Peter Alley using the mag cart at Willington with the dovecot in the background.

A weekend at Willington

Firstly, apologies to everyone for the delays in posting. The day job got in the way, but I am hoping to catch-up with several posts over the next week or so.

In late April, we spent a pleasant weekend surveying at Willington, just south of Bedford, for the local group that have been investigating the medieval complex, of which the dovecote and stable block survive.  Both listed monuments and are looked-after by the National Trust. We had to obtain a Section 42 licence from Historic England to undertake the work.  The site has been surveyed once before: an earth resistance (“resistivity”) survey was undertaken in 1997.

Peter Alley using the mag cart at Willington with the dovecot in the background.

Fig. 1: Peter Alley using the mag cart at Willington with the dovecot in the background.

The area available for survey was not very large, a mere seven of our 40x40m grids covered the area we needed to investigate and we completed the mag survey in a day.  Historic sites are, however, often poor candidates for magnetometry due to the large quantities of ferrous material, such as nails, that get left behind.  We decided, therefore, to undertake a GPR survey as well.  Many thanks, once more, to SEAHA for lending us their Mala system.  The GPR survey took us two days to complete a slightly smaller area.  We also undertook some survey inside the buildings.  That will be the subject of another post.

Results of the magnetometry survey at Willington.

Fig.2: Results of the magnetometry survey at Willington.

The first impression of the survey, shown above, is how “noisy” it is.  The large quantities of ferrous metal fragments, and possibly even relatively magnetic bricks, along with two roads and some services, make for a very messy result.

Interpretation of some of the main features showing in the survey.

Fig. 3: Interpretation of some of the main features showing in the survey.

The northern road marked on the image above is the modern drive to the buildings to the SW. The older road which curves around the stables dates back at least as far as the Ordnance Survey maps in the 1880s, and was still in use in the 1970s as it is marked on the maps of that date.  The one feature our survey shows, which does not appear on the early OS maps from the 1880s and 1900s, is that the older road forks to the SW.  This split, however, shows clearly on the Google Earth image, which dates from 2009, shown below.

Google Earth image showing the fork in the old road as a parch mark.

Fig. 4: Google Earth image showing the fork in the old road as a parch mark.

This extra road does, however, show on the maps of the 1920s and must have been built sometime in the early 20th century.

As the survey progressed I only surveyed in one of the manhole covers, which is marked in Fig. 3.  One can see how clearly these show in magnetic data.  The large and very noisy area represents a pond which is shown on many historic maps including the OS maps from the 1920s, but has been filled-in by the 1970s map.  A more thorough investigation of the map evidence that I was able to do online might tie down these dates more closely.

In general, it is hard to see any details of the buildings we know existed from the 1779 estate map.  I have re-drawn it for comparison here (Fig. 5).

Fig 5: The 1779 Estate map redrawn.

Fig 5: The 1779 Estate map redrawn.

The GPR survey took two days and we covered two 74m long strips.

Fig. 6: the GPR starting the survey near the stable block.

Fig. 6: the GPR starting the survey near the stable block.

The GPR results are affected by changes in ground and surface water.  It can be quite difficult to get surveys split over several days to match, and this is the case here.  I have created time-slices 4 ns thick.  It would be possible to spend quite some time adjusting the slices and the contrast to make the edges match more cleanly than is shown here.

GPR time slice 1: 8 to 12 ns.

Fig 7: GPR time slice 1: 8 to 12 ns.

The topmost time slice (Fig. 7), merely shows the two road surfaces, the modern one to the north and the older one to the south.

GPR time slice 2: 12 to 16 ns.

Fig. 8: GPR time slice 2: 12 to 16 ns.

The second slice (Fig. 8) shows the southernmost road more clearly.  The lighter band to the SE is partly where we stopped for lunch:  as the sun dries the upper layers we can see this in the GPR results.  But this doesn’t explain why there is a change back.  I doubt this is archaeological as it follows the line of the GPR transects.  We are, however, probably still quite close to the surface at this point.

GPR time slice 3: 16 to 20 ns.

Fig. 9: GPR time slice 3: 16 to 20 ns.

The third time slice (Fig. 9) is starting to get down a bit as we are beginning to see through the road. The area to the east of the stable block is still remarkably noisy.  If we compare this image with the 1779 map (Fig. 5) we can see that “noisy” parts of the time slices are roughly where there are buildings, and the quiet parts of the slices are where we have empty spaces on the map. Sadly, however, we cannot really see any buildings clearly in the GPR data.  There is a broad stronger response (shown in dark tones) heading towards the entrance of the stables (may be a track?) and a linear feature running away from that at roughly a right angle which is much narrower.  Maybe this is an earlier wall?

GPR time slice 4: 20 to 24 ns.

Fig. 10: GPR time slice 4: 20 to 24 ns.

The fourth timeslice (Fig. 10) shows a little bit more towards the northern corner of the survey.  The thin line is almost certainly a modern service.  There is, however, a broader linear feature running SW towards the stables.  Perhaps this is another track?

GPR time slice 5: 24 to 28 ns.

Fig. 11: GPR time slice 5: 24 to 28 ns.

The last time slice (Fig. 11) shows a scatter of strong reflections but generally very little.  The signal has “attenuated”, and we are getting little sensible information.  Some parts of the road is still showing as an “echo” of the upper features.  One useful thing, however, is the strong linear feature heading towards the southern corner of the stables.  This is probably a modern service but it does show clearly on the radargram (Fig. 12).

Screen grab of one radargram in RadExplorer.  the strong reflection in Fig. 11 is indicated by the red arrow.

Fig. 12: Screen grab of one radargram in RadExplorer. the strong reflection in Fig. 11 is indicated by the red arrow.

Why do we care?  This strong reflection can be used for hyperbola matching, i.e., drawing a curve on-screen which matches that in the data indicated by the red arrow in Fig. 12.  This process provides a speed for the radar waves through the soil at this site.  I tried several spots and radargrams, and a speed of about 8.5 to 9cm per ns seems about right.  Using RadExplorer I can then see the strong reflector (?pipe) is about 60cm deep.  Assuming the top of our top slice is about ground level, each of our time slices is about 12cm deep.

A maximum depth of only about 60cm is not all that much for GPR, and we must presume that the soils are not that conducive to this sort of survey.  Having said that, I would be surprised if the top levels of the archaeology were any deeper than 60cm.

Unfortunately, the surveys have not provided the impressive and exciting results that we have obtained at some sites which is a shame.  Both the magnetometry and the GPR surveys show noisy areas which match well with the areas of buildings shown in the 1779 Estate Map, but we do not have any clear indications of surviving building remains, although there are a couple of possible track ways.

 

Digging a hole

A while back WAS and CAGG combined forces to do a survey next to Datchworth church.  The mag survey showed very little in the area which is to become an extension of the graveyard, but the resistance survey showed a possible circular feature.  I reproduce the survey here for convenience.

Results of the resistance survey.

Results of the resistance survey.

We were then asked if WAS could put in an exploratory trench to see what, if anything, this circular feature was.  I spent a happy few hours watching a machine strip the topsoil for us.

Opening up the trench.

Opening up the trench.

At first, it seemed as though we had nothing.  The finds were certainly exciting…

The first day's finds.

The first day’s finds.

Yup, two bits of relatively modern brick in the topsoil.  On the Saturday and Sunday we started to clean up, trim the section and so on, expecting to find nothing at all.

The trench from the west.

The trench from the west.

Then as we were cutting the end square, we found a piece of samian platter (of Roman date) and some sand-tempered coarse wares which might be medieval.  These were at the bottom of the topsoil.  Below them, however, was a ‘cut’ feature.

Cut-like feature at the west end of the trench.

Cut-like feature at the west end of the trench.

This, however, had nothing in it, not even a fleck of charcoal.  What about the circular thing?  There appears to be variations in the amount of gravel in the subsoil.  The circular thing appears to be where there is more silt and less gravel.  I am not convinced it is man-made.

We’ll be going out again next weekend to finish the recording and check things over a bit more, but all in all, there doesn’t appear to be all that much there.

Looking forward to 2016

Firstly, I’d like to wish all our readers, and especially all our volunteers, a very happy New Year and all the best for 2016.

This past year has been very productive with our big survey at Verulamium, but also other surveys around the region.  We have had an article published in issue 310 of Current Archaeologyand another article awaiting publication in the journal Archaeological Prospection,  as well as a short piece in the International Society for Archaeological Prospections newsletter.  I have given lectures about our work to the St Albans and Hertfordshire Architectural and Archaeological Society, the Römisch-Germanische Kommission in Frankfurt, the Norton Community Archaeology Group and to the Chess Valley Archaeological and Historical Society.  Several more bookings are in the pipeline.

One excellent addition to our work since August has been access to SEAHA’s Ground Penetrating Radar (GPR).  This has been an excellent addition to the range of techniques we can regularly employ on our surveys providing some superb results at Verulamium and Bushey Hall.  Hopefully, we can continue to access this machine in 2016, and perhaps even raise funds to purchase our own.

What does 2016 hold in store for us?  There are a number of sites which we are hoping to survey:

Verulamium

We are hoping to continue the survey of the Gorhambury estate side of the town in August.  We should be able to complete the mag survey of the Theatre Field and begin on the field to the north of the drive.  GPR surveys, perhaps resistance cart surveys, and even a magnetic susceptibility survey are all desirable as well providing we can access the equipment.

Willington (Bedfordshire)

We have been asked if we would be willing to undertake some surveys near the dovecot and stables at Willington, Bedfordshire.  The stables and the the dovecot are looked after by the National Trust and are listed buildings.  The area around them has been part of a community-based project for a few years looking for the Tudor manorial complex which goes with the standing buildings.

The dovecot at Willington.

The dovecot at Willington.

The stables at Willington.

The stables at Willington.

Hogshaw (Buckinghamshire)

Following our two successful days last February, it would be good to go back and finish the bits of the survey area we were unable to do last time, and perhaps try the GPR out over some of the areas where walls had been detected previously.

Surveying at Hogshaw.

Surveying at Hogshaw.

Wing

We have been asked if we would like to work with the Wing Heritage Group to do some surveys around this very important village with some great Anglo-Saxon archaeology.  It seems an exciting chance to try our equipment out on a different period of archaeology, and to work with a very active local group.

Ashridge

The Berkhamsted and District Archaeological Society have been working on the site at the Ashridge Management College for a few years.  They have asked if we would be willing to undertake surveys in the gardens.

Little Hadham

We undertook a survey at Little Hadham on a late Iron Age and Roman site a couple of years back.  The site merits more work to extend the survey.

Ashwell End

The site at Ashwell is like the road that goes for ever on.  It would be good to complete Great Buttway and move into nearby fields.

These are just some possibilities!  Hopefully I have remembered the main sites…  From a personal note I’d like to do some more work at Broom Hall Farm, and in the eastern half of the county more generally.  We seem to have been slowly drifting west!

Another aspect of our work also needs addressing, and that is writing-up the reports.  We need to develop ways to work on mapping the results of our surveys collaboratively, and to investigate archive materials, especially at Verulamium.  Help with drafting reports, again especially at Verulamium, is going to be needed.  All this makes me think that we probably need to put in for some more grant money to allow us to expand our equipment and software, and to develop ways of sharing the post-fieldwork analysis and reporting.

Last, but not least, up to now CAGG has only existed as an informal network based around our mailing list and the blog.  Should we be working towards creating a more formal group? Should planning and prioritizing surveys be placed in the hands of a steering committee? How to we maintain our identity of a cross-arch soc group, rather than being just another arch soc? Ideas and thoughts on a postcard to Kris!

I would like to thank everyone involved with our work.  Without you, there is no group and there are no surveys.  It has been a privilege to work with you all.

Here is to another successful year. May our anomalies be clear, our batteries charged and enthusiasm unwaning!

 

Cholesbury, Buckinghamshire

At the request of the Buckinghamshire Archaeological Society‘s Active Archaeology Group, members of CAGG, the Chess Valley Archaeological and Historical Society and the BASAAG got together at Cholesbury to survey a field close to the hillfort.

Cholesbury Camp is one of the hillforts that is being looked at as part of the Chilterns Hillforts Project.  It is a scheduled ancient monument and therefore has legal protection requiring a licence from Historic England to undertake a survey within the protected area.  The field we worked in, however, lies just outside the hillfort and is of potential interest.  The historic map data I have been able to access, just shows this as an empty field.  Over many years, however, marks in the grass, dew and frost had led the owners, Bob and Mary, to wonder if there was anything under the surface. They kindly allowed us to come and survey for the day, and between us we undertook magnetometry, resistance and ground penetrating radar surveys.

As the field was not nicely aligned on the Ordnance Survey grid, I had to set-up a “floating grid” using the dGPS.  Unfortunately, the guide for how to do this provided by Leica is terrible and it took a while to work out what to do.

Putting in the grid. Image (c) Mike Smith.

Putting in the grid. Image (c) Mike Smith.

The mag survey went quickly once we had got started.  The field was only four grids in size, and only one of those was a whole 40x40m.  By early afternoon the job was done.

Pauline Hey (CAGG, BASAAG) operating the magnetometer.

Pauline Hey (CAGG, BASAAG) operating the magnetometer.

The resistance survey was undertaken by John Gover and members of CVAHS.  They completed a number of 20x20m grid squares at 1m intervals, although at times we were tripping over each others lines and cables!

Members of CVAHS operate a TRCIA resistance meter.

Members of CVAHS operate a TRCIA resistance meter.

The Ground Penetrating Radar on loan from SEAHA was operated by members of CAGG and completed an area 76m by 40m.  We had the option of a second day if the results warranted it.

Jean Bluck (CAGG/BASAAG) operating the Mala GPR.

Jean Bluck (CAGG/BASAAG) operating the Mala GPR.

The grass was short enough to not be too much of a problem.  Unfortunately, the moles were less forgiving…

Jim West (CAGG/CVAHS) pushes the GPR over some unforgiving mole hills.

Jim West (CAGG/CVAHS) pushes the GPR over some unforgiving mole hills.

Muddy wheels from the mole hills.

Muddy wheels from the mole hills.

The location of the survey can be seen in the next image.  The oval of trees follows the line of the ramparts of the hillfort.

The location of the field next to Cholesbury camp.

The location of the field next to Cholesbury camp.

Undertaking magnetometry surveys in relatively small fields is often less successful than one would hope.  The metal associated with fences, along with other ferrous material can lead to quite “noisy” surveys.  The image below shows the magnetometry plot overlain on the Google Earth image.  As can be seen, all around the edges of the area surveyed are strongly magnetic features shown in black or white, associated with the fences, gates and so forth.

Results of the magnetometry survey.

Results of the magnetometry survey. The mag plot has been clipped to +/- 4nT, with black as the positive readings, white negative.

There are, however, a few potential archaeological features.  There are a number of irregular areas of high magnetism, especially towards the north (see labelled plot below), which might just be the remains of bonfires. Excavations by Day Kimball within the fort in 1932 revealed some industrial features including some hearths and some bloomery slag from iron working. These features have readings in the range of -4 to +12nT, which does not seem strong enough for iron working, but may be from other processes involving fire.  One clear linear feature can be seen, with ranges from -3 to +5nT, which is also visible on the GPR time slices, along with a couple of fainter and less convincing linear features.  These are all labelled on the next image.

The principal features in the mag survey.

The principal features in the mag survey.

The resistance survey was initially downloaded into Snuffler and then the data exported as text files, which were in turn imported to TerraSurveyor. The grids were range matched to make the edges merge nicely, despiked to get rid of odd data, interpolated, smoothed and clipped.  The resultant image shows a few possible features but may, as is often the case in our region, reflect the underlying geology as much as anything.

The resistance survey. The image has been clipped to 16-32 ohms, black is high.

The resistance survey. The image has been clipped to 16-32 ohms, black is high.

There are a few possible features, including two curved / circular ones and a possible platform.  In the raw data, these look a little less convincing.  The “circular feature” (see below) is very close to the end of a linear feature seen in the GPR data.  It would be helpful to extend the survey to pick up more of these features.  The most convincing feature is the large low resistance area (i.e., wet like ditch fill) with a high resistance area running along the edge (i.e., dry like a bank).  This lines up nicely with an enigmatic spur which extends from the ditch and bank circuit of the hillfort on its western side.  We may have found a bit more of the fort’s earthworks.

The resistance survey with labels.

The resistance survey with labels.

The GPR data was processed using Jeff Lucius and Larry Conyers’ free software.  Once converted from Mala format to GSSI format, time slices were in 4ns bands from 6–10ns, 10–14ns and so on.  The top five slices showed features.  Below that the GPR signal was attenuated and nothing can be seen.

Top GPR time slice, 6-10ns.

Top GPR time slice, 6–10ns.

The top time slice, which is essentially the topsoil, shows hints of what is coming below but is largely dominated by two areas of strong responses along the NE edge of the grid.  Yup, its them moles again…

Second GPR time slice, 10--14ns.

Second GPR time slice, 10–14ns.

The second time slice starts to show a regular grid work of lines.  Some are especially strong, e.g., the one that runs roughly parallel to the SE edge of the plot.  That one also shows in the mag data, unlike the others.  The lines, however, do not seem to easily resolve into buildings.

Third GPR time slice (14--18ns).

Third GPR time slice (14–18ns).

The third time slice shows how shallow many of these linear features are. The one to the north of the second time slice is not really visible, although the one to the south that shows in the mag data persists quite strongly.

The forth GPR time slice (18--22ns).

The fourth GPR time slice (18–22ns).

By the fourth time slice, the plot is largely noise with the occasional  feature showing, the most obvious one being the linear feature mentioned above.

The fifth time slice (22--26ns).

The fifth time slice (22–26ns).

The last slice I bothered plotting shows the GPR signal almost completely attenuated and only a few strong features remaining.

What are we to make of this?  The linear feature that shows on the mag and the GPR surveys must be something both magnetic and reflective.  It could be a brick wall, or perhaps a land drain filled with brick rubble?  The remaining regular grid-work of lines doesn’t really resolve into clear building outlines.  My best guess (and it is a guess!), is that these represent field drains leading down towards the ditch of the hillfort.

Sadly, nothing in the surveys really leaps out at one as “wow, we found…” There are tantalizing hints and some possibilities, but in general it is a little disappointing.  It is great to see the strength of using multiple techniques, however, and how the combined data sets help with the interpretation.

Many thanks to everyone who came out and helped, to Peter Marsden for organizing it, and especially to Bob and Mary for not only letting us play in their field, but also for supplying tea, cake, and somewhere dry to sit!

Insane?

The sun was shining, we have a GPR and a Park to play in, so why not? Mike, Peter and I headed out to Verulamium Park to do a little more GPR, although with a difference. We used a 25cm spacing between lines instead of 50cm. We managed a 40m x 60m block.  Without taking into account turning around, or getting to the spot and back again, that is 9.6km of walking, or just under six miles in old money. The results were great, however.  Later, I will do a posting comparing the 0.5m spacing with 0.25m to see if the extra 4.8km was worth it.

First things first.  Where in the Park were we?  I am especially interested in Insula XXIV simply because the Wheeler’s did not dig anything there and we have very good mag results.

Location of the mag grids in the northern half of Insula XXIV.

Location of the mag grids in the northern half of Insula XXIV.

Here is a more detailed view of the mag results.

Detail of the magnetometry results from the northern half of Insula XXIV.

Detail of the magnetometry results from the northern half of Insula XXIV.

The stark black-and-white feature in the NE corner is a practice cricket wicket made of concrete and green carpeting. It overlies the Roman road.  The diagonal feature in the NW corner is the other Roman road, and at least two buildings show clearly as white lines against a dark background.  The western 40m square was surveyed by Ralph Potter at 0.5m intervals.  The middle square and the western 20m of the eastern square were surveyed yesterday.

I used Larry Conyer’s program to slice the data we collected.  The slices start at 8ns and go down in 4ns thick pieces.  Ground surface in the Mala radargrams is usually at about 7ns and so the first slice is very much the top surface of the field.

Slice 1: 8 to 12ns.

Slice 1: 8 to 12ns.

The cricket wicket can be very clearly seen in the top-right corner.  The change across the plot at about 27m is where we went for lunch.  But what on Earth is the big circle?  Here is a clue…

GPR in the Park.

GPR in the Park.

Yup, it is the centre circle of the soccer pitch.  The second slice reveals the striping we have often seen in the Park.

Slice 2: 12 to 16ns.

Slice 2: 12 to 16ns.

I am guessing these are cultivation marks before the land was turned into the Park in the 1930s.  How old they are I am unsure, but could they be the residue of cultivation ‘strips’ as seen at Ashwell?

In the third slice, we begin to see the Roman archaeology.

Slice 3: 16 to 20ns.

Slice 3: 16 to 20ns.

Notice how there is a white line running through the lower building cutting the black lines representing internal walls.  This is a wall which has been partly robbed.  We also appear to have some surviving floors as shown by the larger black areas in the building.

Slice 4: 20 to 24ns.

Slice 4: 20 to 24ns.

In the fourth slice down we can now see the road running across the top-right hand corner in a similar, but not identical position to the cricket wicket.  I wonder if we might be seeing evidence of a hypocaust in the middle of the lower building?

Slice 5: 24 to 28ns.

Slice 5: 24 to 28ns.

In slice five we can see the outer walls of the building are much fainter suggesting shallower foundations, but the main wall which was only a light line previous is now showing more clearly.  This is presumably what is left at the bottom of the robbed wall.  There is also a building along the road which was not visible in the higher time slices.

Slice 6: 28 to 32ns.

Slice 6: 28 to 32ns.

Slice six is very black-and-white.  This is because the radar signal is now very weak and the slices are either showing something or nothing.

All in all, a very nice set of results.  As always, these are my rather rough-and-ready time slices.  With some experimentation and practice they could be better.

Here are Ralph’s grid square (processed by Mike Langton of Mala) and our new one roughly plotted together in Google Earth.

Ralph's and our blocks of GPR data plotted together using Google Earth.

Ralph’s and our blocks of GPR data plotted together using Google Earth.

I think it would be worth surveying the whole of Insula XXIV if we can!

 

Processing mag data (Part 1)

I thought it would be useful to outline how I have processed the magnetometry data from our surveys.

The first stage is to download the data from the Foerster datalogger.  I use their simple and free program IFR Dataload to do this.  Foerster sell a more complex program for processing data from the Ferex, but I find TerraSurveyor easier to use, as well as having the convenience of being able to process data from the resistance meters and other magnetometers.  When I download the files, you can give them a suffix, and they are numbered in sequence.  Usually I use some like “gorday4_” standing for Gorhambury survey, day four. I then end up with a sequence of files: gorday4_1.fdl, gorday4_2.fdl and so on.  FDL is Foerster’s own file format. You can look at each square in Dataload.

Screen grab from Dataload (best seen full size).

Screen grab from Dataload (best seen full size).

Although one can play around with the image at this point, there is no reason to.  If the square is a partial, it is worth making a note of its dimensions.  Each grid square is then exported as a “Text table” which is a fairly simple ASCII file.  I just number each text file sequentially, e.g., gor001.txt, gor002.txt and so on.  If I know that two or more text files are eventually going to be combined into one grid in TerraSurveyor, e.g., when we have surveyed a partial grid square in bits, I use gor003a.txt, gor003b.txt and so on.  You can see the importance of keeping good field notes or processing the data very soon after the day’s survey.

The next stage is to import the text files into TerraSurveyor.  If it is a new site, we need to create it.

Creating a new site in TerraSurveyor.

Creating a new site in TerraSurveyor.

Having created the new site, we have to import the Foerster text files.  First click on the import button.

The download button is TS.

The download button is TS.

This will make the import window pop-up. A Foerster template is not currently part of the default installation, you’ll need to get it from David Wilbourn or myself. Once you have it, just select it from the list.

Selecting the Foester text table import template.

Selecting the Foerster text table import template.

In the next screen, you need to navigate to where you saved the text files exported from dataload and then choose which ones you want to import.  On the first day it will be all of them, but subsequently just the grids since the last time you processed the data.  After that, just keep clicking next and accept all the defaults until the data is imported. The TS grid files will take the name of the text files, so gor001.txt becomes gor001.xgd.

Choosing text files to import.

Choosing text files to import.

The next stage is to assemble the grid into a composite.  Either click on “Assemble grids” to create a new composite, or select the existing one and click on “Open Grid Assembly” (all in the navigation window).

Grid assembly (part 1).

Grid assembly (part 1).

In the Grid Assembly window you can see thumbnails of the individual grids.  These can be drag-and-dropped into the grid in the correct pattern.  The direction of first traverse is always from left to right, so if we are surveying south-to-north on the first line of the grid, north will be to the right.  To make the grid bigger, use the blue arrows on the right.

Grid assembly (part 2).

Grid assembly (part 2).

Having completed adding the grids, click on Save or Save As.

Grid assembly (part 3).

Grid assembly (part 3).

At first, the composite just looks uniform gray.

First look at a new composite.

First look at a new composite.

This is because the extreme values from pieces of iron in the ground are being plotted as black and white.  The majority of the values which are much closer to zero are squeezed into a small number of mid-grays.  To see the pattern in the majority of the data, we need to clip the display.

Clipping the display values.

Clipping the display values.

The clip button on the processes toolbar on the left is indicated above.  You can see from the graph in the pop-up window how the values are compressed.  I have entered values of -9 and 9 for the clipping.  Depending on the site, you may need to clip down to as far as +/- 1.5nT.

In the image above I have cheated a bit as the composite has already been clipped.  You can see the archaeology, but the image is very stripy.  This is caused by the sensors being imperfectly compensated, by walking in zig-zags, and by sensor drift.  It is perfectly normal in mag data and can be removed using the destripe command in TS.  Most processing packages call this zero mean traverse as the process alters each traverse (a line of data within a grid square) so that its mean is zero.  TS offers ZMT, but also zero median traverse which is often more robust.  TS, therefore, bundles all the options in the destripe window.

Zero median traverse.

Zero median traverse.

I have indicated the position of the destripe button in the above image.  Often, just accepting the default values is fine but sometimes large numbers of large values can mess this up, and so I set absolute values for the process.  In this case I have used +/- 10nT.

Destriped and clipped.

Destriped and clipped.

The resulting image is rather flat.  This is because we have performed the processes in the wrong order.  We should only clip the final image, not the data on which other processes are working.  I did this in the wrong order so that you can see what was happening at each stage, and also to demonstrate the Modify command.  TS, very sensibly, does not alter the base values.  This allows us to edit the processes via the Modify command indicated above.

Modifying and re-ordering processes.

Modifying and re-ordering processes.

In this case we just want to move the destriping command down to below the clipping command.  Clipping should always be the last command.

Interpolating data values.

Interpolating data values.

The resulting image is pretty good and for quite a while that is all I would do.  Jarrod, however, persuaded that better images could be obtained by interpolation and smoothing.

Interpolating data values.

Interpolating data values.

The first step is to interpolate some new values.  The Foerster collects readings 10cm apart along the traverse, and the traverses are 50cm apart.  This is a rather unbalanced grid.  The traverses are the y-axis in TS and so by selecting the Interpolate button (shown above) and choosing to double the values on the y-axis, we create a grid which is now 25cm by 10cm.

Applying a low pass filter.

Applying a low pass filter.

The next stage is to smooth the data a little. Obviously, we don’t want to smooth the data so much we get rid of the archaeology! We use a low pass filter.  Click on the button indicated above and select low pass in the window.  I use values of x=7 and y=3.  This is because x=7 is 70cm and y=3 is now 75cm, i.e., close to the ideal of a circular “window” for the filter.

The resulting image will look rubbish.  This is because the processes are again in the wrong order.  Using the modify command, the processes should be in the following order (from bottom to top!)

  1. Destripe
  2. Interpolate
  3. Low pass filter
  4. Clip

I have done things a bit backwards so that you can see the effect of each stage.  Normally, I would just do everything in the right order from the start.  If you are adding new squares to an existing composite, the processes will be automatically applied when you save it at the grid assembly stage.  Normally one would only do all this once a survey.

In the next posting I will explain how I deal with partial grid squares.

Apologies to anyone who looked at this soon after I posted it as something went wrong and all the work I did this morning vanished.  I had to re-write half of it again!

Bank Holiday Monday

The August Bank Holiday lived up to its reputation.  It rained, pretty much all day.  According to our rain gauge we had another centimeter and the roads were all flooded once more.  So, folks, that it is for Gorhambury for the next 11 months.  Plenty to do, however, with data processing, analysis and interpretation.

Today Ellen and I went back to site to collect the last remaining pegs.  Ellen organised us into walking back and forth across the field systematically to see if we could find my missing survey book.  On our second pass…

A very damp notebook!

A very damp notebook!

It was very soggy, but thanks to being a “rite-in-the-rain” product it looks OK and is now drying in the airing cupboard.  Yay!  Inevitably, as we were walking around picking up pegs and finding the book, it started to rain and we were soaked by the time we left.

On Sunday, I had some fun making timelapse videos.  Here is a condensed version of surveying three squares.

I did a poor job of setting it up for the GPR, but here it is anyway:

I have quickly processed the GPR data from using the large 160mhz antenna. We surveyed a block 20x40m which went over the “sinuous ditch” in the hopes we might be able to see it more clearly. We have picked up the walls that we saw with the 450mhz antenna.  In the radargrams (the vertical slices which is how the data is collected) I think I can see the ditch… but I need to check with someone more experienced at reading them.  Here are the time slices.  Note that we used a 1m spacing between lines which leads to a cruder picture.

Time slices from a 20x40m block at 1m spacing using the 160mhz antenna.

Time slices from a 20x40m block at 1m spacing using the 160mhz antenna.

I thought it would be useful to show one radargram from the day 15/16 block.  This is line 1893 which was surveyed from north to south, i.e., from the top of a block (as I usually present them) to the bottom.  Here is a screen grab from “radexplorer” showing unprocessed data.

An unprocessed radargram of line 1893.

An unprocessed radargram of line 1893.

It is very difficult to see much.  This is because most of the signal is the loud “noise” near the surface.  The first thing to do is tell the software where ground level is, i.e., the start of the first ‘loud’ reflection shown as black band at the top of the radargram.  Then we need to apply ‘gain’.  This is simply amplifying the lower reflections which are much weaker than the ones near the surface.

Line 1893 after the ground level has been input and gaining (using the autogain function in radexplorer).

Line 1893 after the ground level has been input and gaining (using the autogain function in radexplorer).

We can see much more now, but there is still a great deal of banding.  The bands are, essentially, echos and can be removed with the “background removal” function.

The processed radargram of line 1893.

The processed radargram of line 1893.

There is a great deal going on in this line of data.  Note, however, how the vertical column of echos under where I have put the label “wall” starts down into the radargram where as the column of echos where I have put “ring” starts at the top.  The “ring” is a small incipient mushroom ring which I noticed as we pushed the GPR over it.

A small incipient mushroom ring.

A small incipient mushroom ring.

If we look at the upper time slices we can see the ring:

A time slice from near the surface showing line 1893. The dark blob near the centre of the line is the mushroom ring.

A time slice from near the surface showing line 1893. The dark blob near the centre of the line is the mushroom ring.

For the wall, however, we need to look at a deeper slice.

Slice 3, 14-18ns.

Slice 3, 14-18ns.

In this slice we can see a small rectangular building showing faintly in the data. This building explains some of the things we can see in the radargram.  Remember that north is to the left of this radargram.

The arc we can see at the bottom of the top slice starting at about 27m west and going to 34m west is the northern edge of a very big mushroom ring.

A large mushroom ring.

A large mushroom ring.

The interaction between the fungus and the grass is quite complex.  The rings are showing in the radargrams probably because they are retaining water.  This can make the grass grow lush, or in extreme circumstances kill the grass.

A mushroom ring killing the grass.

A mushroom ring killing the grass.

Some of the rings merge and become quite complex.

A complex mushroom ring.

A complex mushroom ring.

And some of them cause mad photographers to get to their knees.

yummy circle

Yet another mushroom ring.

Yes, I became a bit obsessed.

Although we have finished for this season, there are lots of other things on the horizon, as well as working through all this great data.

I know I have said it before, but it is worth saying again.  Many thanks to everyone who contributed.  We collected great data, and we have found some really intriguing things.