Tag Archives: GPR

Being completist

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

The team are very patient with my need to be neat and tidy and do the silly little bits around the edges.  Today was perhaps an extreme example.  Due to yesterday’s little hiccup, I set-up the res kit and, with Graham’s help, surveyed about a sixth of a grid square, then packed it all up again.  Now I can sleep easy.  On a more ambitious note, I have finally put all the res grid squares into one large composite.  The survey now consists of nine hectares, which is about 360,000 individual readings.  Most Earth Resistance surveys use what is known as a twin-probe configuration.  That means that there are two mobile probes on the frame, and two stationary probes on the end of a long cable, normally about half a meter apart.  One mobile probe, and one remote probe set-up an electric circuit part of which is the soil.  The other mobile probe and the other remote probe measure the resistance.  The problem with this “standard” set-up is that when you have to move the remote probes, the readings for the same spot will change.  This leads to endless struggles to “grid-match” each set of squares.  Since 2016 I have gone over to using a pole-pole configuration.  This is basically the same except the remote probes are a long way away (I aim for about 30m or more) and a long way apart from each other (I aim for more than 20m). This helps enormously with the grid matching.  Grids completed on different days of the survey will match quite nicely, usually.  Where this is not true is when (a) there is a lot of rain in the middle of a survey and (b) when the survey is split over multiple years.  In the case of our 9ha block, this has been completed over four seasons.  Unsurprisingly, one can see the edges.  TerraSurveyor has a function called “periphery match” which will, sometimes, do an excellent job of grid matching.  In this case, it was pretty good.  Figure 1 shows the survey with a periphery match applied.

Figure 1: the res survey as of the end of Thursday.

If you click on the image and see it full size you can see the detail of many buildings.  Unfortunately, the range of values makes seeing some buildings quite hard.  A high-pass filter is a background trend removing tool that makes some buildings show more clearly (Figure 2).

Figure 2: the res survey, high-pass filtered.

There are still many features, many buildings, that one can only see when looking at smaller blocks.  With such a big area getting everything to look clear is going to be impossible, I fear.

From tomorrow the res will be working in Church Meadow where we hope it will map the remains of St Mary du Pré.

The GPR team is getting really close to finishing the theatre field (Figure 3).

Figure 3: the GPR survey so far.

The team have some fiddly bits around the edges to complete, one missed block, and one block we want to re-survey at 50cm intervals as there is a building and road in it.  Fingers crossed, two more days should do it.  Then, for a bit of last day fun, the GPR will also have a look at St Mary du Pré.

The mag team completed another eight blocks today.  Since moving to Church Meadow, they have managed to survey four hectares in three days which is very impressive, especially given that one block had to be repeated due to a sensor freeze.  Having lots of whole grids and no partials makes such a difference.  The team are now just 2.5 ha away from completing a square kilometer of mag at Verulamium.  Figure 4 shows the mag results in Church Meadow.

Figure 4: Church Meadow mag survey after day 3.

Three things are of note.  Firstly, earlier today I wondered which of the two raised areas in the field was Watling Street.  Looking at the survey results, it looks like the road splits in two near the edge of our survey.  Perhaps it is two phases?  I’m not convinced.  Secondly, we have some marked linear features showing that almost look like enclosures.  These are, however, low magnetism suggesting and might be yet more pipes, but not of metal this time?  Again, I’m not convinced.  They might well be archaeological features.  I will have to survey all the pipes I can see in the field.  Lastly, as we get closer to the town to the south, there are many more little black blobs.  Seasoned readers of this blog will know that usually little black blobs in mag data are often pits.  In this case, I wonder if we are starting to pick up the edge of the cemetery which is likely to have lined Watling Street?  In the Roman world, the richer you were, the closer you wanted to be buried to the road and the town.

Tomorrow is the antepenultimate day of the 2019 Gorhambury survey season.  The weather looks good so fingers crossed all goes well.

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Pipe dreams

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

Saturday night I said the weather was predicted to be “unsettled”.  Well on Sunday at 10am it was raining cats and dogs.  (I wonder why cats and dogs?  Why not ducks and pigeons?, or frogs and mice?)  I was determined to set-out grids in Church Meadow and so I soldiered-on.  Up until now, I have stuck to the 40m grid based on the OS for all the fields we have surveyed in Verulamium Park and in Gorhambury.  Church Meadow, however, is a long thin field at approximately 45º to the OS grid.  Additionally, the fence along Gorhambury drive is very straight for much of its length.  I decided, therefore, to use a floating grid to minimise partial grid squares and wheel spinning.  The lack of an “end line” function is the Foerster’s Achilles’ heel.  We must have wasted hundreds of hours spinning the wheel due to that one simple omission.  Although the data processing involves some extra steps and jiggery-pokery to get the plot in the right place, it seems worth it in this case.  I think my decision was vindicated when the team completed nine complete 40x40m grids despite the wet start to the day, and the lunchtime deluge. Figure 1 shows the team in action.

Figure 1: the mag team in action in Church Meadow. Image © Mike Smith.

Figure 2 shows the location of Church Meadow.

Figure 2: the location of Church Meadow, outlined in red.

Looking closely at the Google Earth image reveals some features in the field (Figure 3).

Figure 3: Features visible on Google Earth.

With these features we had high hopes.  Figure 4 shows the results of the mag survey from the first day.

Figure 4: Mag survey results.

Sadly, the plot is dominated by the two pipelines running through the field (Figure 5).

Figure 5: Heavy metal. Image © Mike Smith.

One of the pipes clearly runs straight through the building seen in the GE image. There are, however, some archaeological features to be seen (Figure 6).

Figure 6: mag survey in Church Meadow with some labels.

It is very frustrating that we can see the walls in the mag data to the SW and between the pipelines, but the image is so dominated by them that it is hard to make sense of anything.  Hopefully the res or the GPR will show the details better.  The ditch is interesting, however.  Could this be the vallum monasterii? It could, perhaps, be related to Watling Street, or it could simply be the remains of the earlier route of Gorhambury drive.  It will be fascinating to see where it goes.

The Earth Resistance team completed an excellent six blocks of data.  Figure 7 shows the whole res survey.

Figure 7: the whole Earth Resistance survey after Sunday 18th.

With good luck and a fair wind we should reach the hedge line on the next survey day.  Figure 8 shows the grids completed on Sunday.

Figure 8: The grids completed on Sunday.

Not a great deal is showing in those grids apart from the faint line across the top corner.  Let’s look at the mag data from that area (Figure 9).

Figure 9: the mag data from the same area as Figure 8.

The light line in the res data is matched by the dark line of “the sinuous ditch”, which is exactly what we would expect.  The sinuous ditch is, we think, the town’s aqueduct.  We should pick-up much more of this on Wednesday.

The GPR team have been working down the western edge of the town with the end in sight.  Soon, soon, they hope, they can escape the theatre field and its rugged terrain (Figure 10).

Figure 10: GPR and the rugged terrain.

The GPR team have been doing a lots of sawtooth edges as well as extreme hill-climb GPR.  Figure 11 shows recent results.

Figure 11: the western edge completed between Thursday and Sunday (colour section).

GPR, perhaps even more than Earth Resistance, is affected by the weather and ground conditions.  It seems very difficult to get different days to match-up.  I tried three methods with this data collected over four days, and none were perfect.  This image was created by just treating everything as one big data set. It doesn’t help that each tweak to see what works takes half an hour to process!

Looking back over the first three weeks, we have managed to achieve quite a bit despite dry weather, wet weather and endless partials.  Many thanks to everyone who has been involved, especially those stalwarts who come most days (you know who you are!).

Nigel wonders what next week will bring…

Figure 12: The Thinker. Image © Mike Smith.

 

The Great Escape

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

The mag team have pulled-off the Great Escape.  They waved a tearful goodbye to the field they have been working-in all of this season.  Today saw the completion of the available area of Prae Wood Field.  In fact, we did more than we agreed to because I did not know they had grubbed out a hedgerow since the images on Google Earth were taken last summer. Figure 1 shows the entire mag survey from Abbey Orchard (we must finish that!) to Prae Wood.

Figure 1: the entire Verulamium magnetometry survey as of 17/8/2019.

The complete survey now consists of over 93ha of mag data, which is about 18,700,000 readings.  No wonder my computer is feeling the strain!  The Prae Wood field survey is 10.7ha in extent.  Figure 2 shows the whole field.

Figure 2: the Prae Wood Field magnetometry survey.

Compared to the riches of surveying inside the town, the field provided slim pickings.  We did find an enclosure and a large ditch at the eastern end of the field, and some enclosures towards the west.  Much of the field, however, is a remarkably blank canvas, magnetically speaking.  As always, absence of evidence is not evidence of absence, but in this case there is not much to encourage further work here.  Figure 3 shows the western area completed today.  Well done Ruth, Jim, Dave and Pauline, along with Rhian and Ellen on previous days.  Tomorrow sees the great move to Church Meadow.

Figure 3: the western end of Prae Wood Field.

The one new and obvious feature in the field is the big black blob.  It is quite large, about 10m across, and moderately magnetic (-3.5nT to about 8nT).  My best guess this is a ploughed in pit.  Hertfordshire is full of pits, for chalk, gravel or clay (and sometimes all three in the same field).  This one would be quite modest compared to many.

The Earth Resistance team consisted of Ellen and Anne, assisted in the morning by myself until I went to help with the GPR after lunch.  We completed another six grid squares as we work our way north. Figure 4 shows today’s data.

Figure 4: the Earth Resistance survey. Area completed today outlined in red.

Towards the north of today’s area is a nice little apsidal building orientated NW-SE.  Although it reminds me a little of an early church, apses were not uncommon in the Roman world, and we may not be seeing all the building.  It can be seen a little more clearly if we apply a high pass filter (Figure 5).

Figure 5: the 2019 area after the application of a high-pass filter to remove the background trends.

We have seen this little building before in the GPR data, but not quite so clearly.  Once thing that I am finding very intriguing is why some buildings show in the mag data and the res/GPR data, and some buildings do not show at all in the mag data.  Figure 6 shows the mag data from this area.

Figure 6: the mag data from the same area as Figs 4 and 5.

As you can see, there is no sign at all of the apsidal building in the mag data.  Compare this to the buildings further to the south which line the road and show very clearly in both mag and Earth Resistance data.  There is a mystery to investigate!

The GPR team were on some of the steepest slopes at Gorhambury (Figure 7).

Figure 7: John Ridge (SAHAAS) gallantly pushes the GPR up the steep incline.

Not often do I hear survey teams hoping not to find something, but afraid I might make them resurvey the area at 0.5m transect spacing, the team were keeping their fingers crossed that nothing exciting showed!  The radargrams on screen seemed to be fulfilling their hopes (Figure 8).

Figure 8: example radargram from today.

The data were duly processed in GPR Slice.  I present just one slice, No. 4 (Figure 9).

Figure 9: Time slice 4 from today’s survey.

Although there are areas of high reflectance (in red) and low (in blue) I suspect this is mainly to do with geology on the side of this steep dry valley.  The underlying mag data shows a few features (Figure 10).

Figure 10: the mag data in the same area as the today’s GPR survey.

Although there are a couple of pit-like features, including one quite strong one near the northern edge of the block, there isn’t that much to suggest there is much going on here.  Not enitrely surprising given the geology.

Today’s weather was highly variable.  We had some rain just before lunch, but sunny skies in the afternoon (Figure 11).  Fingers crossed for tomorrow.  The afternoon’s weather is currently forecast to be “unsettled”.

Many thanks to everyone for an excellent day’s progress.

One hundred and eighty!

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

As much as I would enjoy getting three triple tops, in this case the 180 refers to the number of posts on this blog.  We have had almost 50,000 views and the text is now almost 100,000 words.  That’s more than a PhD thesis…  There are also over a thousand images in the media library.  Granted, some of those are of dogs and sunsets, but there’s nowt wrong with that!

Just a quick posting tonight as I have been struggling with some awkward data processing.  Although we lost half a day to rain, all three teams did an excellent job before the heavens opened.

The res team managed an excellent four grids today which took them over the cross roads.  The results are shown in Figure 1.

Figure 1: Today’s resistance results.

The buildings to the east of Street 11 show very nicely.  The north side of the cross-roads does not seem to show a building, and it is this spot which the mag data has a very strong feature that I have speculated previously might be a burnt down timber building.  The street running to the SE appears to have been robbed.

The mag team completed another strip of grids in Prae Wood field.  Figure 2 shows progress so far.

Figure 2: the mag survey of Prae Wood field.

An examination of the results from today does not (Figure 3), sadly, show any more enclosures.  What a shame.  Hopefully, weather and equipment willing, the field should be completed soon.

Figure 3: the mag survey at the western end of Prae Wood field.

The GPR team have been working away down the western side of the town completing sawtooth-edged grid after grid.  I can barely keep-up.  Correlating my lists of coordinates and the sketch plans of grids / line numbers takes a bit of time, but I am getting there.  A GPR report will be available soon…ish!

Many thanks to everyone who put up with the rain today.  Fingers crossed for nicer weather tomorrow.

Half-way point

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

It is amazing (and slightly scary) that we have already reached the half-way point in the 2019 Gorhambury survey season.  To give the surveyor a chance to get a little ahead of the game, we pulled the mag team off Prae Wood Field this morning and got them to help the GPR crew and to do some res.  After lunch, the mag finished Prae Wood and the GPR completed their 80 x 80m block.

The mag completed the last two triangles in the far eastern part of Prae Wood, and re-did one square for which the mag cart had, for some inexplicable reason, developed a horrible stagger error (Jim puts it down to cosmic rays).  I wouldn’t have been too bothered, apart from the fact that one edge of our enclosure passes through that square.  I have started to lay-out the grids for the western edge of Prae Wood field.  Hopefully, by next weekend we can be out of that field and into Church Meadow.  Figure 1 shows the whole mag survey of the field, and Figure 2 the detail of the eastern extremity.

Figure 1: the mag survey of Prae Wood field.

Figure 2: detail of the mag survey of Prae Wood field.

Not much new has shown-up in the eastern extremity of that field, but the enclosure we found a couple of days ago is now a little clearer than before.  The mass of modern noise is unfortunate.

The res survey was extended up to the hedge line in the theatre field, with hints of what is to come.  Just to the north of the hedge line their are many buildings which the res should pick-up nicely.  Figure 3 shows the entire res survey, and Figure 4 a closer view of the second block of the 2019 survey so far (the strip to the west).  As always, the edges are because the blocks have been processed separately and joined-up in Google Earth.  I’ll start to put everything together soon.

Figure 3: the Earth Resistance survey after day 10.

Figure 4: detail of the western edge of the survey showing the new data.

The res survey is now 8ha in extent.  Not counting grids we have had to do twice for various reasons, this is 320,000 measurements for the 0.5m mobile probe separation survey.  (We only started using the 1+2 survey method last summer.)

GPR this year, and last, can be a little challenging (Figures 5 and 6).

Figure 5: Dave Minty (WAS) pushing the GPR in week 1. Image © Mike Smith.

Figure 6: Jim West (CVAHS) pushing the GPR today. Image © Mike Smith.

For most of this week, we have been using a 1m transect spacing with the GPR in order to finish the field this season.  We can do this because (a) the features we know about are big such as the 1955 ditch; (b) we aren’t expecting any stone buildings in this area and (c) if we do find something interesting, it will show in the 1m data, but just won’t be very clear.  Today saw the GPR reach the bottom of the dry valley across which the aqueduct dog-legs, and also joins up with last years survey.  Figure 6 shows the entire GPR survey (very crudely!).

Figure 7: the entire mag survey after day 10.

Figure 8 shows today’s block along with the neighbouring ones.

Figure 8: the day 10 GPR block, slice 4.

The eagle-eyed amongst you would have spotted a long building running SW–NE with its short end on the big black blob.  Yup, we have a building.  Oops.  Well, at least that proves that we can see buildings even with 1m transect spacing, although it does look a bit dot-to-dot.  What are the other things?  Figure 9 shows the mag data.

Figure 9: Mag data in the area of the Day 10 GPR survey. Red square marks today’s block, blue rectangle the building.

The mag data shows us that part of the big black blob is the aqueduct.  We have been speculating whether the aqueduct had some sort of structure to carry it across the dry valley.  This may help us address this question.  The long building, however, does not show in the mag data at all.  We have come across this previously.  The further buildings are from the core of the town, the less likely it is we will see them in the mag data.  This is because, I think, the surrounding soils are less magnetic than in the core of the town.  The mag survey processes out these broad scale background changes in magnetism.  We need to undertake a magnetic susceptibility survey!

Across the SE corner of the mag plot is a long linear feature which is the 1955 ditch shown with green arrows in Figure 9.  As can be seen, some parts are strong and easy to see, other parts are much fainter.  Are we dealing with parts of the ditch that have been filled-in, or parts that were never really dug in the first place?  I suspect the former, but it is only a suspicion. Towards the south part of the 1955 ditch in Figure 9 are two strongly magnetic features in line with the ditch, either side of a low area of magnetism.  This is where Street 11 appears to cross the ditch as shown by the red arrow in Figure 9.  The street barely shows in the mag data but Figure 10 shows how clear it is in the GPR data.

Figure 10: today’s GPR block with some labels.

We have had a very successful two weeks.  Rain has only lost us a few hours (so far… touch wood!) although the wind yesterday was trying.  Many thanks to everyone who has made this all possible.

 

 

 

 

Like a welcome summer rain

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

Langston Hughes probably didn’t have geophysics in mind when he wrote that. I thought we might lose today altogether, but actually it was fine until the afternoon. Then the showers got a bit much and we headed home mid-afternoon. Hopefully, however, it might mean the Earth Resistance survey can make some progress soon.

The mag team are still working their way around the edges of Prae Wood field.  They are making great progress despite the rain and the machine crashing today and loosing a whole grid of data.  Figure 1 shows the whole field.

Figure 1: The Prae Wood field mag survey.

The SE of the field has quite a few services (the dark black and bright white lines) but lets zoom in a little (Figure 2).

Figure 2: the SW corner of the mag survey in Prae Wood field.

As might be expected near to the buildings there are blobs and bits of metal and all that.  There is, however, what might be a sun-rectangular enclosure.  I don’t think I am making it up…  Figure 3 shows the possible outer boundary.

Figure 3: possible enclosure in the mag data.

It looks pretty good to me!

The GPR team were aiming to do another 80 x 80m block at 1m intervals, but the rain stopped them short.  Using the GPR on the steeper bits of hillside is quite a challenge (Figure 4)!

Figure 4: Pushing the mag up the hill.

The collected three 40x40m blocks and the first 10m of the fourth before the rain drove us all away.  Figure 5 shows the GPR blocks from the last two days (left) and last year (right).  Figure 6 shows the mag data from the same area.

Figure 5: the GPR data.

Figure 6: the mag data in the same area as the GPR data in Figure 5.

One of the obvious things in both data sets is the “1955 ditch” running across the survey area north-south.  There are some subtle things happening too.  For example, in Figure 5 there is a strong blob in the ditch which I have marked with a red arrow.  In Figure six we can see a circular feature in the mag data in the same place.  I’m not sure what it is (yet), but it is one of a thousand details we can see when comparing the two data sets.

Tomorrow is forecast to be very windy.  Let’s hope we don’t end-up gently flying over North Herts!

 

Partial madness

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

Apologies for not posting the results from yesterday’s survey.  Prae Wood field is an odd shape, leading to a plethora of partial grids for the mag, and the consequent head-scratching as to how it all fits together back at base last night.

Starting with the mag data.  The team have had two days of multiple partial grids, many of which have to be split into several smaller blocks in order to be surveyed.  At times, the sea of flags looked a bit like bunting at a village fete.  The team, however, have made excellent progress and we should finish the field in a few days (weather willing).  Figure 1 shows the results for the whole field.  Mainly, we have found pipelines (the black-and-white linear features) and clearly something big happened in the field near the gate at the south edge (an old building, perhaps?).  Traces of more interesting archaeology are hard to come by, and I have pointed out one with the red arrow.  Slim pickings in this big field, but important to see the empty spaces.

Figure 1: the mag survey in Prae Wood field after day 7 (day 3 of mag survey).

The Earth Resistance survey took a back seat on Wednesday with Graeme and I completing a token single square after lunch.  Today, we did not even try to do any more, the ground is so dry and hard.  Tomorrow the frame is off being repaired.  The single square we did manage showed nothing at all (Figure 2, top left square).  It lies in the valley between the buildings that line streets 23 (with all the buildings that can be seen Figure 2) and street 25 which we have not yet surveyed with the res.

Figure 2: Earth Resistance survey after day 6.

The GPR team, completed a block 40 x 80 at half meter transects yesterday.  As they did not find any buildings, and generally the evidence seems to suggest that the NW side of the town is free of them, we swapped to 1m transect spacings today enabling the team to cover an area 80 x 80m.  We would very much like to complete the GPR of this field this season.  This afternoon the team was joined by Sandy Walkington, President of the Arc and Arc (Figure 3).

Figure 3: Sandy Walkington (SAHAAS) operates the Mala GPR.

Figure 4 shows the 40 x 160m block surveyed over the last three survey days (on the left) next to some blocks surveyed last year.

Figure 4: the GPR data from the last three days of survey.

Although nothing stunning jumps out, unlike the buildings to the east found last year, there are some things to note.  There are clearly some linear features showing in the NW block of data.  Figure 5 shows the mag data from the same area.

Figure 5: the mag data from the same area as Figure 4.

The large dark linear feature that turns a right angle in Figure 5 is our old friend, the 1955 ditch, the late 1st century boundary of the town (Fig. 6, red arrows). There is a second, much fainter, linear feature running parallel to the ditch, that clearly must be related to it (Fig 6, blue arrows).  Note the three faint circular features (Fig. 6, green arrows).

Figure 6: Figure 5 with colourful arrows (see text).

Looking back at the GPR data (Figure 7) we can see all these features reflected in the GPR data, albeit subtly.

Figure 7: GPR data with arrows (cf. Fig 6 and text).

The three circular things would appear to be depressions or pits into which slight more magnetic topsoil has collected.  These is going to be much correlating of features between surveys on the horizon!

Finally, the St Albans and Hertfordshire Architectural and Archaeological Society (aka the “Arc and Arc”), the oldest archaeological society in Hertfordshire will celebrate its 175th anniversary next year.  They have recently launched an updated website.  John Dent, Arc and Arc member and CAGG volunteer right from the beginning, has his 15 minutes of fame on the front page (Figure 8), pushing the GPR at Gorhambury in the first season of our survey in 2015.  Go John!

Figure 8: The Arc and Arc’s new front page featuring John Dent and the GPR.

Tomorrow may, or may not be a bust.  The weather forecast has heavy rain over night but dry during our working day.  What the reality will be, who knows!

Congratulations everybody for some excellent surveying.