Tag Archives: Foerster

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?

 

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One hundred and fifty

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

We managed a full day today, and I’m just about keeping up! The mag team completed two grids yesterday, half of one in the aforementioned deluge. Today they completed 11 grids: three partials and eight complete ones. Way to go! Well done everyone. Figure 1 shows the survey so far.

Figure 1: the mag survey after day 9.

One really does wonder if that break in the mag data is an entrance.  It doesn’t seem like it on the ground.  I have downloaded the LiDAR data but haven’t had a chance to process it yet.

The GPR crew finished their 80x40m block, and then did some of the next “sawtooth” section, another 14m worth.  Figure 2 shows the time slices.

Figure 2: day 9, time slices 3 to 6.

Nothing jumps out at one, although there are some curious “light” lines in the fourth slice (top-right) which are parallel to the aqueduct.  Figure 3 shows that slice in context.

Figure 3: GPR survey after day 9, slice 4.

After all the rain I thought it would be worth trying the Earth Resistance survey (Fig. 4).  I spent the morning laying in grids for the mag, but managed some survey in the afternoon.

Figure 4: Earth Resistance survey in action.

Although the rain has softened the surface, it won’t have penetrated 50cm yet, and I was concerned that there would be no contrast at that depth.  I decided to survey a grid where we knew there was a building.  Fig. 5 shows the comparison between the GPR survey and the two squares of res I managed to complete (thanks Anne!).

Figure 5: Earth resistance survey compared to GPR results.

Given the drought, the results are pretty good.  It would be interesting to compare these to results from a normal English summer!

Tomorrow isn’t looking great.  We might get some work done in the morning.  Fingers crossed.

Many thanks to everyone who helped out today.  Especially big thanks to Mike, Ellen, Jim and Ruth who take on the responsibility of shipping the equipment back and forth.

By the way, this is the 150th blog post…

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!

 

 

 

A light wind swept over the corn, and all nature laughed in the sunshine.

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

Not much sign of corn, but the wind was blowing over the parched grass.  It was a welcome relief to be working in cooler conditions, although the weather was still beautiful.

Figure 1: St Albans Abbey.

The mag team completed an excellent eight grids in Mobbs Hole (Fig. 2).

Figure 2: the mag survey after day 6.

As before I have overlain the survey on the 2006 imagery in Google Earth which shows the Fosse most clearly.  The hints of a line along the inner edge.  Maybe this is the remains of a palisade trench?  Disappointingly little otherwise.  One thing to note is that the “noise” from random ferrous trash is more prevalent to the south of the old fence line than to the north.  I wonder if the NW corner of this field was pasture previously?

The GPR crew completed their two grids.  Figure 3 shows some time slices.

Figure 3: Day 6 GPR results.

Not a great deal showing apart from in slice 4 (in the top-right corner of Figure 3) which clearly shows the aqueduct.  This is about as clear as I have ever seen it in GPR data.  Awkwardly, the direction of the transects is close to the direction of the aqueduct.  Figure 4 shows the slice in context with the others we have surveyed this year.

Figure 4: GPR day 6, slice 4.

The edges do not match because I keep playing with the settings in the software.  One day, I’ll slowly process the whole lot so that we get a nice final result.  One day (more like several months…).

Barney and Becca came and helped with the GPR in the morning.  After lunch, we blew the dust off our Bartington and did a couple of squares.  I rather liked this image of Barney.

Figure 5: Barney and a Bartington.

The survey moves on and we cover more ground.  Many thanks to everyone who comes and helps expand the area we have covered.  Luckily, most of the area you can see in the last inage has already been done!

Figure 6: Ellen and Mike operate the GPR.

 

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!

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.

Back to Durobrivae

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

Firstly, many apologies for the time it has taken to write this report.  The data were quite complex, and the day job comes first.  This is a monster blog post, so make a cup of tea and settle back somewhere comfy!

Last year CAGG teamed-up with local groups to undertake some survey at the Roman ‘small town’ of Durobrivae, near Peterborough. If you have ever driven on the A1(M) past Peterborough you would have passed by the site. Our aim was simply to determine which geophysical survey techniques would give good results at this site.  The answer was: all of them!  A short note on the results has just been published in the International Society for Archaeological Prospection‘s newsletter.

We decided to add to our original survey by undertaking another three days work from 4th to 6th November.  Unfortunately, it decided to rain in the morning of the first day and so we lost some time.  We had enough helpers to run the mag, two Earth Resistance meters (the Welwyn Archaeological Society‘s and UCL‘s) and the Malå GPR we had on loan from SEAHA.  A small group of us returned on 26th November to expand the magnetometry survey and undertake a topographic survey using the dGPS.   The first three figures just show how much we have done so far.

Fig. 1: the complete mag survey as of the end of November 2017.

Fig. 2: the complete GPR survey as of the end of November 2017.

Fig. 3: the complete Earth Resistance survey as of the end of November 2017.

In the following post I am going to firstly discuss the western block of data, and then the eastern block over “the tumulus”.

Last year we completed a 80m x 360m transect of mag data across the town.  We also completed two blocks of GPR data, one 80×80, and one 80x40m.  This year we wanted to fill in the gap between those two blocks so surveyed another 80x40m block giving us one contiguous 80x160m survey.  Unfortunately, matching GPR grids is quite difficult, especially when there is a year between when they were collected, and therefore quite different ground conditions.  I did, however, manage to produce some “OK” time slices by applying a zero-mean traverse to each line of GPR data.  Hopefully, I will be able to create better slices in future, but these will do for now.  I also noticed that Larry Conyers had produced a much clearer plot of the temple by using a much thicker time slice.  I usually aim for 3ns thick slices.  Larry, however, used 8ns slices.  Here I have compromised by using 5 1/4 ns slices with a 50% overlap.  Fig. 4 is a composite of 12 slices starting at the surface.

Fig. 4: twelve time slices of the western area. Each slice is 5.27ns in thickness.

Apart from Ermine Street cutting across the top right hand corner, the first three slices are not really showing anything much of interest. Let us now look at the individual slices in more detail.

Fig. 5: GPR time slices 4 and 5.

In Figure 5, left, we can see the temple (A) starting to show as an area of lower reflections.  Larry Conyers was able to demonstrate that the interior of the temple building was clear of rubble, and thus there is little to reflect the radar waves.  In slice 5 on the right, we can see a linear feature (B) to the west of the temple.  This lines up perfectly with a strongly magnetic feature and is therefore a narrow cut feature like a ditch or possibly a wall foundation.  There are faint hints of buildings with robbed-out walls at C and D, showing as light areas of low reflections.  Similarly, at E, we can see some of the buildings alongside Ermine Street.

Fig. 6: GPR time slices 6 and 7.

In Figure 6, left, at A, we can see one of the minor side roads off Ermine street starting to show.  The building at B is still visible (just), and we are can start to see a wall parallel to the linear feature we saw previously (Fig. 5, B).  This suggests to me that we are dealing with a cut feature rather than a robbed wall.  Slightly deeper, in Fig. 6, right, at D we can see more buildings alongside Ermine Street.  The square building at E is now showing more clearly.  The large building at G is beginning to show quite clearly at this depth.  I am puzzled, however, that the road coming from the east seems to end in a sharp angle at F, as though something has cut through it.

Fig. 7: GPR time slices 8 and 9.

In Fig. 7, left, at A we can now see the building to the north of the temple very well.  It would appear the walls have been robbed but some of the floors left intact.  At B, the large building to the east of the temple is showing very clearly now. The wall along the west side of the temenos of the temple (C) is showing clearly at this depth.  Starting to show, but more clearly in the next depth slice at D, is a long wall running across the site.  There seems to be an almost entrance-like feature in it at the western end.

Fig. 8: GPR time slices 11 and 12.

Skipping a slice and moving to No. 11 (Fig. 8, left), we can see the wall to the west of the emple at A very clearly.  The possible floor of the building to the north at B still shows.  In the deepest slice I have generated, we have a curious series of curved features at C.  I have no idea what these are.  Answers on a postcard, please, to…

Fig. 9: the Earth Resistance survey (lower half) overlain on the GPR data (faded out a little).

Figure 9 shows the Earth Resistance survey.  We added a single line of grids on the eastern edge of the block we did last year.  There is a strange speckly effect in the new strip.  This block of grids we did with WAS’s TRCIA meter.  The resistance values were very high, and the meter had to keep swapping range which, apart from slowing us down in the field, may be the cause of the rather odd looking results.  The main result in the new strip is the high-resistance line running WSW to ENE which is probably a road.

A comparison with Stephen Upex’s transcription from aerial photographs is quite informative (Fig. 10).

Fig. 10: Transcription of the aerial photographs for the temple complex by Stephen Upex.  The image has been rotated to match the geophysical surveys. © Stephen Upex, reproduced with permission.

Some of the details between the aerials and the geophysics agree quite well.  The temple itself, and the temenos are pretty good.  What about the circular shrine?  I reprocessed the GPR data from just that section using 6.5ns thick slices this time.  I have produced an image of slice 6 (16.04 to 22.49ns) in the variety of palettes offered by GPR Slice (Fig. 11).

Fig. 11: GPR time slice of the area to the immediate north of the temple in a variety of palettes.

The building to the north of the temenos shows quite well.  There are hints of a circular structure just to the north of the main temple building lying underneath a robbed rectangular building.  The circular feature shows quite well in the last palette, and the antepenultimate one.  The rectangular building is clearer in the second slice where black is showing areas of low reflections.

There is clearly a great deal which can be teased out of this data, but let us move on!

The second area we surveyed was over the so-called tumulus towards the western side of the town.  The mound showed quite nicely last year when the evening mist rolled in on the last day (Fig. 12).

Fig. 12: The mist shows the location of the “tumulus” beautifully.

Our aim was to survey the mound using all three techniques.  Unfortunately, the half day we lost to rain resulted in not covering quite as much ground as we hoped.  The mag results were especially interesting, hence our return to expand the survey area a few weeks later.

Fig. 13: the magnetometry survey of the eastern area over the “tumulus”.

Figure 13 shows the magnetometry results.  Ermine Street and the minor road running off it show well.  The town wall also shows clearly.  The zig-zag look to the wall is not “stagger” in the usual sense of the odometer being incorrect, but a result of the cart going up and down a steepish slope resulting in the sensors not being vertical.  There are indications of more long, thin buildings coming off Ermine Street at right angles, and plenty of other pits, ditches and other features.  The really curious aspect though, is the empty space in the middle, under the “tumulus”.  This seems to have a polygonal linear feature around it, showing as a magnetic positive and therefore either a cut feature, or a brick-built wall. To the east / south-east of the tumulus is a largely open area, somewhat fan shaped in appearance.  How very curious.

I wanted to check the relationship between the results and the topography so I undertook a topographic survey with the dGPS taking readings every six paces (just under 5m).  This differs from using the UAV.  The GPS survey will give us a digital terrain model (DTM) which is the actual surface, but at a cruder resolution.  The UAV will give us a digital surface model (DSM) which gives the surface and thus maps the tops of stinging nettle patches and so on, but at a much higher resolution.  We saw this at Darrowfield. Neither method is better than the other, it depends on ones aims, but using the UAV is certainly very much quicker in the field!

Fig. 14: dGPS topographic survey of the area around the tumulus.

As can be seen from Figure 14, the tumulus shows as an elongated feature running SW-NE.  My guess is that this shape is a result of plough damage.  How does this relate to the mag results?

Fig. 15: the topography with the mag data overlain on it and made partially transparent.

In Figure 15 I have overlain the mag data on the topography, and then made it partially transparent.  As can be seen, the mound is smack in the middle of the polygonal magnetic feature.

The Earth Resistance survey adds a little to the picture (Fig. 16).

Fig. 16: the Earth Resistance survey overlain on the mag data.

The resistance data shows little in the way of positive features apart from an area of high readings towards the south.  There does seem to be a oval of low resistance readings, normally indicative of a ditch-like feature.  There are faint hints of this in the mag data, but they are obscured by other magnetic features running up to the polygonal feature.  If the ultimate origin of the “tumulus” is a prehistoric burial mound, perhaps the res survey is showing us the outer ditch surviving, in part, below the Roman levels?  Fig. 17 shows the resistance survey with contours from the topo survey.

Fig. 17: contours overlain on the Earth Resistance data.

We managed three 40x40m blocks of GPR data.  Unfortunately, the GPR had a glitch in the second grid resulting in a single line of very high values.  This glitch caused some problems in the processing.  I have tried to get rid of them, but the line still shows, especially in the lower slices.  Fig. 18 shows the composite of 16 slices.

Fig. 18: all GPR slices from the eastern survey.

Surprisingly little shows in this survey.  The two main features are the rectangular building which shows from slice 5 onwards, and the general lack of anything much under the tumulus part from some general reflections suggesting some hard material (stones, rubble?) under the mound.  Let us look at slice 6 in more detail.

Fig. 19: GPR time slice 6.

The building towards the south of the image is fairly clear.  I wonder if it might be a bath house?  The area of higher reflections under the mound have faint hints of straight lines and rectangles, but this only shows in this one slice and my guess is that these are fortuitous rather than archaeology.  How does the GPR data relate to the topography?

Fig. 20: GPR time slice 6 with the contours superimposed.

As can be seen from Fig. 20, the higher reflections do not lie below the main part of the mound but slightly to one side.  The building lies outside the polygonal feature seen in the mag data.

A slightly deeper time slice (Fig. 21) shows the strip buildings along Ermine Street starting to show.  They appear to be missing their back walls which might be one impact of ploughing.

Fig. 21: GPR time slice 8 with topographic contours.

I made a crude interpretation map in Google Earth by marking the polygonal feature from the mag data, the building from the GPR data, and, with some guess work, the outer feature from the Earth Resistance data (Fig. 22).

Fig. 22: rough interpretation of the three data sets.

It is impossible from the data to tell if the outer feature from the res survey goes under or around the building, so I may have been a bit generous there.

So what is it?  One possible interpretation could be that we have a prehistoric feature with a mound and a ditch, presumably a round barrow.  The ditch silts up before the Roman occupation.  The mound is then fenced off and kept completely clear of structures or negative features like ditches and pits.  A building is constructed to the south of this mound, and a viewing area to the east.  Stephen Upex, solely on the basis of the aerial imagery, suggested that the feature was prehistoric, and re-used as either a amphitheatre in the Roman period or a small castle in the medieval period.  With the new data, I think we can rule out the castle (unless it was much more substantial at that date).  Although the phrase “ritual” is greatly over-used in archaeology, maybe in this case we are looking at an earlier mound which continued to be venerated into the Roman period?  Baths are often associated with religious sites.  At this stage, this is purely guesswork at the end of a long blog post.  This feature is, really fascinating and quite enigmatic.  Extending the earth Resistance and GPR surveys would, obviously, be very helpful.

The landscape around Durobrivae is fascinating from an archaeological point of view.  The nearby Roman fort is only known from aerial photographs, and just across the river and the Nene Valley Railway lies the site of Castor (Fig. 23) with its huge Roman building complex.

Fig. 23: Castor as seen from Durobrivae.

Last, but not least, many thanks to all those who helped push the mag and the GPR, and who aerated the grass with the resistance frame, or flew UAVs to map the topography.  Although the site is a long way for all of CAGGs volunteers, the site is both stunning and intriguing and, I think, worth the effort.  We hope to return to collect some more data soon.

Fig. 24: collecting Earth Resistance data with WAS’s machine.