Tag Archives: Ground Penetrating Radar

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.




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!

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.

Worn down

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

After three days of using the beast, the Earth Resistance team are feeling worn down. Not with the effort, but with boredom! We have completed a 60x40m block over the “House on the Hill” and that is enough. Tomorrow we return to “normal res” hoping to cover the same size area in a single day, but with just a single depth.

Some of you may remember that the Earth Resistance meter was brand new last year.  The meter came with six steel probes to allow for various configurations, of which we have used three extensively, and three very occasionally.  Clearly we have been busy if Fig. 1 is anything to go by.

Fig. 1: the probes before and after some use.

A small problem with today’s Earth Resistance survey was resolved with some programming in the statistical programming language R.  Here are the six depth slices along with the Wenner array image.

Fig. 2: the multiple depth Earth Resistance survey.

I am very pleased with the result, and it will be interesting to compare the images with the GPR time slices for approximately the same depths.  I think we can safely say we have “done” the house on the hill.

The GPR completed another 40x80m block up on the hill to the south of us. Firstly, a composite of the time slices.

Fig. 3: The GPR time slices for day 9.

Despite some strong reflections, nothing much resolves itself into anything very intelligible.  Slice 1 to 3 are basically the topsoil, and slice 10–12 are nonsense from the bottoms of the profiles.  Slice 6 does show an empty space where the 1955 ditch is.  A composite of days 8 and 9 in Google Earth, plus the mag, makes this quite clear.

Fig. 4: GPR results from days 8 and 9 showing the 1955 ditch.

Fig. 5: the magnetometry data with the area of the day 8 and 9 surveys shown as a yellow box.

As we get up here on the heights, there is less in the way of obvious buildings showing on the mag survey, but there are some intriguing enclosures.  Elsewhere at Gorhambury was have found buildings in these areas not visible on the mag surveys.  There next few days could be very interesting, or very disappointing!

Many thanks to Anne, Pauline, Jim, Adrian, Dave and Mike for all their help today.