Tag Archives: geophysics

Being completist

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

The team are very patient with my need to be neat and tidy and do the silly little bits around the edges.  Today was perhaps an extreme example.  Due to yesterday’s little hiccup, I set-up the res kit and, with Graham’s help, surveyed about a sixth of a grid square, then packed it all up again.  Now I can sleep easy.  On a more ambitious note, I have finally put all the res grid squares into one large composite.  The survey now consists of nine hectares, which is about 360,000 individual readings.  Most Earth Resistance surveys use what is known as a twin-probe configuration.  That means that there are two mobile probes on the frame, and two stationary probes on the end of a long cable, normally about half a meter apart.  One mobile probe, and one remote probe set-up an electric circuit part of which is the soil.  The other mobile probe and the other remote probe measure the resistance.  The problem with this “standard” set-up is that when you have to move the remote probes, the readings for the same spot will change.  This leads to endless struggles to “grid-match” each set of squares.  Since 2016 I have gone over to using a pole-pole configuration.  This is basically the same except the remote probes are a long way away (I aim for about 30m or more) and a long way apart from each other (I aim for more than 20m). This helps enormously with the grid matching.  Grids completed on different days of the survey will match quite nicely, usually.  Where this is not true is when (a) there is a lot of rain in the middle of a survey and (b) when the survey is split over multiple years.  In the case of our 9ha block, this has been completed over four seasons.  Unsurprisingly, one can see the edges.  TerraSurveyor has a function called “periphery match” which will, sometimes, do an excellent job of grid matching.  In this case, it was pretty good.  Figure 1 shows the survey with a periphery match applied.

Figure 1: the res survey as of the end of Thursday.

If you click on the image and see it full size you can see the detail of many buildings.  Unfortunately, the range of values makes seeing some buildings quite hard.  A high-pass filter is a background trend removing tool that makes some buildings show more clearly (Figure 2).

Figure 2: the res survey, high-pass filtered.

There are still many features, many buildings, that one can only see when looking at smaller blocks.  With such a big area getting everything to look clear is going to be impossible, I fear.

From tomorrow the res will be working in Church Meadow where we hope it will map the remains of St Mary du Pré.

The GPR team is getting really close to finishing the theatre field (Figure 3).

Figure 3: the GPR survey so far.

The team have some fiddly bits around the edges to complete, one missed block, and one block we want to re-survey at 50cm intervals as there is a building and road in it.  Fingers crossed, two more days should do it.  Then, for a bit of last day fun, the GPR will also have a look at St Mary du Pré.

The mag team completed another eight blocks today.  Since moving to Church Meadow, they have managed to survey four hectares in three days which is very impressive, especially given that one block had to be repeated due to a sensor freeze.  Having lots of whole grids and no partials makes such a difference.  The team are now just 2.5 ha away from completing a square kilometer of mag at Verulamium.  Figure 4 shows the mag results in Church Meadow.

Figure 4: Church Meadow mag survey after day 3.

Three things are of note.  Firstly, earlier today I wondered which of the two raised areas in the field was Watling Street.  Looking at the survey results, it looks like the road splits in two near the edge of our survey.  Perhaps it is two phases?  I’m not convinced.  Secondly, we have some marked linear features showing that almost look like enclosures.  These are, however, low magnetism suggesting and might be yet more pipes, but not of metal this time?  Again, I’m not convinced.  They might well be archaeological features.  I will have to survey all the pipes I can see in the field.  Lastly, as we get closer to the town to the south, there are many more little black blobs.  Seasoned readers of this blog will know that usually little black blobs in mag data are often pits.  In this case, I wonder if we are starting to pick up the edge of the cemetery which is likely to have lined Watling Street?  In the Roman world, the richer you were, the closer you wanted to be buried to the road and the town.

Tomorrow is the antepenultimate day of the 2019 Gorhambury survey season.  The weather looks good so fingers crossed all goes well.

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Pipe dreams

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

Saturday night I said the weather was predicted to be “unsettled”.  Well on Sunday at 10am it was raining cats and dogs.  (I wonder why cats and dogs?  Why not ducks and pigeons?, or frogs and mice?)  I was determined to set-out grids in Church Meadow and so I soldiered-on.  Up until now, I have stuck to the 40m grid based on the OS for all the fields we have surveyed in Verulamium Park and in Gorhambury.  Church Meadow, however, is a long thin field at approximately 45º to the OS grid.  Additionally, the fence along Gorhambury drive is very straight for much of its length.  I decided, therefore, to use a floating grid to minimise partial grid squares and wheel spinning.  The lack of an “end line” function is the Foerster’s Achilles’ heel.  We must have wasted hundreds of hours spinning the wheel due to that one simple omission.  Although the data processing involves some extra steps and jiggery-pokery to get the plot in the right place, it seems worth it in this case.  I think my decision was vindicated when the team completed nine complete 40x40m grids despite the wet start to the day, and the lunchtime deluge. Figure 1 shows the team in action.

Figure 1: the mag team in action in Church Meadow. Image © Mike Smith.

Figure 2 shows the location of Church Meadow.

Figure 2: the location of Church Meadow, outlined in red.

Looking closely at the Google Earth image reveals some features in the field (Figure 3).

Figure 3: Features visible on Google Earth.

With these features we had high hopes.  Figure 4 shows the results of the mag survey from the first day.

Figure 4: Mag survey results.

Sadly, the plot is dominated by the two pipelines running through the field (Figure 5).

Figure 5: Heavy metal. Image © Mike Smith.

One of the pipes clearly runs straight through the building seen in the GE image. There are, however, some archaeological features to be seen (Figure 6).

Figure 6: mag survey in Church Meadow with some labels.

It is very frustrating that we can see the walls in the mag data to the SW and between the pipelines, but the image is so dominated by them that it is hard to make sense of anything.  Hopefully the res or the GPR will show the details better.  The ditch is interesting, however.  Could this be the vallum monasterii? It could, perhaps, be related to Watling Street, or it could simply be the remains of the earlier route of Gorhambury drive.  It will be fascinating to see where it goes.

The Earth Resistance team completed an excellent six blocks of data.  Figure 7 shows the whole res survey.

Figure 7: the whole Earth Resistance survey after Sunday 18th.

With good luck and a fair wind we should reach the hedge line on the next survey day.  Figure 8 shows the grids completed on Sunday.

Figure 8: The grids completed on Sunday.

Not a great deal is showing in those grids apart from the faint line across the top corner.  Let’s look at the mag data from that area (Figure 9).

Figure 9: the mag data from the same area as Figure 8.

The light line in the res data is matched by the dark line of “the sinuous ditch”, which is exactly what we would expect.  The sinuous ditch is, we think, the town’s aqueduct.  We should pick-up much more of this on Wednesday.

The GPR team have been working down the western edge of the town with the end in sight.  Soon, soon, they hope, they can escape the theatre field and its rugged terrain (Figure 10).

Figure 10: GPR and the rugged terrain.

The GPR team have been doing a lots of sawtooth edges as well as extreme hill-climb GPR.  Figure 11 shows recent results.

Figure 11: the western edge completed between Thursday and Sunday (colour section).

GPR, perhaps even more than Earth Resistance, is affected by the weather and ground conditions.  It seems very difficult to get different days to match-up.  I tried three methods with this data collected over four days, and none were perfect.  This image was created by just treating everything as one big data set. It doesn’t help that each tweak to see what works takes half an hour to process!

Looking back over the first three weeks, we have managed to achieve quite a bit despite dry weather, wet weather and endless partials.  Many thanks to everyone who has been involved, especially those stalwarts who come most days (you know who you are!).

Nigel wonders what next week will bring…

Figure 12: The Thinker. Image © Mike Smith.

 

Fighting the wind

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

The weather forecast was for a windy day, and it was correct! At one point, the wind was making the flag on the cathedral look as stiff as a board, but the sun made the the yellow cross glow. Even from the far side of the Roman town it was quite striking.

Logistics led to an unusual format for the day.  Jim, Ellen, Pauline and Dave continued to expand the mag survey in Prae Wood Field.  Meanwhile, Kris, Mike, Anne and Julia firstly finished yesterday’s block of GPR data, curtailed because of rain, and the went on to complete six Earth Resistance survey blocks.

Figure 1 shows the whole of the mag survey in Prae Wood field, and Figure 2 a detail of the eastern end.

Figure 1: Mag survey after day 9.

Figure 2: Eastern end of the mag survey after day 2.

You may well ask why we have a funny diagonal edge to the survey at the eastern end.  This is because there is an electric fence creating a paddock for horses.  As we have gone a little further than we intended, we will just take what we can get.  The big new find is a ditch running across the end of the field.  I have marked this in Figure 3 with red arrows.

Figure 3: Mag survey with arrows (see text).

We have no way of knowing what date this feature is.  The first thing I will have to do is check the historic maps.  It does, however, look like much more than a field boundary.  It is 2 to 3.5m wide.  The blue arrow in Figure 3 indicates a much slighter feature than runs at a right-angle to the big ditch.  Just to the west of the ditch is a strong magnetic feature that I have marked with a yellow arrow.  The form of the feature (a bigger blob next to a smaller blob) is reminiscent of the pottery kilns we have found on the south side of the town.  The magnetic values (c. -8nT to +130nT) is also in the right sort of range for pottery kilns.  Some work I have published previously shows kilns with a maximum range of about -27nT to +180nT.  It is typical that the area where we have started finding potentially interesting features is also where there is the most modern interference from services and so on.

The GPR survey just completed the block left over from yesterday.  A quick look at the data showed no surprises.  The good news, however, is we are just one day’s worth of 1m transects from joining-up with the survey to the north.  The survey is shown in Figure 4.

Figure 4: the GPR survey after day 9. The red box marks the block completed on days 8 and 9.

The bare strip to the west of the GPR data in Figure 4 is what is left to be surveyed. That to the east has been done (I just haven’t loaded them onto the GE image).  The eagle-eyed amongst you may have noticed that the GPR blocks do not quite match-up as well as they used to.  This is a bit complicated but is basically because the OS have up-dated their guidelines for converting OS National Grid coordinates to lat and long.  I work in the National Grid, but Google Earth works in lat/long.  I’ve changed to using the OS’s official conversion webpages, partly because I can up-load them in bulk saving me a great deal of cutting-and-pasting.  My crude use of GE to display the results, however, involves dozens, if not hundreds of image over-lays.  As a result, I have a great deal of work to do to update all the slices from all the GPR blocks to the new coordinate conversions.  Arrgghhh.  The joys of doing a survey over five seasons.

The entire Earth Resistance survey is shown in Figure 5.

Figure 5: The entire Earth Resistance survey after day 9.

As can be seen, we have covered an impressive area now, about 9ha in total.  The edges in the image are the different years which have been processed slightly differently and crudely put together in Google Earth.  I will be joining them all together soon and trying to make a more seamless image.  Figure 6 shows the western 2019 block.

Figure 6: the 2019 western block of res data (the lighter strip to the west).

Most of what we managed today is the open dry valley between the buildings on on Street 23 (seen in the latest data) and Street 25 which we have yet to survey.  Tomorrow, however, we will start hitting the buildings on the latter street, and especially the cluster of buildings which lie on the junction of Streets 25 and 11 on the western corner of Insula XXXI.  In today’s data (the upper six blocks on the 2019 strip), we picked up the western half of a large building which lies back from the road with a wing running up to the road with a sequence of smaller rooms.  Fascinating stuff.

Tomorrow we are going to start off with GPR and Earth Resistance giving me time to lay-in more grids for the ever-efficient mag team.

Many thanks to everyone who braved the gales today.  We got some great results despite the weather.

 

Waffles in Ashwell?

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

Gil Burleigh asked us if we could survey a field in Ashwell. The field is quite small at just 1ha. Some nice finds had come from this field including a nice scabbard chape (Fig. 1) in the 1970s. The field was ploughed for a short period in the 1980s, and North Herts Museums undertook a fieldwalking survey in 1986 which retrieved pottery from many periods with concentrations of Roman, Medieval and post-Medieval material and a thinner scatter of Iron Age and Anglo-Saxon pottery.

Figure 1: scabbard chape © North Herts Museums.

Members of North Herts Archaeological Society excavated three small test trenches over the weekend of the 16/17th March and found archaeological features in two of them. Soil test trenches has also revealed archaeological features in test pits SA01 and SA02 (although they were misinterpreted as just deeper topsoil! Figure 2).

Figure 2: Location of the NHAS test pits (in red) and the soil test pits (SA01 to SA05). © CAGG.

As the site is pasture over chalk, it seemed likely we would get a good result.  Magnetometry was the obvious technique to try (Fig. 3).

Figure 3: Nigel “compensates” the magnetometer. Photo © Mike Smith.

We managed to complete the whole field in a day with the mag, which was excellent progress.  I was exciting to see what we had detected and processed the data that evening (Fig. 4).

Figure 4: the mag survey © CAGG.

My initial reaction was “ugh”.  Why was the data so horribly noisy, like some sort of giant waffle maker? Although we can see some features, especially on the western edge, we cannot see all that much going on, and nothing around where the trenches had revealed archaeological features. Can we explain this?

Peter Alley flew the site with his UAV.  The photos from his flight can be used in two ways: they can be stitched together to produce an orthomosaic (basically an aerial photograph corrected for distortions) or to map the topography.  The orthomosaic is fun, but doesn’t help us solve our problem (Fig. 5).

Figure 5: Orthomosaic from the aerial survey © Peter Alley.

Mapping the topography shows the basic form of the site, basically a flat field on a slope (Fig. 6)!

Figure 6: the topographic survey derived from the UAV images. © Peter Alley.

If we use a hill-shade, however, we can see the micro-topography of the field.  Lighting the field from the SW creates Figure 7.

Figure 7: topography lit from the south west. © Peter Alley.

We can see from Figure 7 that even though the field has not been ploughed since the late 1980s, the plough scars remain running up and down the slope.  If we move the light to shine from the NW we get Figure 8.

Figure 8: topography lit from the NW. © Peter Alley.

Now we can see plough scars running across the slope!  This is, in part, the origin of the noisy data: the cross cutting ploughing has left a surprisingly uneven surface, like the proverbial giant waffle iron.  We should assume this is mirrored by scars in the surface of the underlying chalk and archaeology. One further possible cause of the problem are thirty years worth of ant hills (Fig. 9).

Figure 9: ant hills at Ashwell. © Kris Lockyear.

These aspects of the site, in part, explain the noisy looking data but do not completely explain the lack of contrast between the fills of the features and the chalk.  Gil reported that the metal dectectorists were also having problems with large numbers of responses in this field, so the mystery isn’t completely solved.  A plane did crash in this field during World War Two, but I cannot see how that has created this problem unless the whole field went up in flames.  Ideally, we would take some samples and test the magnetic susceptibility of them.

There are, however, some features in the data, despite the noise.  I have outlined the ones I can see in Figure 10.

Figure 10: magnetometry survey results with interpretation shown in yellow (cf. Fig. 4). © CAGG.

Two of the possible features run down and across the slope in a manner similar to the plough scars, and we must be cautious in their interpretation as a result.  The corner that is protruding from the western field boundary is much more interesting.  It looks like we may have clipped one side of an enclosure. (We seem to specialise in enclosures that don’t enclose things.)  It looks like it probably runs under Ashwell House next door.  In the middle is a “dark blob”.  Initially, I thought this was likely to be metal as it is so close to the edge of the field where the fencing creates a strong response.  Checking the values in TerraSurveyor, the range of values towards the east of the blob is from -2nT to +12nT.  If one goes right to the edge where one can see the impact of the fence on either side, the values jump to about -4nT to +26nT. Contrary to my initial thought, this might well be an archaeological feature in the enclosure.

On the Saturday we completed a 40x40m block of Earth Resistance survey in the NE corner of the field.  Not a great deal showed!  We decided to persist on the Sunday so that we could include the area of the “enclosure”.  We had some unusual help (Fig. 11).

Figure 11: Unusual help with the Earth Resistance survey. © Kris Lockyear.

As has become our standard method, we used the Earth Resistance meter on the 1+2 setting.  In other words, every time we stick the mobile probes in the ground the machine takes three readings.  One uses the two outer probes which are a meter apart to measure down to about a meter or so.  The other two readings use the inner probe to take two side-by-side readings with a 0.5m probe separation, looking about 50–70cm into the ground (Fig. 12). Given the topsoil is only about 30cm deep, this should be fine.

Figure 12: the Earth Resistance survey underway. Photo © Gil Burleigh.

The results, were, underwhelming… (Figs 13 and 14).

Figure 13: Earth Resistance survey with 0.5m mobile probe spacing.

Figure 14: the Earth Resistance survey, 1.0m mobile probe spacing.

I can only conclude that the difference in moisture retention between the features and the chalk was minimal.  Peter Alley made the excellent suggestion that the banding we can see is possibly related to the layers within the chalk geology.

We have one last source of data to examine.  In 1986 Keith Fitzpatrick-Matthews undertook a field walking survey here for North Herts Museum Service.  Obviously, he didn’t have the luxury of a GPS to log his finds or lay in his grid.  We can, however, roughly position the results on the map with a bit of careful editing in a drawing package.  Figures 15–17 show the three most common types of pottery: Romano-British, Medieval and post-Medieval.

Figure 15: Romano-British pottery distribution © North Herts Museum Service.

Figure 16: Medieval pottery distribution © North Herts Museum Service.

Figure 17: Post-Medieval pottery distribution © North Herts Museum Service.

The post-Med distribution doesn’t seem to fit our survey results.  Both the Roman and Medieval ones, however, are temptingly close to the feature on the western side.  By making the distributions transparent we can see how these match (Figs. 18 and 19).

Figure 18: Romano-British pottery overlain on the mag data © North Herts Museum Service and CAGG.

Figure 19: Medieval pottery overlain on the mag data © North Herts Museum Service and CAGG.

Both the Romano-British and Medieval pottery distributions lie just to the north of the “enclosure”, down slope from it. This is what one would expect if the material collected on the surfaces derives from the fills of the features. We can only be sure of the date of this feature by further excavation.

Although this survey has not produced the beautifully clear results we have had from other sites such as Kelshall, it has shown some interesting features.  It is also a good example of the value of combining different data sets, in this case field walking, aerial photogrammetry and magnetometry.

Many thanks to everyone who helped with the survey and fetched and carried equipment.  Also, thanks for Gil for suggesting we have a look at this site and David Short for allowing us access to the site.  As 19 images seems an odd number to finish on, I thought I would sign off with another photograph of one of David’s magnificent sheep (Figure 20)!

Figure 20: A sheep. Photo © Kris Lockyear.

A chilly day at Little Hadham

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

Firstly, apologies to those waiting for the results of the GPR survey in the churchyard at Ashwell undertaken just before Christmas.  I need to do some more data processing to see if we have some graves or not.

Meanwhile, the site at Little Hadham has been one to which we have returned on-and-off almost since the beginning of the project.  (Use the drop-down box on the right of this page to see previous posts on this site.)  The site clearly extended beyond the area we had surveyed so far, and so CAGG once more teamed-up with members of the Braughing Archaeological Group (itself part of the East Herts Archaeological Society).  We could only access one of the fields, the one we first surveyed in April 2014.  Our aim was to extend the survey to the edges of the field, and to examine one group of features we had detected with the mag previously with an earth resistance survey.

The morning was cold and crisp with a hard frost.  The thermometer in Jim’s car registered minus 3 Celsius.  Brrrrrr…  Unfortunately, the frost melted quite quickly and muddy wheels on the mag became a problem (Fig. 1)!

Figure 1: Nigel wheels the mag across the field.

The mag team completed six grids, five of which were partials.  Given the amount of time spent trowelling the mud off the wheels, this was a good haul.  We have four partial grids left to complete the field.  The results are given in Figure 2.

Figure 2: the mag survey.

The areas completed yesterday were the block to the far east of the survey area, and the incomplete strip of partials on the southern edge.  In the new area we can see the ditch previously detected (shown by the red arrow) carrying on across the site.  It is fainter in Millfield to the west of the road, but is still evident.  Near the hedge is another clear line of a ditch (marked with the yellow arrow).  This one is worryingly straight and almost parallel to the  field boundary.  It maybe more modern than some of the other features.  The curvy, more irregular ditches (shown with the blue arrow) may be something like a farmstead with boundary ditches.  Picking apart the phasing of all these features is going to be difficult and would require some targeted excavation.

As we had a good sized team we also undertook some Earth Resistance survey (Figure 3).  We targeted one of the possible farmstead enclosures.

Figure 3: Katie Burgess and Peter Baigent (BAG) using the RM85 Earth Resistance meter.

The team completed five 20x20m grids at a 0.5m reading spacing.  The results are shown in Fig. 4, and the underlying mag in Fig. 5.

Figure 4: the Earth Resistance survey results.

Figure 5: the mag results in the area of the Earth Resistance survey.

I had been hoping — rather optimistically — that the resistance survey might pick-up some structures.  Sadly, it did not.  There are, however, correspondences between the mag and res results.  The ditch with the right-angle corner in the mag survey shows well, if slightly more diffuse, in the resistance data.  Similarly, the long curving ditch also shows well. I have indicated one end of it with a blue arrow in Figure 6.

Figure 6: Resistance survey results with arrows.

More curious, however, is the change from low to high resistance along a straight line indicated with a red arrow in Figure 6.  This corresponds exactly with the diagonal line in the mag data which cuts east-nor-east west-sou-west across the D-shaped enclosure. I’m at a loss to know what this represents.  It maybe a reflection of the various cut features in the underlying geology.

At the end of the day we were treated to a beautiful moonrise and sunset.  Not quite the blood wolf moon seen some 12 hours later (when I was tucked-up and asleep in bed!).

Figure 7: Moonrise. Shame about the electric cables!

Many thanks to everyone who turned-out on a freezing but beautiful day.  This site continues to repay our attention, and it worth the effort.  We should try and survey some of it in the summer, however!

Neat and tidy

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

Due to being rained off on our last day, a small team of us decided to go out and finish off some things on Bank Holiday Monday. Many thanks to Pauline, Judith, Ruth, Dave and Jim for turning out to do “just one more grid.”  I think it must be a geophysicists ailment that we always would like to be able to just a little bit more…

The mag team completed an impressive ten grids including two awkward partials.  Figure 1 shows the entire survey at the end of the 2018 season.

Figure 1: the mag survey after day 19.

The team have managed to add 19 ha to the survey in the last month.  Figure 2 shows the southern area that we have been surveying this week.  (This field is, confusingly, called “Prae Wood”.)

Figure 2: the southern area (Prae Wood) after day 19.

The team have picked-up an area of intense ferrous noise.  This looks like a small historic period site.  We will have to check out some old maps to see if we can work out what that might be.  The one hiccup in a brilliant last day of work is a single line of data where the sensor froze.  It is very annoying and I’ll have to find some way of fudging that until next summer!

The Earth Resistance survey had one last little block left to make the plot look all neat and tidy.  Many thanks to Pauline and Judith for helping me fill that in (Figures 3 and 4)!

Figure 3: Kris, Judith and Pauline (out of shot) extended the resistance survey. Image © Mike Smith.

Figure 4: the main block of Earth Resistance data collected 2016–2018.

The data collected shows some faint indications of buildings in that corner (Figure 5).

Figure 5: the northern area of the res survey. The NW corner was completed on day 19.

Although my trick of spreading the remote probes wide apart has worked on the whole, this year there is a bit of an edge.  This is because we started with a block in the SW corner, worked eastwards, and then when we had got to the corner, worked back along the hedge line westwards.  Between when we started this block and yesterday we have had in excess of 100mm of rain (or about 4 inches in old money) so it isn’t surprising this shows in the results.

We have now cleared away all the pegs and flags, packed-up the machines and left Gorhambury for another year.  It is a beautiful place to work and we are very grateful to Lord and Lady Verulam and their family for allowing us to extend the survey, to those who work the estate and put up with us getting in the way, and to the estate managers, especially Stuart Gray. Thanks to the Institute of Archaeology, UCL, for lending us the dGPS and the res meter, and SEAHA for the loan of the GPR.  I hope everyone involved thinks the results are worth all the effort. Most of all I would also like to thank all the volunteers who came this year, whether you only managed a day or two, or you came for the whole season.  You are what makes this project so much fun!

 

“The way I see it, if you want the rainbow, you gotta put up with the rain.”

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

As I start this entry of the blog, the rain is splashing against my windows as was predicted by the Met Office. Although we might question Dolly Parton’s grammar, the sentiment seems true enough.  Yesterday, however, was a superb day with all three techniques collecting data across the site.

After yesterday’s excellent results, the GPR crew had great expectations.  The only problem was a tree in the way under which the shepherdess had put hay when the grass in the field was dead from lack of rain.  Unfortunately, sheep mean sheep droppings (Figures 1 and 2).

Figure 1: Mike on sheep poo removal duty.

Figure 2: Check out those wheels!

Luckily for everyone concerned, I think the effort was worth it (see Figure 3)!

Figure 3: GPR time slices from Day 18.

I could misquote Dolly along the lines of putting-up with sheep poo if you want excellent GPR results but I might be pushing my luck…  The many buildings are quite obvious in this data set.

Figure 4 shows this grid in context of the other GPR grids in this area.

Figure 4: GPR results including the day 18 data (SW corner).

We have added a very large number of new buildings to the map of Verulamium.  As I was only just starting with GPR data when we started collecting it in 2015, the processing keeps changing a bit from block to block. One of my jobs is to start from scratch and reprocess the whole thing so that the maps are consistent.  Should keep me busy for a while.  Figure 5 is a crude mosaic of images just showing the entire area surveyed so far.

Figure 5: crude mosaic of GPR time slices at the end of the 2018 season.

This represents 19ha of GPR data collected at 0.5m transect intervals.  Just pushing the machine along the lines, not including getting to the block, setting-up, moving strings etc. is 380km.  It also means 380km of radargrams!  No wonder the data takes-up 33gb of my hard disk and consists of over 70,000 files.

The mag team completed nine 40x40m grid squares which is 1.44 hectares.  Excellent progress!

Figure 6: the mag team in the southern field.

Figure 7 shows the whole of the 2018 survey (along with a big chunk of Verulamium).

Figure 7: the mag survey after day 18.

Even though we have been using the machine for some years now, and it does have its frustrations, when all is going well we can really cover some ground.  The season was planned for 20 days: we lost 3 days to rain, and most of a day to testing the mag at the start.  Despite this, the team have managed to collect 17.7 hectares of mag data.  Without actually getting to the grids and back (which is quite a bit of walking in itself), the team have pushed the cart 88.5km over the past four weeks.

Figure 8 shows the southern area in more detail.

Figure 8: the southern area of mag data after day 18.

The blue arrows in Figure 8 indicate the lines of old field boundaries.  These can be seen on old maps such as the 1699 parish map.  The yellow arrows mark ferrous objects.  Some are very big, but there are a scatter of smaller ones too.  Last, but definitely not least, there are a few magnetic features which may be archaeological, such as pits.  I have picked a few out with red arrows.  Although they look small at this scale, they are probably 1m to 2m across, a quite respectable size for a pit.

Although large mainly  blank areas are disappointing to collect, they are important nonetheless. The immediate environs of Verulamium are extremely rich, archaeologically. The field lies:

  • 360m W of the busy area of buildings recorded by the GPR discussed above;
  • 600m NE of the major Iron Age settlement at Prae Wood;
  • 600m N of the fields at Windridge Farm where metal detecting rallies have taken place;
  • 500m NW of the major cemetery at King Harry Lane;
  • 1,100m SE of Gorhambury Roman villa;
  • 1,000m NE of the new villa found at Windridge Farm.

Also, the Fosse, which is preserved in the woodland along the NE edge of the field, is a really very impressive earthwork.   We just seem to have hit an empty bit of landscape between all these sites!

The res survey now covers some 6.58ha, that is about 263,200 earth resistance readings.  Not into the millions like the mag and GPR, but this is res after all!  Figure 9 shows the entire survey.

Figure 10: the entire Earth Resistance survey after day 18.

At this scale the roads show very nicely as do some of the more substantial buildings.  Figure 11 is the area surveyed in 2018.

Figure 11: Res survey after day 18.

Given that the fields were baked hard and the grass was dead at the start of the season, I am pleased we managed any Earth Resistance survey at all this season.  The team yesterday put-up with my geophysics OCD and completed right into the corner by the theatre. We then doubled-back and started filling-in between the top of the survey block and the drive.  We have picked-up some parts of buildings seen in grids to the south, but in general along the edge the deep colluvium, as shown by the sunken nature of the drive, is to some extent masking the archaeology.

Many thanks to everyone on the team who made the 2018 season such a success.  A especially big thanks to those who helped move the equipment about including Ellen, Mike, Jim and Ruth.

For those who haven’t been involved but would like to join future surveys, do get in touch.  We are a friendly group, and provide on-the-job training.

And finally… (as they used to say on the news)