Day 54

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

Yesterday was the last day of the 2017 season at Gorhambury. Apologies for the slight delay in posting… we went down the pub for supper!  We have completed 54 days of survey at Gorhambury over the three seasons.  As well as the 35.7ha of magnetometry we completed last season, we have now completed 14ha of GPR survey.  Just pushing the machine along the strings is about 280km.  We didn’t manage any usable Earth Resistance survey in 2015, but we have done quite a bit in 2016 and 2017.  Last year we had terrible problems with the very hard and dry soil.  As a result, many grid squares have been done twice.  We have, however, completed a 5.6ha survey at 0.5m intervals.  That is 224,000 resistance readings, or sticking the machine into the soil 112,000 times.  That doesn’t include three days with the beast which is a further 9,600 survey points resulting in 67,200 readings.  Here is the story in pictures.

Fig. 1: crude mosaic of GPR time slices showing the extent of the survey so far.

Fig. 2: the 2016 and 2017 Earth Resistance surveys at Gorhambury.

Figure 1 shows a (very crude) mosaic of time slices for Gorhambury just to show the entire extent of the survey.  There is going be a great deal of work reprocessing these to get the best out of them and to get the various blocks to match.  I also need a faster computer.  I tried out the kriging option last night and it took several hours to process the data, but the images were no doubt sharper.

Figure 2 shows the 2016 and 2017 Earth Resistance surveys.  At the moment the two seasons are just images put together in Google Earth.  I need to see if I can join the two into one big survey and get the edges to match properly.  I also need to see if I can get rid of the line caused by the deluge this season.

Firstly, let us look at the Earth Resistance survey results.  On the last day we redid four squares from last year, and then completed five awkward partials around the corner of the enclosure for the theatre.  Why did we redo those four?  Figure 3 shows last year’s survey with the block marked.  The hot dry conditions gave very noisy and unsatisfactory results.  I thought it was worth a morning’s effort to get those re-done.

Fig. 3: the 2016 resistance survey showing the duff grids.

Now the improved grids.  Note that the slight difference between the two surveys is due to minor differences in how I processed the data.  I will produce a more standardised plot.

Fig. 4: detail of the area surveyed at the end of the 2017 season.

Fig. 5: the Earth Resistance survey with the blocks from 2017 high-pass filtered.

There are some interesting things happening at the north-eastern corner of the plots, and into the area to the north we haven’t surveyed.  My guess is that the stratigraphy is probably deep and complex in this area.  Let us compare this area to the magnetic data (Fig. 6) and the GPR data (Fig. 7).


Fig 6: the magnetic survey with the area of the Earth Resistance survey from days 17 and 18 indicated by the cyan outline.

Fig. 7: the GPR survey with the 2017 Earth Resistance survey area indicated in red.

Fig. 8: the GPR survey with the res data overlain on it.

From all three data sets we can see that there is a lot going on in that bit of the field near the drive and the theatre, but it also appears there has been a good deal of robbing to add confusion to the picture.

How about the GPR team?  A month of nice weather with a bit of rain has made the grass green and lush.  Lovely for sheep, but a pain to push the GPR through.  They completed two 40x40m grids on a hot humid day, excellent progress in the conditions.

Firstly, here are a set of time slices (Fig. 9).

Fig. 9: GPR time slices from the area surveyed on Day 18.

The vast majority of the area surveyed appears to be empty.  There are hints of earlier agricultural practice but not much else apart from the top edge where part of a building can be seen.  This connects to the area surveyed earlier in the season.  I reprocessed the earlier version of that block today using kriging to give a sharper image.  Here are the slices together (Fig. 10).

Fig. 10: the GPR survey from day 18, along with a re-processed block from day 12.

We clearly have a nice L-shaped building.  I suspect this is Insula XXX, Building 4, Niblett and Thompson Monument No. 461 which is known from aerial photographs from 1976.  They only have part of the plan, however, and ours looks quite different (hence my doubts).  The mag data shown in Fig. 11 shows some slighter, more ephemeral buildings to the SW along the line of the SW-NE street 25 which can be seen very easily.

Fig. 11: the mag data in the area of the GPR survey.

The SSW–NNE running street 25 which runs along the NW side of the building we have been discussing, shows very clearly indeed but street 10, which is supposed to have run WNW–ESE just to the south of our new building and the “House on the Hill”, does not show at all in either the mag or the GPR data.  It was observed in excavations by Frere near the modern road, but not this far west.  In terms of the town plan, we seem to have two lines of buildings running SW-NE, one along Street 25, and another along street 23, but a large open area with nothing very much it in apart from a few quite large pits.  One can almost see some alignments in those pits.  Are we seeing backyard areas divided into blocks?

Although our season at Gorhambury has come to an end, we will be undertaking surveys elsewhere, and probably in Verulamium Park once more.  I started this posting with some numbers, so I thought I would end with some as well.  This is the 131st posting on the blog.  Those postings take-up 583mgb of our 3gb free allowance and include 693 images.  There have been 68 comments, but we have been protected from 9,402 pieces of spam!  The blog has been viewed 32,150 times by 11,016 separate visitors (in practice, this means that number of IP addresses).  Our best month was the first season at Gorhambury in 2015.  This August has been down on the previous two (2015: 1.8k, 2016: 1.7k and 2017: 1.4k) but the average number of visitors per day has gone-up over the year and we are likely to reach 9,000+ views by year’s end.

I would like to thank everyone who helped this season once more, both with pushing machines, moving strings, laying-tapes and moving equipment.  You are all stars in my eyes, and I think we have created a stunning survey.  We all got a bit tired towards the end, especially in the rather hot and humid conditions over the couple of days, including CAGGs loyal follower, Fergus (Fig. 12).

Fig. 12: Fergus sleeps off a busy day on site.

If anyone is interested in joining in with some of CAGG’s activities, drop us an email.  We are a friendly bunch, and on-the-job training is given.


Buildings galore

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

Before looking at today’s results, I thought everyone might be interested in seeing the reaction to our survey at Verulamium at the Near Surface Geophysics Group conference last December. Not being someone who twits, I hadn’t realised that a webpage of postings from the conference had been put together at You have to scroll down a long way, but look out for the comment by Magnitude Surveys.

Today saw the GPR crew tackle an awkward block that has the hedgerow half way across it.  Here are nine time slices from it.

Fig. 1: Nine time slices from the survey on day 17.

There are some very clear buildings.  Look, for example, at slice 7 (right hand column, middle slice).  In proof of the only universal law, look at the first “sawtooth” on the south side of slice 8 (bottom left hand corner).  There is a lovely little apse just peeping out into the plot.  Typical… the building is under the hedge.

Let us see where this block fits in the overall GPR survey (Fig. 2).

Fig. 2: all areas surveyed using GPR at Gorhambury. Today’s block is in colour.

The absolute area we have now surveyed is quite impressive thanks to the efforts of members of CAGG.  Well done all.  Let us now have a look at a couple of time slices from today.

Fig. 3: GPR time slice 4 from the day 17 survey,

The fourth time slice (Fig. 3; roughly 0.4 to 0.5cm below surface), mainly shows the road running diagonally SW to NE.  The cross-roads just to the north of today’s block is a very busy locality with lots of buildings clustered around it. The block just to the south of today’s survey also has some pretty substantial buildings.

Fig. 4: time slice 8 from the GPR survey on day 17.

Time slice 8 (Fig. 4, roughly 0.8 to 1.0m below the surface) shows some of the buildings alongside the road to the south of the hedge line.  The road is squeezed in between the building found earlier in the block to the south and the new rooms found today.  We also have a little more of the building to the west.  The two lonely walls in the southern block would seem to be related to that building too.  All-in-all, some very nice results.

The Earth Resistance team of Ellen, Pauline and Graham headed north to tidy-up the top-edge of last year’s survey.  They managed a surperb eight 20x20m grid squares.  Here are the results.


Fig. 5: Earth Resistance data from day 17.  The pink line marks the 2016 survey.  The lone grid square on the northern edge was re-surveyed.

Fig. 6: the Earth Resistance data from day 17, high pass filtered.

As can be seen, especially from Fig. 5, we have picked-up some more details of the nice large building in the middle of the plot, as well as other buildings such as the small one at the western end of the strip of grid squares.  At the eastern end we have a large square high resistance feature.  A surviving floor, perhaps?

Fig. 7 shows the mag data from this area.

Fig. 7: the mag data from the same area as Figs. 5 and 6.

Not much sign of the small building to the west, or the “floor” to the east in the mag data, although the “floor” seems to be associated with quite a few walls.

The “sinuous ditch” cuts across the line of today’s plot (seen as the broad dark linear feature entering Fig. 7 top centre, and heading to the SE).  This is almost certainly the town aqueduct as it lies along the 300ft contour. Comparison to Fig, 8, however, shows we we do both mag and res…  no sign of the aqueduct in the res data at all.

Fig. 8: today’s Earth Resistance data overlain on the mag plot.

One last push tomorrow and we are “done” at Gorhambury for 2017.  Many thanks to everyone who has worked so hard, and also thanks to the Earl of Verulam for allowing us access.


Just two more days

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

I did consider using antepenultimate again, but I thought you might think me pretentious…

Everyone worked extremely hard today.  Mike and Jim on the GPR finished yesterday’s block and managed another 40x40m grid square.  No easy task over the long grass and thistles.  Ruth, Dave and Julia completed five earth resistance squares, including two that had to be done in two parts and joined together in the software later due to an inconvenient hedge!  Good job everybody, and many thanks.

Here is the Earth Resistance survey, both normal, high-pass filtered, and the magnetometry data from the same area.

Fig. 1: the Earth Resistance data at the end of day 16.

Fig. 2: the Earth Resistance data at the end of day 16, high pass filtered.

Fig. 3: the magnetometry data for the area shown in Figs. 1 and 2.

I hate to say it, but our five squares, including the two annoying partials, appear to lie between the buildings along the road to the north of the hedge line, and to the north of the buildings we found yesterday. We do, however, have a nice tidy area surveyed now.  We couldn’t have left quite such a silly hole in our survey data.  Tomorrow we head north to survey along the northern edge of the block we did last year.

The first block of GPR data from today was a continuation of yesterday’s

Fig. 4: the GPR time slices from the block completed on day 16.

Nothing very exciting jumps out from the plots, although there are some things to check out.  Slice 6 (second from top on left) has a strange upside-down M shaped feature (in black) and slice 8 (bottom-left) has something semi- or sub-circular near the northern edge.

How about the second block?

Fig. 5: the GPR time slices from the second block completed on Day 16.

Not a great deal there either.  Sorry guys!

Tomorrow sees the GPR crew filling-in an odd gap between last year’s survey and this years.  The plus side is that the mag shows lots of buildings, so tomorrow’s results ought to be much more interesting!

Many thanks to everyone who worked so hard in the sun today.  Just two more days.


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

The rather odd title will become clearer! Today saw Graham, Dave and Ruth on the Earth Resistance meter, Jim, Mike and Robert on the GPR, and myself rushing around with the dGPS. I managed to find time to do a little topographic surveying, the purpose of which will be revealed in due course.

First, the resistance survey.  The team managed an excellent seven 20x20m grid squares.  Thankfully, all whole grids today, although we are back in partial-land tomorrow.  Here are the “normal” and the high-pass filtered versions of the data.

Fig. 1: Earth resistance survey at the end of day 15.

Fig. 2: Earth resistance survey at the end of day 15, high-pass filtered.

Two things can be seen from today’s survey.  Firstly, there are some nice buildings showing on the western edge of the survey area, especially in the ‘sticky-out’ bit (technical term that) which was the last grid square we did.  They show great.  The second thing that arises, is: what on earth is going on the the south-west corner?  The irregular lines of high resistance running down slope from SW to NE do not look like archaeology, so are either geology or erosion.  Very curious!  I need to drape the results onto a topographic map (probably the LiDAR data) to see what the relationship is.

The mag data (Fig. 3) also shows the buildings on the west, but not so clearly.

Fig. 3: Mag data in the same area as Fig 1 and 2. The 2017 survey area marked in red.

The strong linear things in the SW corner do not show at all.  In fact, it is rather bland!

Lastly, the GPR data (Fig. 4).  Please note that the image is made of a variety of time slices created by two separate systems. Eventually, I will process it all to make a nice clean image, but this will do for now.

Fig. 4: GPR data in the same area as Fig 1 and 2. The 2017 survey area marked in red.

Interestingly, the plans of some of the buildings look clearer in the GPR than the res, and for others it is the other way around!  The “muddier” looking walls in the western side of the GPR plot are because I am using inverse weighting to interpolate the results as the kriging in the new software is impossibly slow. The old software’s kriging routine was much faster and gives crisper looking walls.  I probably need a faster computer!

The GPR team finished one block they started yesterday, and started a second block today.  Here are the slices.

Fig. 5: GPR survey time slices. Yesterday’s second block and today’s first block.

Fig. 6: GPR survey time slices, today’s second block.

You can be forgiven for being underwhelmed by the results.  I am a bit puzzled.  The radargrams look quite busy in places, but the slices look very dull.

Now comes the explanation of the strange title of this post.  Let us first look at the slices in position (Fig. 6), and then at the mag data for the same area (Fig. 7).

Fig. 6: a crude mosaic of the time slices in Google Earth.

Fig. 7: the mag data in the same area as Fig. 6.

In Fig. 7 you can see a small square of in black on the lefthandside, probably representing a small square enclosure.  I had hoped this was another Romano-Celtic temple like the one we surveyed at Durobrivae.  Not much sign of anything is showing in the GPR data, however.  Shame!  I had hoped for a small temple overlooking the Insula XVI temple and looking across the valley to the one at Folly Lane.  Sadly, not to be.  The little square enclosure remains a mystery.

Many thanks to everyone who helped out today. Tomorrow is our antepenultimate day.  Fingers crossed for good weather.

One more week

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

As we start our final week of the 2017 season at Gorhambury, our minds turn to what we can finish this year. I have a master plan for the Earth Resistance meter which aims to complete a sensible looking block of data to join to last year’s survey.  Thanks to the amazing efforts of Ellen, Pauline and Julia we managed not only to complete two awkward blocks with trees, nettles and thistles in them, but another four blocks too.  This means we are two grids ahead of schedule compared to the master plan.  Worry not…  I have plenty I would like to get done to fill the time available!

The next three images show the Earth Resistance data, firstly as normal (Fig. 1), then high-pass filtered (Fig. 2) and finally the mag data from the same area.

Fig. 1: the Earth Resistance data at the end of day 14.

Fig. 2: the Earth Resistance data at the end of day 14, high-pass filtered.

Fig. 3: the mag data for the area covered by the Earth Resistance data shown in the previous two figures.

One interesting feature is the building which is to the south of the obvious east-west road (Street 11 in Niblett and Thompson, Alban’s Buried Towns) in between the two easternmost clumps of trees.  We did not know about this building because neither the mag or the GPR really like fighting with nettles and thistles.  A res meter wielded by someone with good boots and trousers can venture where other machines fear to tread!

The other interesting feature is the lack of anything much on the western edge.  The pattern looks like erosion and water-borne sediments.  This is one of the dry valleys which come down the slope from the deep south.  We are still unable to say whether there were no buildings in that area, or if they have been eroded away or buried.  My guess is the former, but it is really only a guess.

The GPR team in the deep south did an amazing job completing the saw-tooth section into the far corner, and starting the next line.  Well done Mike, Jim and Robert.  Excellent job.  They are getting tantalizingly close to my mystery enclosure which shows in the mag.  Here are two sets of time slices.

Fig. 4: All 12 time slices for the survey in the deep south.

Fig. 5: All 12 time slices for the second block of GPR survey.

In Fig. 4 one can see the impact of splitting a survey over several days (let alone years!).  The second slice shows that today’s data has very strong reflections early on.  That is the impact of the rain over our days off, and the generally damp conditions today.  It is interesting — well at least to me it is! — that the literature on the various survey techniques talk a great deal about the impact of rain and weather on Earth Resistance survey, but don’t really mention the problems of GPR surveys run over multiple days.  The lower slices (e.g., slice 9) do show the boundary discussed in the previous post continuing south to the Roman wall.

The second block (Fig. 5) seemed as though it was going to be exciting.  The radargrams are full of strong reflectors and we are starting to go over the enclosure seen in the mag data.  The slices are less than exciting, however. There are a few strong reflections, e.g., near the top of slice 7, but nothing too much to write home about.

I had hoped to get this posting done quickly tonight as I have a report to finish, but the new software refused to play ball for ages and now it is too late and I am too tired!  Only four days left for the 2017 survey season at Gorhambury.



Just in time

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

Not long after we arrived home after a busy week on site, the heavens opened. We’ve had 7.2mm of rain in the last 24 hours. Hopefully, not enough to create problems with the Earth Resistance survey, unlike the deluge we had the other week.

Mike Smith sent me this entertaining picture.

Fig. 1: Red Flag © Mike Smith.

The Earth Resistance meter was manned by Ellen, Pauline, Graham and Fergus (CAGG’s mascot).  They completed an excellent seven 20x20m grid squares including two partials with a big oak tree in the middle.  Figs. 2 and 3 show the data “normally” and high-pass filtered.

Fig. 2: the Earth Resistance survey.

Fig. 3: the Earth Resistance survey, high-pass filtered.

The main feature of interest in the new area is the nice building in the top left-hand corner of the plot.  The apsidal end of a room facing NE shows especially clearly.  This building was known previously, partly from an aerial photograph taken in 1977 and partly from the English Heritage survey undertaken in 2000.  We have much more of it, however.  The buildings also show nicely in the mag data (Fig. 4).

Fig. 4: the magnetometry data for the area of the Earth Resistance survey.

We will fill in the odd missing squares on Wednesday.

The GPR was in the deep south of the Gorhambury side of the town, up on the hill near the King Harry Lane roundabout with marvellous views down across the theatre and the River Ver with the fields of ripe wheat on the opposite hill slope.  That is after you have caught your breath…

The GPR team completed the block I was hoping to get done, and started on the next one slated for Wednesday. Good job!  Here is the composite of six time slices (Fig. 5).

Fig. 5: The GPR time slices 3 to 8. See labels for times and approximate depths.

Slice 4 (Fig. 5, top right) appears to show a wall running NNW–SSE with a rounded corner.  Here it is on Google Earth.

Fig. 6: time slice 4.

Slice 6 (Fig. 5, middle right, and Fig. 7) starts to show a clear building in the NE corner. This is supposed to be along a road which runs approximately parallel to the modern road, but which I cannot see in the geophysical data. The wall seen in the previous image is still visible on the northern side but does not show on the west.

Fig. 7: time slice 6.

Slice 8 (Fig. 5, bottom right and Fig. 8) shows the wall again on the western side as well as the northern.  Curiously, the corner appears different. Comparison to the mag data (Fig. 9) is informative.

Fig. 8: time slice 8.

Fig. 9: the mag data in the area of the GPR survey.

The line of the wall is clearly following the magnetic anomaly in this area, with two large circular anomalies on the corner.  I had thought the anomaly was a ditch, but now I am less sure.  It is clearer to see the relationship if one traces over the wall line from slice 8 (Fig. 10).

Fig. 10: the mag data showing the wall line from GPR time slice 10 marked in yellow.

The differences in the corner can be seen if one then looks at slice 4 with the wall line from slice 8 marked (Fig. 11).

Fig. 11: the wall line from slice 8 marked on slice 4.

As can be seen from Fig. 11 the wall lines match perfectly apart from the corner.  Was it rebuilt at some point, or do the reflections in slice 4 represent some wall collapse?  I’ll need to examine the radargrams carefully to decide what is happening.

The ditches are marked on the “Urban Archaeological Database”.  Isobel Thompson informs me that they have been interpreted as field boundaries for “Little Wynyards” in the 17th century.  Perhaps a vinyard? As Isobel points out, there are the Vintry Gardens near the abbey which are in a walled enclosure.  Just one more avenue of research to pursue to be able to interpret our complex data set.

Many thanks to Pauline, Ellen, Graham, Mike, Jim and Nigel for all their help. We’ll be back on Wednesday for the final week.  Doesn’t time fly when you are enjoying yourself!

Excellent progress

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

Today saw another good sized block of GPR data collected and another six 20x20m grid squares of Earth Resistance data. I skived off to give a talk at Verulamium Museum (and welcome to anyone visiting the blog for the first time having heard my lecture!).

Today’s results saw the GPR and Earth Resistance meter swap roles, in the sense that the Resistance data, although good data, showed very little but the GPR picked-up some spectacular buildings on the ridge.  First the resistance data.

Fig. 1: the total Earth Resiatnce survey. Today’s data is in the middle where we are filling in the gap working westwards.

We have picked up the road as a very solid linear feature just north of the hedge line that bisects the theatre field.  Then it stops. Hmmmmm.  It stops where there is a slight dry valley.  Robbing? Erosion? Or just never that solid at that point?  The high-pass filtered data is not much help in this case.

Fig. 2: the Earth Resistance data, high-pass filtered.

The difference between the upper and lower halves of the plot are just differences in processing between last year’s survey and this years.  At the end of the season I’ll join the two together and attempt to make the two seasons match.  For completeness, here is the mag data.

Fig 3: the mag data with the 2016 and 2017 Earth Resistance survey areas marked.

Now to the GPR data.  The team collected a 57x40m block, and then a “sawtooth” section along the hedge line.  The new software produces a map of the lines so one can check that they have been set-up correctly in the program.

Fig. 4: the map of the transects. The red ones are the zigs, the blue the zags. Note the total length of the transects shown in the title.

The initial processing showed some clear buildings, but which colour scheme to choose?  Here is a selection with the sixth slice.

Fig. 5: Colour options.

Whichever colour scheme you favour, the busy line of buildings on the western edge are very clear.  Fig. 6 shows the six most informative slices.

Fig. 6: the time slices for the day 12 GPR data (see labels for depths. etc.).

There is a road running across the NW corner of the plot, with multiple buildings alongside it.  Quite an impressive set of rooms and corridors.  In the middle of the plot there appears to be a partially surviving structure close to the fence line.  Slice 6 is the most informative, so Fig. 7 shows it in context.

Fig. 7: time slice 6 from the day 12 data.

The last image (Fig. 8) shows the mag data from the same area.

Fig. 8: the mag data for the same area as Fig. 7.

The large high contrast black and white blob in the corner of the mag plot is a metal water trough.  The buildings along the road show quite well, but the GPR plot provides more detail.

Many thanks to Ellen, Pauline, Graham, Nigel, Mike and Jim for all their efforts today.  Tomorrow, the res will continue to work westwards, whereas the GPR is heading back to the depth south at the top of the hill.