because some pesky Romans built a wall between them.  There is, however, a gate we can use which is moderately convenient, which we have dubbed “The Manikin Gate”.

Figure 5: the Manikin Gate.

The weather tomorrow morning looks OK according to the Met Office, but we may be rained off again in the afternoon.  On the bright side, it might be possible to break out the earth resistance meter soon!

 

 

 

Sawtooth Saturday and other tales

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

The lack of posts doesn’t mean we haven’t been out working…

Enough of that!  The lack of posts is simply that we had a friend staying and a BBQ and beer won over working on the blog.  Sorry…  Well, not very sorry.

The mag team have been working southwards across the “Fosse field”, the proper name for which is Mobbs Hole.  The area we have covered in around Verulamium is getting pretty large (Fig. 1)!

Figure 1: total area surveyed to date.

The survey has now started to clip the edge of the Fosse itself. In Figure 2 I have used the 2006 imagery in which one can see the Fosse clearly as a soil mark.

Figure 2: the mag data on the 2006 imagery in Google Earth.

Progress is excellent for four and a half days of survey.  Figure 3 shows the survey in more detail.

Figure 3: the mag data after day 5.

The edges of the Fosse can be seen in the mag data, mainly as a lighter line.  This is because the topsoil will be thinner over the lip of the ditch as soil has eroded down into the fill.  There is a line of large dark “blobs” along the lip of the Fosse.  Although these might be something interesting, I suspect they are tree-throws (i.e., the hole made by a tree being blown over).

The field system which shows in the upper half of the survey is interesting.  Jon Mein has kindly shown me the 1799 map of the parish and these boundaries do not match those mapped then.  There is a line of woodland following the line of the Fosse a little to the south of the area we have reached.  The track which runs along the northern edge of Mobbs Hole was a much more important road at that date.

Wheeler thought that the Fosse represented the “first Roman city” at Verulamium and cut several sections across it (Figures 4, 5 and 6).

Figure 4: Wheeler and Wheeler 1936, plate 109, detail showing the location of the sections excavated by the Wheelers.

Figure 5: Wheeler and Wheeler 1936, plate 18. Sections across the Fosse.

Figure 6: Wheeler and Wheeler 1936, plate 78. Sections across the Fosse.

We now know that the early Roman town was based down towards the forum where the Museum now is.  The Fosse, however, does appear from Wheeler’s finds to be first century.  But what was it for?  Hopefully, the geophysics within the line of the Fosse may give us a clue.

One aspect of the landscape I had not appreciated was that the dry valley which the aqueduct has to dog-leg across as shown in our survey (the V-shaped long linear feature within the town walls shown in Figure 2), becomes quite a major feature to the west of the walls.  The northern arm of the Fosse lies on the crest between the dry valley and the valley of the Ver, and then when it turns to form the southern arm it has to cross that valley.  Figures 7 and 8 compares the plate published by the Wheelers showing the view from the crest to a panorama I took the other day.  The hedge line shown in the Wheelers’ plate is no longer there.

Figure 7: view southward across the Fosse as published by the Wheelers in 1936.

Figure 8: Panorama of Mobbs Hole (click to see full sized).

Back inside the town walls the GPR crew completed an awkward couple of blocks on “sawtooth” Saturday and another 80 x 40m block on Sunday.  Figures 9 and 10 show some of the time slices.

Figure 9: the GPR from day 4.

Figure 10: GPR time slices from day 5.

On neither day do we have some of the beautifully clear buildings we have seen previously.  There does seem to be a small square structure showing on day 5: the NW edges showing in slice 8 (the sixth image in Figure 10) and the bottom edge in slice 10.  Figures 11 and 12 show slices 5 and 6 from days 2 to 5 in Google Earth.

Figure 11: GPR days 2 to 5, slice 5.

Figure 12: GPR days 2 to 5, slice 6.

Although we have not got lovely clear buildings, the “blobby” bits do have a generally SW — NE orientation which matches the town grid.  It is, however, very difficult to interpret.  Looking carefully, however, there is more than immediately catches the eye.  Lets look at the day 5 data more closely.

Figure 13: GPR day 5, slice 6.

It all seems rather dull.  Now plot that on top of the mag data.  Look at the point the red arrow is indicating in Figure 14.  Figure 15 just shows the mag data.

Figure 14: Day 5 GPR overlain on the mag data.

Figure 15: the mag data from the same area as Figure 14.

The feature which shows quite clearly in the mag data does show quite faintly in the GPR.  The moral: lovely clear buildings are wonderful, but sometimes there is more there than you think.

We have been enjoying our two days off, and will be back at it on Wednesday.  The forecast is for it to be a bit cooler.  Luckily, our guard dog has been keeping a close eye on the flags…

Figure 16: Guard Dog.

Many thanks to everyone involved.  The heat has made it quite hard work, but the survey looks ever more amazing with each new grid square!

 

Back and forth

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

Today was like most days, back and forth along the strings. We had the added delight of temperatures into the 30s centigrade. Just a little toasty. It is (hopefully) going to be cooler tomorrow.

The mag team consisting of Jim West, Ruth Halliwell, Dave Minty and Rhian Morgan continued extending the block of mag data southwards.  The results are shown in Figure 1.

Figure 1: the magnetometry data at day 3.

As you can see, there are a lot of linear features, some very faint. In the theatre field I was clipping the image to +/- 5nT.  Here I have clipped it to +/- 2nT.  They look like field boundaries, and the one to the west heads for the corner where the gate is.  They don’t, however, match the boundaries shown on the 19th century OS maps.  I’m probably going to have to shell-out for the tithe map of St Michaels.  Tomorrow the crew are going to keep heading south and up slope, and soon they should be hitting the Fosse.

Figure 2 shows quite how dry the site is.

Figure 2: John Ridge and Anne Petrie using the GPR.

The GPR team consisting of Mike Smith, John Ridge and Anne Petrie, occasionally hindered by yours truly, managed our standard 80x40m block, although in the heat in felt like a great deal more.  Time slices 3 to 11 are given in Figure 3.

Figure 3: time slices 3 to 11.

As with yesterday’s data, it is annoyingly blobby.  In a way this is probably a good thing as it means that not everything has been ploughed right down to its foundations.  The fourth slice (top row, middle slice) seems to show a wall running SW–NE across the middle part of the plot.  There is something going on here, but it is very unclear.  The next three Figures show slices 4, 5 and 6 from today and yesterday superimposed on the Google Earth image.

Figure 4: days 2 and 3, slice 4.

Figure 5: days 2 and 3, slice 5.

Figure 6: days 2 and 3, slice 6.

Hopefully, as we get closer to Watling Street to the north, we will start picking-up some clearer structures.

I think everyone was grateful for the end of the day.

Figure 7: at the end of the day.

Tomorrow, however, is sawtooth Saturday…

Figure 8: a plethora of survey flags.

Many thanks to everyone who helped today.  It was hard work in the sun!

El Scorchio

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

It was hot today. Not a cloud in the sky in the morning (Figure 1).

Figure 1: the GPR crew setting-up under a cloudless sky.

We did, however, manage to complete our target of two 40x40m GPR grids and seven mag grids, so a good day’s work all round.  I also put in enough grid pegs to keep people busy for the next two days.

The mag team of Jim West, Graham Spurway and Rhian Morgan continued working in the “Fosse Field” outside of the town walls (Figure 2).

Figure 2: Rhian Morgan (CVAHS) operates the magnetometer.

The results are encouraging.  As well as more of the very faint features we detected yesterday, we have some much clearer linear features, almost certainly ditches, as well as a couple of other more amorphous ones which may well be pits (Figure 3).

Figure 3: Magnetometry results after day 2.

The linear features look like field boundaries but they do not bear any relationship to the current boundaries. Indeed, just to the north of the area we have surveyed, past the track which can be seen in the image, is a large lynchet caused by many years of ploughing.  We would appear to be seeing something pre-dating the laying out of the park at Gorhambury.

The GPR team of Mike Smith, Dave Minty, Nigel Harper-Scott and John Ridge, having filled-in the annoying gap yesterday, have moved to the north end of the site near where we park.  Slices 3 to 8 are shown in Figure 4.

Figure 4: GPR time slices 3 to 8.

Unlike yesterday, we didn’t have quite such clear buildings.  There does, however, seem to be a rather knocked-around one in the western half of the block.  The mag isn’t all that clear in this area ether apart from a ditch running NNW–SSE.  The next two figures show two slices in context.

Figure 5: Time slice 5.

Figure 6: Time slice 6.

With the eye of faith one can just about see the ditch in the GPR data, but the large areas of high reflections are hard to interpret clearly.

By the end of the day, we did start getting some clouds.

Figure 7: Clouds and stripy GPR.

I’m lovin’ those stripes in the brown grass!

 

Gorhambury, year 4 day 1

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

It really does not seem like the fourth year we have returned to Gorhambury, and the sixth year of the project, but it is true.  In our first year at Gorhambury we had GPR for the first time and mag, but the res would not work.  In the second season we had all three, although we had some problems with the res and dry weather.  Last year we had nothing for the mag to do, but the res was working well.  This year the drought has made doing res impossible.  Oh well, life would be boring if everything went to plan!

The mag survey is expanding into the field through which the Fosse runs.  Figure 1 shows the location of our first two grids.

Figure 1: the location of the first mag grids of 2018.

They are looking a bit lonely up in the that top corner, but it won’t take long for them to start fitting into the wider pattern.  Figure 2 shows the grids closer up!

Figure 2: day 1 mag results.

Nothing too stunning in our first two grid squares.  Quite a few bright black-and-white spots which are tractor bolts or horseshoes.  The eastern grid, however, does have two quite faint parallel lines which turn a right angle.  My first though was agricultural marks but they do not lie at angle which would make sense in comparison to the field boundaries.  It will be helpful to see the wider pattern when we have surveyed some more.

The big success with the mag is that it is fixed!  We took the morning to do a test grid square and process it to make sure that it all worked OK.  Many thanks to Pat for bringing the mag down from Tamworth and helping survey the test square.

At the end of last year we had an annoying two grid square gap in the side of the GPR survey.  We decided to fill this in first.  In Figure 3 I have outlined the block we surveyed today in red.

Figure 3: GPR grid location for day 1.

As GPR data is three-dimensional, getting the grids to match is quite a challenge.  At the moment, I have just been processing each block individually and crudely adding them together in Google Earth.  One day I’ll have to do it properly!

Figure 4 shows eight of the amplitude slices from today’s survey.

Figure 4: day 1 time slices.

The slice in the top-left corner shows the first hints of archaeology under the ploughsoil.  In the second slide walls and a road are starting to show quite clearly. Note how some of the building walls show more clearly at a deeper depth than the road, especially the small square one which is on its own.

In Figure 5 I have just taken one time slice, a5 (third one on the top row) and put it in context in Google Earth.

Figure 5: day 1 GPR results.

I have labelled some of the main features.  One pleasing thing is that the building on the eastern edge of the block we surveyed today matches perfectly with the block we surveyed last year.  Yay for differential GPS!  One interesting thing is that there seems to be a wall cutting across the line of the “1955 ditch”, the first century boundary of the city.  We do have structures built over the line of the ditch in the southern part of the town, and Frere thought the boundary went out of use in the early second century, around about AD 125.

Hopefully we will be able to greatly expand the mag survey tomorrow, and we are going to start filling in the GPR survey in the northern part of the theatre field.

Many thanks to everyone who turned out today in the hot weather.