Tag Archives: GPR survey

Rain didn’t stop play

… but may be it should have done!  Last year, on the last day of the survey, it poured and we cancelled.  Today, we thought “it is only a little drizzle!”  On occasions, drizzle was more of a deluge.  At one point I was about as far from the cars as it is possible to get when on site, and I got soaked.  Thankfully, Ellen went and fetched a dry tee-shirt and my waterproof coat.  Thank you Ellen, you’re a star!

Peter, one of our volunteers from SWHAS and WAS, has bought himself a UAV fitted with a camera.  He has been having some practice flights over the workers and the site.  When I have worked out how to edit the video down to a sane size, I’ll post one of those, but meanwhile here is one of the stills.  The UAV will prove a very useful tool.

Verulamium Theatre.

Verulamium Theatre.

One of the main reasons we persisted in the rain was the fact that we are so very close to completing the mag survey of the Macellum field.

The magnetometer survey of the Macellum field.

The magnetometer survey of the Macellum field.

Here is a closer detail of today’s survey.

The mag survey after day 36.

The mag survey after day 36.

As can be seen, Watling Street has rejoined the drive.  There appear to be many buildings opposite the theatre which isn’t a surprise in the heart of the town.  Street 24, which runs NNE from the theatre, has the macellum on the east side of it.  This building has been partially excavated and has a complex building history of five phases going from the mid-first century to the early fourth.  It has an almost equally complex excavation history: it has been examined by Grove Lowe (1847), John Harris (1869), Kathleen Kenyon (1934) and finally by Miss K. M. Richardson in 1938.

The resistance survey continued.  The wet surface was both a boon and a problem.  The first two grid squares went very smoothly.  Then we moved all the probes and so forth and the machine started to play-up.  After lunch, Peter came to the rescue and worked out that there was water where it oughtn’t to be, cleaned and dried connections and so forth, and all was well again.  We managed another three grids including one in 35 minutes.  Here is the survey.

The res survey after day 36.

The res survey after day 36.

The same area as the previous image showing the mag data.

The same area as the previous image showing the mag data.

In the first of the two images, I have not “edge matched” the grids so that you can see the ones which are a problem.  Three of the grids we did today fixed existing problems.  Although we had not planned to work tomorrow, I am hoping we might manage five more squares to fix the problems and give us a nice tidy survey.  Archaeological geophysicists are obsessed by “nice tidy surveys.”

Comparing the mag and the resistance surveys, the end of the “sinuous ditch”, seen snaking in from the top of the mag survey, can be seen in the resistance survey but it seems to continue further to the east.  The clear building on the northern edge of the resistance plot also shows pretty well in the mag survey.

Lastly, we learnt one lesson today.  The GPR doesn’t work well in the rain!  We surveyed a block near the rectangular enclosure I thought might be a temple.  Here is the mag:

Rectangular enclosure, near the southern side of the town, seen in the mag data.

Rectangular enclosure, near the southern side of the town, seen in the mag data.

Speculation has been rife as to what this may be, so we tried using the GPR.

Day 36 GPR data.

Day 36 GPR data.

The terrible striping is caused by the rain.  We will have to re-do this block another year.  We can see, however, a square in the centre of the block, and almost another square around it. Before we get too excited, however, comparison of the two surveys shows that the squares in the GPR data lie outside the NNE edge of the enclosure, and in fact, partly show as light white lines in the mag data.  Yet another question to be investigated more fully next year.

Many thanks to everyone who suffered the rain today.  You are all stars.

Tomorrow will be our last day.  I won’t post the results until Tuesday, however, as we are going for a celebration meal in a local pub.

Record breaking

It was an odd day, weather wise. Largely dry with just one quick, light shower, windy at times, sunny spells… Luckily nothing interfered with the fieldwork!

The res team consisted of myself (when I wasn’t putting in grids for people), Ellen, Tim and Pauline.  They pulled out the stops and managed a record-breaking eight grid squares.  Area-wise, that is what the GPR covers in an average day, but for resistance survey at 0.5m intervals, that is very good going.  Well done everyone.

The resistance survey at the end of day 32.

The resistance survey at the end of day 32.

Today’s grids behaved themselves and make the four odd ones from yesterday stand-out even more.  I did make sure that some of the connectors were off the ground today.  How annoying. We may have to re-do those four grids.  The survey did, however, show the buildings along the road in the SE corner beautifully.  The big question… where now?  North to the sinuous ditch?  South for more shops?  West to cover the cross roads?  Only four days surveying left, and we have to assume that we won’t cover eight squares every day.

The mag team also had a very successful day in the Macellum field.

Detail of the mag survey showing the Macellum field.

Detail of the mag survey showing the Macellum field.

We can just see a hint of the cross-roads running NE-SW across Watling Street.  The ‘1955 ditch’ barely shows.  With the eye of faith one might see it in the high readings along the edge of the cross-road, but very much with the eye of faith.  Is the ditch just so built over we cannot see it?  Or was it never built here?

With just four survey days left to go, the team is getting close to finishing the field, but I think we are a day or two short of being able to do that.

The entire mag survey to date.

The entire mag survey to date.

Way down across the field, the GPR team tackled another fiddly staggered bit along the hedge line.  In the next three images I have made the previous days’ surveys partially transparent.

The day 32 GPR, slice 3 (12.5 to 15.5ns).

The day 32 GPR, slice 3 (12.5 to 15.5ns).

The day 32 GPR, slice 4 (15.5 to 18.5ns).

The day 32 GPR, slice 4 (15.5 to 18.5ns).

The day 32 GPR, slice 35 (18.5 to 21.5ns).

The day 32 GPR, slice 5 (18.5 to 21.5ns).

The curious shallow valley to the west of the surveyed area (‘valley’ seems a strong word for it!) that runs down the hill towards the temple is just as devoid of buildings or other recognizable archaeological features as the mag data.  In all three time slices not a great deal shows.  Was this valley always empty?  Or has the archaeology been eroded away, or even buried?  Difficult to say,  There is, however, a long narrow building just to the right of the middle of the surveyed area almost parallel with the hedgerow.  It seems fairly ephemeral, but it definitely there and one corner was picked-up in last year’s grid to the south.

Although the GPR hasn’t covered as much as the mag, we have still collected a mass of data.

Montage showing the area surveyed with the GPR to date.

Montage showing the area surveyed with the GPR to date.

It certainly takes-up a large chunk of my hard disk.

Many thanks to everyone who came out today and worked so hard.  A very successful day all round.  Our next survey day is on Thursday.

And so it begins

Today saw a fair sized group for the start of the survey of the Gorhambury side of Verulamium.  Having completed the Park side of the Roman town in January 2014 we were keen to do the other half.  Our thanks are due to the Earl of Verulam for not only allowing us to work on his land, but for acting as intermediary on our behalf between the various ‘stakeholders.’

We are going to be running the magnetometer mainly, and will aim to do as much of the town as we can.  The first day was a little bitty and the rain didn’t help, but we have already got some nice results.

Day 1: the area south of the theatre.

Day 1: the area south of the theatre.

The temple which lies to the SW of the theatre shows in the mag data to the west of our blocks, but only faintly,  The large black ‘blob’ is very interesting.  It is about 8m across and ranges from -7 to 24nT.  A big pit, maybe?  The dark linear feature clearly runs parallel to the walls of the temple, but again, what is it? It is about 10m long and is quite magnetic with a range of -11 to 36nT.  Near the middle of our plot are a series of white lines representing buildings with flint footings cut through more magnetic material, either burnt or very organic.  These are probably the footings of buildings already known, although seem here in more detail (M451–454 in Niblett and Thompson Albans Buried Towns). Not bad for the first day!

As well as the magnetometer, we have the use of SEAHA‘s Mala Ground Penetrating Radar and will be running that alongside the mag.  It is much slower as it is only collecting one line of data for each pass, not four, but as we have seen before, combining the mag and GPR data can be very useful.  We collected 128 lines of GPR data today, but I haven’t had time to process them yet.  Watch this space.  Finally, we are also collecting some resistance data but that is even slower than the mag.  It is good, however, to be able to combine multiple data sets.

Watch this space over the next three weeks to see how far we get!

Follow up to “a Tale of Two Villas”

Jim West of CVAHS writes:

CAGG teamed up with the Chess Valley Archaeological and Historical Society (CVAHS) in July 2014 to undertake surveys near two Roman villas at Sarratt and Latimer. In mid-September the CVAHS did some further work near the Latimer villa site using resistance and GPR equipment; the latter was operated by Ralph Potter (West Essex Archaeological Group) who kindly agreed to make the trek into Bucks,

The results from the surveys using these sensors are shown in the following images.

Composite image showing the results of the magnetometry, resistance and GPR surveys.

Composite image showing the results of the magnetometry, resistance and GPR surveys.

The four GPR images show the reflected radar from progressively deeper distances from the surface.  The approximate depths are 360-420, 540-630, 720-840 and 900-1050mm for images GPR 2, 3, 4 and 5 respectively.

Image GPR 2 shows the line of the public footpath running diagonally SW – NE; there is evidence of the path in the first three images but why should it leave a trace more than half a metre down?  One conjecture is that the field surface was lower when the path was established several centuries ago.  The GPR images contain no evidence of Roman structures and neither do the magnetometry or resistance images; the GPR survey are has been overlaid onto them to help comparison.

The magnetometry image does show the path and a few strong anomalies but is otherwise unremarkable. Apart from the path, there is no correlation between the GPR and magnetometry images.

The resistance image does not show the path or any other correlation except perhaps the broad V-shaped feature at the top which echoes an absence of features in image GPR 5.

Many thanks to Jim for sending this interesting update to our earlier surveys.

Ralph’s great data

Last Monday and Tuesday we were out in the Park, partly continuing our survey but mainly to teach masters level students from UCL.  Many thanks to Jim West, Pauline Hey, Mike Smith, Peter Lilley and Ralph Potter for acting as unpaid teaching assistants!

Although we did some magnetometry (with both the Foerster and the Bartington) and some resistance, the star of the show is Ralph’s Ground Penetrating Radar (GPR) data.  Ralph provided me with a movie showing his results and the raw data.  I’ve long wanted to learn how to slice GPR data so with Larry Conyers’ program, and Larry’s patient help, I sliced the data myself.  Many thanks to Larry and Ralph for their help and the data.  The first image below shows the location of the GPR survey: a 50m by 40m grid partly over the 1955 ditch but also over some known buildings.

Location of GPR survey undertaken in February 2014.

Location of GPR survey undertaken in February 2014.

The next and long sequence of images show the mag data on its own, and then each of six time slices.  These represent increasing depth although at present all I know is time going from 8 to 32 nanoseconds (ns) in 4 ns slices.

Foerster magnetometer results in the area where the GPR survey was undertaken, February 2014.

Foerster magnetometer results in the area where the GPR survey was undertaken, February 2014.

Time slice for 8-12ns.

Time slice for 8-12ns.

Time slice for 12-16ns.

Time slice for 12-16ns.

Time slice for 16-20ns.

Time slice for 16-20ns.

Time slide for 20-24ns.

Time slice for 20-24ns.

Time slice for 24-28ns.

Time slice for 24-28ns.

Time slice for 28-32ns.

Time slice for 28-32ns.

As can be seen we have some beautifully clear buildings including one which overlies the 1955 ditch.  We also seem to have a road inside the 1955 ditch which I haven’t seen postulated on maps of Verulamium.   One of the many questions which comes to mind is whether the big rectangular area to the west of the plots is only the result of the cricket wickets or is there an underlying building as well?

Just to finish I include one last graph showing the single 20x20m resistance data plot on the GPR data.  There is excellent agreement.  Given how utterly saturated the ground is I am surprised anything shows, but the results are OK, if not stunning.  The dark band is probably the road which runs to the south of the 1955 ditch which has been seen before.

Resistance data overlain on the GPR and magnetometry data.

Resistance data overlain on the GPR and magnetometry data.

Next week we’ll be out again working under the trees to finish off the last few bits.