Tag Archives: Roman

The final results from Alba Iulia (Apulum)

We managed the final ten squares on the last day completing the area I hoped we would cover. We managed 2.6ha in seven days of survey.  The final four grid squares had an old excavation trench in the middle of them and a sea of mud from the spoilheap being bulldozed back into the hole.

Fig. 1: Stefan and Wyatt surveying in the mud.

I have left the grid on the next image (Fig. 2) which gives you some sense of scale.  The grid squares are 20x20m, and site north is to the right.

Fig. 2: the surveyed area with grid for scale.

In the next image (Fig. 3) I have labelled a few of the features.  One can easily see many of the buildings.  Because of the large differences between areas of low readings and areas of high readings I have applied a log-transform to the data to make some of the features more visible.

Fig. 3: the 0.5m survey with log-transformed data.

An alternative (and very common) method of evening-out the differences is to apply a high-pass filter (Fig. 4).  The downside to this is that areas of very high readings will often end-up with a area of very low readings next to it which are purely an artefact of the filter.  It is essential, therefore, to have several images and compare them so that you do not create features where there are none.

Fig. 4: the data after the application of a high-pass filter.

As well as the main survey with a 0.5m probe spacing (thus ‘looking’ 0.5 to 0.7m below the surface, approximately), we were simultaneously collecting 1.0m probe spacing data (which ‘looks’ about 1.0m into the ground), but at only two readings per square meter rather than four.  This extra data did not add that much more to the survey (Fig. 5), but does make the north-south road show more clearly.

Fig. 5: the 1m probe spacing data, high pass filtered.

There is quite a bit to do in terms of interpretation and drawing-up maps of the results, but I am very happy with the new results which have greatly improved an already excellent set of results from 2002/3.

The journey home seemed to go on for ever, but at least I had some entertainingly-named beer on the way!

Fig. 6: Munich Hell.

Many thanks to the whole team, and especially to Stefan, Wyatt, Doru and Ian.


Somewhat tired

On Friday the snow kept on coming and we had to abandon the survey for the day. It gave us the chance to explore the citadel and visit the museum.

Fig. 1: Carol’s Gate, Alba Iulia.

The snow had stopped yesterday morning so we headed out to site once more. During the day the snow cover slowly melted and we managed an excellent eleven grids, although two of those were small partials.  Today, the weather improved…

Fig. 2: Stefan surveying.

I even ended-up with sun-burnt ears!

Having lost time through airline incompetence and snow, we were determined to try and catch-up and we managed twelve 20x20m grids at 0.5m intervals with just the two of us.  To say our feet ached would be an English understatement.  We did, however, get some lovely results.

Fig. 3: the half-meter probe spacing survey at the end of Sunday.

We believe that the thin line on the right hand side of the plot running diagonally is the line of the town wall.  We seem to have picked-up a set of three small buttresses on the inside of the wall.  We have some more buildings at the top of the plot.  Obviously, there is a great deal going on in this data and a detailed interpretation will take a little longer.

Tomorrow our target is the ten grids along the top edge of the survey to complete the block and join the survey up with the earlier excavations.  It is tempting, however, to try and survey a little more at the top to see what those two pairs of parallel lines are doing!

I’d like to thank the whole team but especially Stefan who has worked really hard, and Wyatt who has also helped a great deal, and also Ian Haynes and Doru Bogdan who made the whole trip possible.  The whole team have been great to work with and made what could have been a trying experience great fun.

And now for somewhere completely different

Although this isn’t CAGG related, or Hertfordshire, I thought members of the group might be interested in my latest geophysical adventures.

Some 15 years ago I undertook a survey in Alba Iulia, Romania, for a colleague.   The site was part of one of the Roman cities at Apulum which grew-up alongside the legionary fortress.  The results were pretty good, but I was only taking one reading per square meter.  Since getting the RM85 I have been wanting to return and re-do the survey at higher resolution.  Well, be careful what you wish for!  Last Saturday, I found myself on the way…

Fig. 1: On my way…

Yes, you did read the time correctly.  I flew to Cluj-Napoca via Munich.  Sadly, when I got to Cluj, my luggage was still in Munich.  Thankfully, they delivered it all safe-and-sound the next day but it did mean I lost a half day of survey.

Alba Iulia has changed quite a bit in the fifteen years.  The citadel, especially, has been restored beautifully and now has a series of bronze statues decorating the area.

Fig. 2: scrumping.

Having lost half a day, we got started in the afternoon.  Three whole grids and partial that day, seven whole grids a four partials (including one very silly small one) the next day, and eleven yesterday.

Fig 3: Wyatt helping with the Earth Resistance survey.

Yesterday, going along the first line seemed fine with the wind at my back.  Then I turned into the howling gale and snow…  The effect was like star-trails in a science fiction movie as the snow blew past me horizontally.  Thankfully, the weather got better during the day.

There were some software issues to begin with, but thanks to David Wilborn’s excellent customer support, those were quickly resolved.  The results look pretty good.  In the next image I have applied a high-pass filter to even out the big changes in range that occur in this data set.

Fig. 4: the Earth Resistance survey results at the end of Day 3.

I’ll update you all as I go along.  I have six more days to try and complete the whole survey.  I suspect I’ll be a little tired by the end.

Your foreign correspondent.

Midwinter madness

Who on Earth goes out on midwinter’s day to do fieldwork? Well, we do! Mike, Ruth, Peter and myself met up with Rex and Nick from FRAG to extend the magnetometry and Earth Resistance surveys at Durobrivae.  It was a foggy morning but at least the temperature was well above freezing and, in fact, it was quite pleasant working conditions… for midwinter!

Fig. 1: “… and this bit goes here”. Building the mag at the start of the day, Photo by Mike Smith.

Unfortunately, the mag developed some problems and although we managed two grid squares before it died completely, I am unable to download them at the moment.  Fingers crossed I can rescue the data in due course.

The Earth Resistance survey was more successful.  Given the short day I was only aiming for four grids to extend the previous survey to see if the outer circular feature did actually join-up on the eastern side.  Figure 2 shows the results.

Fig. 2: The Earth Resistance results over the “tumulus” as of December 2017.

As can be seen, the “outer circle” does indeed form a continuous curved feature on the eastern side.  There also seems to be two features running off this curved edge eastwards.  I have marked these in Figure 3.

Fig. 3: the Earth Resistance results. Red lines mark the features mentioned in the text.

We have clipped the corner of a building in the bottom right hand corner of the survey.

How do these match up with the mag data?  Figure 4 shows it for the same area.

Fig. 4: the magnetic data for the same area as Figs 1–2.

The northern linear feature picked-out in Figure 3 shows clearly in Figure 4 as being strongly magnetic (it ranges from -10 to +15nT).  It is probably a ditch full of rich organics and/or burnt material.  The southern linear feature is harder to see in the mag data so in Figure 5 the lines from Figure 3 have been superimposed on the image.

Fig. 5: the mag data with the linear features from the Earth Resistance data superimposed.

The northern line in Figure 5 fits nicely on the magnetic feature as we expected.  The southern line joins-up three magnetic anomalies ending up with a big one which is about 10m by 5m.  It looks like parts of the ditch were filled in with non-magnetic material, and parts with highly magnetic material suggesting several phases of infilling.

It clearly worth continuing to expand the Earth Resistance survey yet further.  It would be good to see what is happening on the western side, but also to see if we can pick-up the plan of the building to the south where we have just got a single corner.

Nerdy bit…

For those of you interested in this sort of thing…  Since getting the new RM85 I have been using a “pole-pole” configuration for the probes.  That means having the remote probes 20m or more away from the edge of the survey area, and separated by at least 10m.  This contrasts with the more usual set-up (called “twin probe”) where the remote probes are often only half a meter apart.  The reason for doing this is that is removes, usually, the need for grid-matching, i.e., getting an image where one cannot see the joins between grid squares.  If the survey is done over only a few days, this works well.  If, however, the two blocks of survey are separated by several weeks of winter weather, the differences can still be seen (Fig. 6). This requires a bit of extra processing, but still not as bad as having to get the grids to match every time one moves the remote probes.

Fig. 6: plot showing the girds of Earth Resistance data before grid matching.

The CAGG team are a sociable group, and we get to sit and eat lunch in entertaining places (Fig. 7).  If you would like to come and join us during our surveys in 2018, please get in touch.  We provide training, and commitment is minimal.  We simply add everyone to a mailing list, and when there is a survey coming-up, we ask who can make it.  Often, we do not want a team bigger than seven people apart from the big survey season at Verulamium in August, and maybe one or two other sites where we would like to run more than one machine.

Fig. 7: the crew having lunch accompanied by the dull roar of the A1(M). Photograph by Mike Smith.

I hope you all have a very Merry Christmas, and wish us all a successful year of surveys in 2018.


Ivinghoe Aston

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

Last Sunday we undertook a small survey in the parish of Ivinghoe Aston, Buckinghamshire. Earlier in the year, following a find by a metal detectorist, the Buckinghamshire Archaeological Society’s Active Archaeology Group undertook a small excavation and recovered a Roman cremation burial.  As is always the way, tucked into one corner was the edge of a second burial.  This was left in situ and we were asked if any of the survey techniques might detect cremations.  Of the techniques we use, magnetometry is the most likely to be able to find cremations but given how small they are, the standard 0.5m spacing between sensors might just be too big.  We can configure our cart to have a 0.25cm spacing between sensors (Fig. 1) at the cost of having to walk twice as far.  In this case, we were not aiming to cover a large area and so the trade-off seemed reasonable.  Jim West (CVAHS)  Pauline Hey (LBDAHS), Peter Alley (WAS) and myself (WAS), along with Jean Bluck (CVAHS and BASAAG), Rhian Morgan (CVAHS and BASAAG) and Piotr Sobisz (the person who found the site) surveyed a 80 x 80m area at 40 readings per meter.  Peter Alley also undertook a survey with his UAV to map the lumps and bumps in the field.

Fig. 1: Jim West operating the mag with a 25cm sensor spacing. Fergus is supervising closely.

The basic results are shown in Figure 2.  This image has had the readings clipped to +/- 2.5nT.

Fig. 2: the magnetometry survey results.

The first and most obvious things are the two strong linear features across the bottom and top left hand corners.  There are also some fainter linear features.  I have marked these in Fig. 3.

Fig. 3: the results with the linear features marked. Yellow: fence lines etc.; red: more subtle linears, possibly archaeology.

The lower feature marked with yellow arrows clearly lines up with a hedge in the next field and is almost certainly a grubbed-out hedge line.  The upper line marked with yellow arrows is less  certain.  It isn’t that strong, and it isn’t that continuous.  I still have my doubts, however.  It may be an old field drain?  There are many subtle linear features marked with red arrows.  These do not make a coherent pattern and may well be earlier drainage or other agricultural work, but some could be something more interesting archaeologically.

But what about the grave?  Fig. 4. marks the position of the excavation trench.

Fig. 4: the results with the excavation trench marked.

You can just see the little red square in the middle of the plot. (It might be worth clicking on the image to see it in more detail, and then clicking on that version of the image to zoom it to maximum size.)  Fig. 5 shows the area in more detail (again you might like to click on it).

Fig. 5: zoomed in view of the area of the trench.

It is interesting to see that the excavation does not show in the data at all.  The second, unexcavated cremation was in the SW corner.  As can be seen from these plots, there are lots of “little blobs”, some of them are stronger than others.  Little blobs which are strongly magnetic, shown with the positive pole in black and the negative in white, are likely to be metal, especially when the negative (white) part is not north of the black part.  In order to see the subtle stuff, we have clipped the image to +/- 2.5 nT (which isn’t much!) but this makes it hard to see the difference between strong and moderately magnetic “anomalies”.  To make this clearer, I have created an image where the strong values (greater or less than 5nT) are in red and green (Fig. 6).

Fig. 6: strong positive and negative magnetic values shown in red and green.

This allows us to discount a number of features, but there are still lots of possible blobs.  If we look at Fig. 5 closely there is a small blob on the SW corner, just about where the second cremation is likely to be.  Using TerraSurveyor, we can measure the magnetic profile of the feature and draw a graph of it (Fig. 7).

Fig. 7: Readings across the feature to the SW of the trench.

From this graph we can see that the burial, if that is what it is, has a “signature” about 75cm across and ranging from -1nT to a maximum of about 5nT.  From this we can suggest some possible extra burials.  I have marked just a few in Fig. 8.

Fig. 8: Red arrow: likely cremation next to the trench; yellow arrows: some possible cremations; blue arrow: larger feature.

There are clearly quite a few more in the survey, I have just marked five.  Unfortunately, the law of equifinality comes into play, i.e., very different things can have very similar magnetic signatures.  All sorts of things might make small, subtle magnetic features, like old animal burrows for example.  There are also a small number of larger features which might be interesting, for example the one marked in blue.  This is bigger than a single cremation, being 2m north-south and 1.5m east-west but has a similar magnetic range (i.e., -1 to -5nT).  The reason why I am curious about it is that cremation cemeteries sometimes also preserve evidence for the pyre site.  Rarely, we also get what are called bustum burials where the pyre is built over a small pit and the pyre remains, along with possibly extra grave goods, and placed in the pit.  Hopefully, the AAG might be able to test a few of these features to “ground truth” the results.

All in all we had a grand day, especially Kai (Fig. 9)!  We have several possible surveys in the pipeline.  If you want to be put on the mailing list, please email us.

Fig. 9: Kai being spoilt.

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 https://storify.com/girlwithtrowel/tweets-from-the-nsgg-recent-work-in-archaeological 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.