Tag Archives: resistance

A picture is worth a 1000 words

The Verulamium magnetometry survey.

The Verulamium magnetometry survey.

I suppose I cannot really get away with that simple a post.

First of all, CONGRATULATIONS to everyone, it is a fantastic achievement and I am so proud of all of you.  Secondly, a big thanks to everyone who turned out for an extra day on Bank Holiday Monday to complete the Macellum field.

How about some numbers?  Well, Verulamium is the third biggest Roman town in Britain, after London and Cirencester.  It is, however, the largest Roman town in Britain which doesn’t have a modern settlement built over most of it.  We have surveyed 64.5ha of the total area of 81ha.  It has taken us 83 working days starting in the summer of 2013, but we didn’t do much at Verulamium in 2014.  It took 12,900,400 readings to cover those 64.5 ha.   That, of course, doesn’t include the grids we did twice because of frozen sensors or other problems. People pushing the cart walked about 322km, not including having to go back to the start for partials, getting to the squares in the first place, or laying in the tapes and strings.

Let us look in more detail at the last bit surveyed in the Macellum Field.

The area surveyed during day 37) high contrast).

The area surveyed during day 37) high contrast).

Several things come to mind.  Firstly, there is very little there!  Towards the NE and along the western side there may be a ditch feature, although it is quite faint.  Other than that, the main (and annoying) thing are the strong magnetic anomalies along the edge of the field.  Some of you may remember the 12″ gas main which runs across the Park… well here it is again.  What I do not entirely understand why there are differences between the negative and positive readings along our grid lines.  Jim and I spent some time making sure I put the composite together correctly, and we are sure it isn’t a survey error.

This end of the field is know to contain two Romano-Celtic temples.  These are known from aerial photographs taken in the hot summer of 1976.  I wonder if this area of the town was kept clear of encroaching buildings, pits, ditches and the like deliberately?  If we turn the contrast down (i.e., clip the image at +/- 40nT instead of +/- 7.5nT, we can see one of the temples close to the hedge as a faint white line.

Low contrast version of the area surveyed on day 37.

Low contrast version of the area surveyed on day 37.

Yet another target for the GPR next year!

Some of the team (many thanks Ellen, Mike and Jim!) helped re-do a number of areas of the res survey, plus one extra bonus square.  The biblical deluge of Sunday night (Lamer Lane was flooded once more) was not ideal.  This is the final area completed in 2016:

The 2016 resistance survey.

The 2016 resistance survey.

It is a pretty good result.  There is almost no use of the “edge match” feature of the software to get the various grids to join neatly.  It could be improved.  The very high contrast of the temple rather makes the buildings faint, but either the creation of selective composites (i.e., processing bit of the survey separately), or use of a high pass filter, would improve that.  The survey is quite big for a res survey: 2.5739ha according to TS (or 2.6ha to sane people who round numbers), which equals about 103,000 resistance readings.  That, of course, doesn’t include the large numbers of squares we re-did due to the dry conditions.

There is a great deal more to do in terms of data processing and interpretation, but I think we all deserve a well-earned rest.  Well, at least until Thursday…!

Three day catch-up

I haven’t managed a Verulamium post for a few days so here is a quick catch-up.

Firstly, the mag has been slowly working its way eastwards along the Macellum Field.  They are getting pretty close to the end of it.

The mag survey in the Macellum field after day 35.

The mag survey in the Macellum field after day 35.

As can be seen, Watling Street stands out very clearly running from near the theatre to the Chester Gate.  There are lots of buildings along the road as would be expected.  Some are less clear than one would hope because they have been partially excavated.  The spec-ally look to the data, almost certainly because of the gravel subsoil, does make it harder to see what is going on here.  The carrot at the end of the stick — apart from just finishing the field of course — is that there are two Romano-Celtic temples known from aerial photographs near the modern road.

The next image is just to show how much of Verulamium we have now completed.  Poster, anyone?

The complete survey so far.

The complete survey so far.

The resistance survey has had a few problems.  The lack of rain has made the top-surface of the field very dry and hard.  It is very slow going, and the data is not as clean as one would like.  Despite the problems, however, some of the buildings along the road, especially at the north side of the plot, are very clear indeed.

The resistance survey after day 35.

The resistance survey after day 35.

Although it doesn’t jump out at one when just looking at the plot, the sinuous ditch does show in the resistance data when one knows where to look!

The GPR team completed some blocks along the hedgeline which I haven’t processed yet… sorry!  They also did one block up next to the Chester Gate to investigate the building here, and one over the sinuous ditch.  The latter did show the ditch but very little else.  Let’s look at the block near the Chester Gate.

The mag survey near the Chester Gate.

The mag survey near the Chester Gate.

This first image shows the mag data.  The building in the middle shows as white lines roughly parallel to the modern drive.  There are lots of other darker features, probably various pits and the sinuous ditch shows to the west.

Day 35 GPR, slice 2 (9.5 to 12.5ns).

Day 35 GPR, slice 2 (9.5 to 12.5ns).

This time slice shows the impact of splitting the survey over two days!  The left hand side was done yesterday afternoon, the right hand side this morning.  The pattern of the ploughing and the tractor’s turning circle in the corner of the field show clearly.  Luckily, the problem is much less acute at lower depths.

Day 35 GPR, slice 3 (12.5 to 15.5ns).

Day 35 GPR, slice 3 (12.5 to 15.5ns).

This time slice now shows the building beautifully.  What a wonky end wall on the north side! There is a long narrow range of rooms to the SW.

Day 35 GPR, slice 4 (15.5 to 18.5ns).

Day 35 GPR, slice 4 (15.5 to 18.5ns).

This time slice shows the SW line of rooms more clearly, although at the southern end they are been partly destroyed.  We can see, however, fainter traces of the walls on the NE, a corridor, perhaps?

Day 35 GPR, slice 5 (18.5 to 21.5ns).

Day 35 GPR, slice 5 (18.5 to 21.5ns).

This time slice does show the “corridor” to the NE much more clearly.  Perhaps it is more deeply buried in the plough bank?   For most of the plot, though, nothing much else is showing.

There has been a little rain this evening.  I have my fingers crossed for more.  Hopefully, tomorrow will be dry so that the mag can plough on eastwards!

As always, many thanks to everyone, especially those working with the res meter.  It is slow and boring at the best of times, but slow+annoying is a great deal to ask.

Resistance isn’t futile after all.

Today was another successful day. We had a short rain shower just before lunch but other than that, all went well.  We ran all three machines with good results.  Firstly the mag survey.  The first image shows the western side of our survey.  The block of data we collected today is in the lower left corner of the image.

The western side of the magnetometer survey.

The western side of the magnetometer survey.

The most obvious feature is the big, bright black and white object.  That is a galvanized steel water trough!  Across the SW corner is a dark diagonal line.  That is another stretch of the 1955 ditch — the first century boundary of the town — which Martin Atkin traced with a very early magnetometer in 1959 and 1960.  It was also the first feature we found in Verulamium Park in 2013.  Running diagonally across the survey from NE to SW is a linear feature made up of many light and dark lines which cuts across the western end of the hedge line and goes into the unsurveyed  grid square.  This is the line of one of the street which has buildings and other features along its length.  If we zoom in we can see more details.

The area surveyed with the magnetometer on day 20.

The area surveyed with the magnetometer on day 20.

The area to the SE of the water trough has a number of buildings shown clearly by white lines (representing a low magnetic response) especially either side of the NE–SW running road.  To the west, however, is very quiet with very few features showing.  The unsurveyed square, which is first on the list for tomorrow, should be very busy with the road and more buildings.  The eagle eyed amongst you will have spotted some vertical lines of uniform grey.  Ugh.  Another sensor freeze.  We will need to redo about a third of that square tomorrow.

The resistance survey continued on and completed another four 20x20m squares at 0.5m spacing,  Pretty good going.

The resistance survey after day 20.

The resistance survey after day 20.

In the resistance survey plot, black represents high resistance and white low.  High resistance features are things like roads and walls which do not hold moisture.  Low resistance features are things like ditches and pits which do hold moisture.  Generally, I have found, the high resistance features are easier to spot and interpret on the plots.  In the case of the image above, the two roads show clearly, especially the one in the NW corner.  The buildings along the roads also show well, including some internal walls.  Interestingly, not much shows in the middle of the insula (a “block” in a modern town).  Are they yard areas behind the buildings?

The GPR team managed an 80x40m block of data, plotted to the west in the following image.

GPR data. The two squares to the west were completed on day 20. This is the 15.5 to 18.5ns time slice.

GPR data. The two squares to the west were completed on day 20. This is the 15.5 to 18.5ns time slice.

The GPR has picked up the buildings along the two roads very nice.  It is also pleasing to see the plots from this year join up with the plots from last year so well.  Using a high accuracy differential GPS makes this quite easy to do!

In some areas we now have three data sets over the same area.

The earth resistance data overlain on the GPR data.

The earth resistance data overlain on the GPR data.

The resistance, magnetometry and GPR data all overlain.

The resistance, magnetometry and GPR data all overlain.

Obviously, combining all this data is going to be a challenge.  Looking at a few specific details is fascinating and informative, but how does one do this on a large scale?  There are techniques called “data fusion” that I am going to have to look into.

Again, many thanks to all the team who turned out today, and watch out for tomorrow’s exciting installment!

At 1pm precisely…

The Met Office predicted rain at 1pm, and at 1pm it began to rain.  We had, however, managed to do four mag grids before lunch, and finished off another one and a res grid in the drizzle.  Many thanks to everyone who turned out on a unpromising morning and helped us complete a fair bit in the time we had.

The bad news first.  The Chess Valley Archaeological and Historical Society were kind enough to lend me their resistance meter and cable to try and work out what is wrong with ours.  Unfortunately, it looks like it might be the wiring on the resistance frame as the results we got today were utter nonsense.  Many thanks for John Glover for lending me the equipment and Jim West for bringing it over.

The five mag grids were, however, extremely interesting.  First the overall image:

The mag survey after Day 14.

The mag survey after Day 14.

Weather permitting, we should be up by the drive by the end of tomorrow, and hopefully will have picked up more of the ‘sinuous ditch.’ Zooming into the area we did today reveals some interesting features.

Detail showing the area surveyed on day 14.

Detail showing the area surveyed on day 14.

An annotated version of the previous image.

An annotated version of the previous image.

We have picked up the cross-roads between streets 11 and 25 very nicely.  Along street 25 there seems to be a line of smaller buildings.  More modest dwellings, perhaps, or maybe workshops and shops?  On the north corner, however, is a weird looking and extremely magnetic feature, probably a building as it seems too rectangular to be something else, and is approximately 20m by 10m in size.  Perhaps the building burnt down, or maybe it is an industrial feature?

Tomorrow we should be able to survey the grid next to our first “uber magnetic” building as well as picking up, we hope, some more of the infamous sinuous ditch.

We have also made the news on the Institute of Archaeology‘s website!

Cornered!

Having lost a day on Thursday the team ventured out in misty damp weather to continue the survey.  It was a lovely day… for mid-November.  The English summer!

‘Partials’, that is grid squares which have bits missing around the edges of a survey area, are always fiddly to survey.  A partial which has no top or bottom line are especially fiddly. Today the mag had almost nothing but partials to survey in the corner between the lane and the theatre.  They managed, however, and were rewarded with two whole squares… up a slope!  Today (Saturday) they have three more partials, and then a swathe of open country.

The results after day 4.

The results after day 4.

Some more buildings can be seen in that corner, although they are hard to see, probably due to the build-up of colluvium caused by soil-creep down the hill building up against the hedgerow.

The sinuous ditch appears to continue, although we need the grid-square in the angle to confirm this.  I realised, just after the last post, that this feature was originally found by the survey undertaken by English Heritage in 2000.  As to what it is, that is going to be the subject of some debate.

We have also hit an area of very high contrast in the westernmost grid square.  Probably another building, but why so magnetic? It will be good to see the square to the south.

The GPR managed another two grids, over the corner of the temple in Insula XVI and then on over the line of buildings.  I’ll try and look at those results in more detail soon.  The resistance meter, with two new plugs, was working better for one square, but went back to being stripy for a second square.  Very annoying as even through the duff data one can see it is something worth doing.

Luckily, the weather forecast is better today.  We were all a bit damp by the time we went home.  Thanks team for soldiering on even in bad conditions.

Technical breakdowns…

Tuesday saw a good-sized team out on site.  Having managed to get a charger for the GPR (thanks Ralph!), the resistance meter was still playing up and we had some problems with the mag initially.  A group from UCL came to use the GPS, and even that didn’t work!  We managed, however, to at least identify the problems and resolve some of them

The mag survey managed another six grid squares despite the late start and got some excellent results.

Results from the second day.

Results from the second day.

The line of buildings we saw in the first day’s survey continues nicely, and in the SW corner we have clipped the SE-NW road.  Also, a little but more of the temple is starting to show.

The resistance meter has some problems, but the two squares we have done do show details of the temple very nicely so once it is fixed, it will be well worth continuing that survey, as slow as it is.

Just a quick posting today as we are about to start day 3!

Datchworth Church

Something a little different this time. The CAGG team were asked a favour by the Parochial Church Council at Datchworth.  Basically, they are running out of space in the graveyard.  A local farmer has offered them some land to extend the cemetery, but because of the possibility of archaeological remains in the area they need to have an assessment done before they can extend the churchyard.  Some ten years or so ago, the Welwyn Archaeological Society did some limited excavations in the area prior to the building of their church hall, and so we were approached to see if we could do a geophysical survey.  The area which will be affected is quite small, so we did the mag survey of the entire field over a day and a half, during which time we managed a resistance survey at 0.5m spacing of the key area.

The resistance survey at Datchworth underway, 19/4/2014.

The resistance survey at Datchworth underway, 19/4/2014.

We undertook the survey over two days, the 13th and 19th April 2014.  The mag survey was finished by lunchtime of the second day and would have been even quicker if it wasn’t for the awkward shape of the field — resulting in every grid (bar one) being a partial.

Results of the magnetometry survey.

Results of the magnetometry survey.

The mag survey shows… not a great deal.  The important area to the north of the current churchyard shows nothing much at all.  To the east of the church there are some potential features, but the most likely area is the northern part of the survey.  This spot also has lots of lumps and bumps and the farmer thinks there may have been a building there.  The clarity of the survey is badly impacted by the large quantities of farm machinery parked there and a farm shed.

Results of the resistance survey.

Results of the resistance survey.

The resistance survey was much slower.  When taking readings every 0.5m, it takes a day to cover the area the mag completes in about 40 minutes!  The high resistance feature (shown in black) that runs along the western edge of the survey, is a bank running parallel to the ditch that runs along the road.  The other features are not visible on the surface.  High resistance features are usually things like roads, paths, walls and sold floors.  In this case, we appear to have a linear feature running N-S to the west of our survey area as well as a second one near the “moat” at the top.  I need to ask around a bit more before committing myself to a particular interpretation, but they are certainly too wide for a wall.