Tag Archives: GPR survey

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.


A little damp

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

Today’s weather was a little changeable. For a while at lunchtime we thought it was going to get the better of us, but it improved a little and we managed to complete five 20x20m res squares and a 80x40m block of GPR with a hole in the middle!  I was less lucky on my way back from Welwyn later in the evening.

Fig. 1: a little light diffraction.

The Earth Resistance survey continues to find the line of buildings along the road to the east of the Insula XVI temple.  The next three images are the data plotted normally (Fig. 2), then with a high pass filter (Fig. 3), and finally the mag data for the same area (Fig. 4).

Fig. 2: Earth Resistance data, day 11.

Fig. 3: Earth Resistance data, day 11, high-pass filtered to show the buildings more clearly.

Fig. 4: Mag data in the area of the Earth Resistance data, up to day 11.

The line in the data to the east of the “house on the hill” marks the edge of the survey before the huge deluge we had a week and a half ago.  Slightly annoying, but inevitable on a large survey like this.  Hopefully, I can process it out.

The GPR was filling-in a gap today which has annoyed me all year long!

Fig. 5: filling in the gap. Today’s block is marked in red.

Here is a composite image of the time slices from today’s survey.

Fig. 6: GPR time slices of today’s block.

Although Fig. 6 shows bits of wall, and areas of high reflection, nothing resolves itself into a clear building.  Partly this is because of the hedgerow: above it there is build-up of soil, below it is likely to be some erosion. However, the mag data does not show much in this area either.  It does appear that the road has been robbed!  The next set of figures show some time slices and then the mag data for the same area.

Fig 7: GPR time slice 7 from day 11.

Fig 8: GPR time slice 8 from day 11.

Fig 9: GPR time slice 9 from day 11.

Fig. 10: mag data from the area of the day 11 GPR block shown in figures 7–9.

Tomorrow the Earth Resistance meter will be working its way westwards, and the GPR will be completing a block against the hedgerow.  Should be some buildings showing there!

Thanks to Ruth, Anne, Jim, Mike and Dave for braving the weather.

A good day

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

Today was definitely a good day. Although it was raining early in the morning, the weather had started to cheer up by the time we were on site, and was glorious in the afternoon.  Ruth, Adrian and I worked with the Earth Resistance meter, now restored to its normal operation.  Jim, Mike and Dave were up on the hill with the GPR.

The Earth Resistance team managed an excellent eight grid squares today, and there is now a link to the 2016 survey. I wonder if it was a reaction to the go-slow of the last three working days?  Fig. 1 shows everything we have done at Gorhambury using the Earth Resistance meter.

Fig. 1: the whole area surveyed with the Earth Resistance meter as of the end of 17/08/2017.

The next image shows the mag data from the same area (Fig. 2).

Fig. 2: the magnetometry data for the same area.

Over the next few days we will be working our way west filling in the gap between the two surveys.  That annoying hedge line will slow us down a bit. The next image shows a high-pass filtered version of the area surveyed in 2017. The filter flattens out the background variation and thus makes the buildings stand out more.

Fig. 3: the 2017 survey with a high-pass filter to bring out the buildings.

Unfortunately, my trick of spreading the remote probes wide apart to remove the edge-matching problem didn’t work so well here, so I have to play with the data some more to get a good images.  Today’s survey picked-up the line of buildings running SSW-NNE at the top of the plot.  These buildings lie along a road which run alongside the temple.  In the next day or so we should pick-up the cross roads with the road which runs parallel and slightly to the north of the hedge line.

The area covered by the GPR in the last three seasons is quite impressive (Fig. 4).

Fig. 4: the complete GPR survey.

Fig. 4 is a nonsense mish-mash of images from multiple depths, dates and even software packages, but it at least gives one an idea of the amount we have done, and serves to remind me about that annoying hole in the data around the hedge line.

The next image (Fig. 5) shows everything we have done in 2017.

Fig. 5: the 2017 GPR survey so far.

Fig. 6 shows the area surveyed today (with the yellow border).

Fig. 6: the area surveyed on day 10.

There is a substantial building about one quarter of the way along the area surveyed. and another one parallel to the 1955 ditch.  It is useful to look at the mag data (Fig. 7).

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

There is a magnetic anomaly in the same place as the substantial building, but I am pretty sure I would not have interpreted it as a building.  Although the building plan in Fig. 6 is obviously incomplete, it is certainly there which is very interesting given its position high on the hill outside the 1955 ditch. The building which shows in the topright corner of the GPR plot, doesn’t show at all in the magnetic data.  We have seen this before when we have got towards the edges of the town.

Many thanks to everyone who helped today.  Tomorrow the GPR will fill the annoying gap and the Earth Resistance meter will work slowly westwards.

Working in the rain

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

Today’s weather was “variable”, to put it mildly.  I ended-up with slightly sun burnt legs, but at lunch time we were shivering under a tree as the hailstones hammered down.  Nevertheless we got quite a bit done, and so many thanks to Dave, Anne, Ellen, Pauline, Mike, Jim, Nigel and Graham for persisting despite the dark clouds and the occasional flash of lightning and thunderclap (thankfully a long way off!).

Some surveys we get results where a great deal of squinting is required and some optimistic mental joining of dots is needed.  Today wasn’t one of those days!  Let us start with the Earth Resistance survey.  We decided it would be interesting to run the meter over the nice house on the hill for which we have mag and two GPR data sets.  The first image shows the magnetometry data from two years ago, the second today’s resistance data.

Fig. 1: the magnetometry data of the area with the “house on the hill”.

Fig. 2: the Earth Resistance data of the “house on the hill”.

The Earth Resistance data has been high-pass filtered to bring out the building nicely, but I think you’ll agree it is pretty clear.  It is interesting that the rooms on the western end of the house are showing much more clearly than they did in the GPR survey that we did two days ago (see this post).

Having wiped the GPR’s memory, we completed the grid square from yesterday and then managed another grid and three-quarters further south. Tomorrow we have to do the jaggedy bits along the edge of the field.  First of all, lets look at the mag data.

Fig. 3: the magnetometry data of the area covered by the GPR survey.

The strong linear feature running diagonally across the lower part of the image is the “1955” ditch, the 1st century boundary of the town which had a section cut across it by Sheppard Frere in 1955. The “house on the hill” can be seen in the middle towards the top.

Now for the GPR.

Fig. 4: the GPR survey at the end of day 3.

The grid below the house on the hill is mighty dull.  There is the faint building we saw yesterday, and one or two other possible features, but nothing too exciting.  The grid towards the bottom though shows as lovely town house facing SE.  The next image zooms in a little.

Fig. 5: the town house in the south of today’s GPR survey.

It looks like it lies on the road which runs along the inside of the 1955 ditch. Sweet!

The new software has a variety of palettes.  You have seen the BW one, and the default colour scheme No. 7.  The software can create a composite of all the palettes.

Fig. 6: a plethora of palettes.

Which do you prefer?  Answers on a postcard, please…

The weather forecast is better for tomorrow.  Fingers crossed!

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!