Tag Archives: magnetometry survey

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.

 

Advertisements

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.

Saving the best ’til last?

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

Unfortunately the weather forecast was accurate yesterday.  Far too much rain for us to go out and play.  Today, however, was very pleasant and at least the pegs went in very easily for a change! I’m afraid that a new toy (Figure 1) resulted in me getting a late start processing data this evening, and so I only have the mag and res to report on.  The GPR team, however, did a sterling job and completed a multitude to sawtooth-edged grids along the western side of the theatre field.  I promise I’ll process all that tomorrow!

Figure 1: Kris’ new multiformat pinhole camera being put through its paces.

The Earth Resistance meter was operated by John and Grahame. We have moved north of the hedge line and are close to the cross-roads around which both mag and GPR have shown multiple buildings.  Figure 2 shows the entire res survey, and Figure 3 a zoomed-in view of today’s area.

Figure 2: the Earth Resistance survey so far.

Figure 3: the area surveyed today and some context.

We have just clipped the edge of Street 11 (in the top-right corner of the new block) and have revealed some wonderfully clear images of the buildings to the south of that street.  These are clearer than the GPR survey of the same area, so it is really pleasing to see.  Excellent stuff!

Over in Prae Wood field, the mag team completed two more strips of grid squares.  Figure 4 shows the survey so far.

Figure 4: the Prae Wood Field survey, so far.

Two more days should, weather and equipment willing, see the team finish the field.  After Prae Wood we are heading north into Church Meadow, so-called because St Mary du Pré lies within that field.  Watling Street also runs through it so we should get some exciting results.

But, back to Prae Wood.  Let us look in more detail at the western end of the survey (Figure 5).

Figure 5: the western side of the mag survey of Prae Wood field.

The dark lines in the new area represent ditches, and it looks like we have some enclosures running across this western end of the field.  Given that the major Iron Age settlement lies in the wood just to the SW, these could be of that date, but they could be Roman or medieval as well.  Once more, we need to check out the historic map data.  Still, after many blank grid we now have features in both the western and eastern extensions of this field.  Saving the best for last?

Many thanks to everyone who turned out today.  Tomorrow looks like we might have to pack-up early due to yet more rain. This season is just flying by.

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.

 

 

 

 

Fighting the wind

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

The weather forecast was for a windy day, and it was correct! At one point, the wind was making the flag on the cathedral look as stiff as a board, but the sun made the the yellow cross glow. Even from the far side of the Roman town it was quite striking.

Logistics led to an unusual format for the day.  Jim, Ellen, Pauline and Dave continued to expand the mag survey in Prae Wood Field.  Meanwhile, Kris, Mike, Anne and Julia firstly finished yesterday’s block of GPR data, curtailed because of rain, and the went on to complete six Earth Resistance survey blocks.

Figure 1 shows the whole of the mag survey in Prae Wood field, and Figure 2 a detail of the eastern end.

Figure 1: Mag survey after day 9.

Figure 2: Eastern end of the mag survey after day 2.

You may well ask why we have a funny diagonal edge to the survey at the eastern end.  This is because there is an electric fence creating a paddock for horses.  As we have gone a little further than we intended, we will just take what we can get.  The big new find is a ditch running across the end of the field.  I have marked this in Figure 3 with red arrows.

Figure 3: Mag survey with arrows (see text).

We have no way of knowing what date this feature is.  The first thing I will have to do is check the historic maps.  It does, however, look like much more than a field boundary.  It is 2 to 3.5m wide.  The blue arrow in Figure 3 indicates a much slighter feature than runs at a right-angle to the big ditch.  Just to the west of the ditch is a strong magnetic feature that I have marked with a yellow arrow.  The form of the feature (a bigger blob next to a smaller blob) is reminiscent of the pottery kilns we have found on the south side of the town.  The magnetic values (c. -8nT to +130nT) is also in the right sort of range for pottery kilns.  Some work I have published previously shows kilns with a maximum range of about -27nT to +180nT.  It is typical that the area where we have started finding potentially interesting features is also where there is the most modern interference from services and so on.

The GPR survey just completed the block left over from yesterday.  A quick look at the data showed no surprises.  The good news, however, is we are just one day’s worth of 1m transects from joining-up with the survey to the north.  The survey is shown in Figure 4.

Figure 4: the GPR survey after day 9. The red box marks the block completed on days 8 and 9.

The bare strip to the west of the GPR data in Figure 4 is what is left to be surveyed. That to the east has been done (I just haven’t loaded them onto the GE image).  The eagle-eyed amongst you may have noticed that the GPR blocks do not quite match-up as well as they used to.  This is a bit complicated but is basically because the OS have up-dated their guidelines for converting OS National Grid coordinates to lat and long.  I work in the National Grid, but Google Earth works in lat/long.  I’ve changed to using the OS’s official conversion webpages, partly because I can up-load them in bulk saving me a great deal of cutting-and-pasting.  My crude use of GE to display the results, however, involves dozens, if not hundreds of image over-lays.  As a result, I have a great deal of work to do to update all the slices from all the GPR blocks to the new coordinate conversions.  Arrgghhh.  The joys of doing a survey over five seasons.

The entire Earth Resistance survey is shown in Figure 5.

Figure 5: The entire Earth Resistance survey after day 9.

As can be seen, we have covered an impressive area now, about 9ha in total.  The edges in the image are the different years which have been processed slightly differently and crudely put together in Google Earth.  I will be joining them all together soon and trying to make a more seamless image.  Figure 6 shows the western 2019 block.

Figure 6: the 2019 western block of res data (the lighter strip to the west).

Most of what we managed today is the open dry valley between the buildings on on Street 23 (seen in the latest data) and Street 25 which we have yet to survey.  Tomorrow, however, we will start hitting the buildings on the latter street, and especially the cluster of buildings which lie on the junction of Streets 25 and 11 on the western corner of Insula XXXI.  In today’s data (the upper six blocks on the 2019 strip), we picked up the western half of a large building which lies back from the road with a wing running up to the road with a sequence of smaller rooms.  Fascinating stuff.

Tomorrow we are going to start off with GPR and Earth Resistance giving me time to lay-in more grids for the ever-efficient mag team.

Many thanks to everyone who braved the gales today.  We got some great results despite the weather.

 

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.