Tag Archives: magnetometry survey

Watling Street

Firstly, apologies to those who turned-up and found themselves unemployed. With ground conditions difficult for the resistance meter, we just couldn’t run all three machines. Thank you for being so understanding.  Looks like it will rain most of tomorrow, so hopefully we can press-on with the resistance survey on Saturday.

The mag survey in the second field, which I am going to call the macellum field for convenience (it isn’t the field’s real name) is going well.  Firstly, an image of the whole town:

The mag survey of Verulamium.

The mag survey of Verulamium.

My long-held dream of having a complete survey of Verulamium is getting close to becoming a reality!  Thank you everyone.

Looking at the new area in more detail:

The area surveyed on days 29 and 30 in the macellum field.

The area surveyed on days 29 and 30 in the macellum field.

Hopefully, everyone can see the clear linear feature running from the SE to the NW.  This is Watling Street, the main road from London to Chester.  As one might expect just near to the gate (which was robbed and then excavated, and lies in the trees), there appear to be lots of little buildings along the road which show as white lines against the mid-grey background. What is more curious is the rather different look of that background: much less even and more noisy. I’m not sure why, yet.  The field certainly feels different: flatter, slightly different vegetation, and is obviously closer to the river.  Perhaps we have moved from chalk to the river gravels?  I must check the geology map…

The GPR has been doing lots of bitty blocks around the edge of last year’s survey area.  Rather than hold-up the posting of the mag data, I thought I would do the GPR later when I have dropped all the different areas on to Google Earth.

Thanks to everyone who helped today, especially Graeme for acting as my driver and go-fer as I limped around the field  (don’t ask!), and Ruth for helping transport all the equipment.

 

Done it!

The mag team have completed the Theatre Field. Yay!

The mag survey of the theatre field, complete.

The mag survey of the theatre field, complete.

Congratulations everybody.  It is quite an achievement.

A detail of the area surveyed on day 29.

A detail of the area surveyed on day 29.

The detail above shows that the very north end of this field is quite busy.  This is unsurprising being right next to the Chester Gate.  The sinuous ditch, or should we just call it the aqueduct?, leaves the town about 50m away from the gate.  Unfortunately, it goes straight into woodland. Hopefully, we may get a chance one day to look for it the other side of the wall.  Rosalind Niblett traced what she thinks is the aqueduct from aerial photographs all the way the valley to Redbourne.

The mag team also started in the next field, but I’ll report on that when there is a little more completed.  The res team struggled with the hard, dry ground conditions and managed another two grid squares.  We will have to stop doing res until we have had some rain.  There is some forecast tomorrow.  The GPR team also completed two areas.  I wanted to post the completion of the field, so I’ll report on everything else this evening.

The mystery deepens

We managed to run all three machines today, although we were short handed on the resistance meter so many thanks to Peter and Ellen for soldiering-on. The unfortunate side-effect of the lovely weather is the top surface of the field is now like concrete!

First of all, the resistance results.

Day 27 resistance results.

Day 27 resistance results.

Day 27 resistance results, high pass filtered.

Day 27 resistance results, high pass filtered.

Not much new showing today, but we are getting a little more of the temple.  Tomorrow’s grids should show quite a bit more, if we can get the probes in the ground!

The GPR has, for the last few days, been working its way along the northern edge of the field in a series of stepped blocks.  With GPR it is not so easy to have a ragged edge.  Mike Smith, our expert GPR cart wrangler, felt sure they had found some stuff today.  Here are the last three days (the three blocks with jagged top edges) with today’s being the easternmost one.

GPR survey, days 25--27 (slices vary).

GPR survey, days 25–27 (slices vary).

Timeslice 3 (12.5 to 15.5ns) is usually a good starting place at this site.  Not much showing in that bottom block.  How about the next slice down?

GPR survey, days 25 to 27. Day 27 (the easternmost block with a jagged north edge), slice 4 (15.5 to 18.5 ns).

GPR survey, days 25 to 27. Day 27 (the easternmost block with a jagged north edge), slice 4 (15.5 to 18.5 ns).

I think Mike is right!  Excellent stuff.  Let us look a little closer at today’s block:

Detail showing day 27 results, 4th time slice (15.5 to 18.5 ns).

Detail showing day 27 results, 4th time slice (15.5 to 18.5 ns).

What is really interesting is not only do we have evidence of yet another building in the western side of today’s block, but the white lines surrounding darker areas on the eastern side suggest robbed walls of a large building.  Tomorrow’s results are going to be very interesting!

And to the topic of the TLA from the post earlier this evening.  Let us recap the story.  Last year we started picking-up a long ditch which ran right across the site.  It had been found in the two transects of mag undertaken by English Heritage in 2000, but at the time they were not linked. Last year we speculated that the ditch ran up the dry valley that the mag team have been laboring up and down the last few days.  On the 19th century maps there is a well marked on the other side of the Fosse field.  In the next image I have marked the line of the ditch.

The sinuous ditch.

The sinuous ditch.

Much to my surprise, the ditch curves around on itself at the western end, perhaps following the contours of the sides/bottom of the valley.  I will have to check that with the dGPS in the near future.  But where is it going?  Is it still an aqueduct as we thought, or something else entirely?

Looking more closely, we can see there are lots of other, smaller linear features in the area, some quite straight, and possibly even another building.  This top-corner of the field is proving very interesting and we will have to overlay the results on the topography to see what is happening.  This bit of the field is anything but flat!

Detail of the western end of the sinuous ditch.

Detail of the western end of the sinuous ditch.

Tomorrow we see the mag team edging closer to completing the Theatre Field, the GPR working along the northern edge towards the theatre and the the resistance meter covering more of the Insula XVI temple.

Almost round the bend

Today saw a slightly smaller team than we have had, but we still managed a good area of magnetometry survey and GPR, and even one small square of Earth Resistance survey.

First, the mag survey.  The team are starting to work their way north along the western edge of our survey area filling in between what we have already surveyed and the third century town wall which is hidden in the trees in the Google Earth image.

The magnetometry survey up to the end of day 24.

The magnetometry survey up to the end of day 24.

Looking at the area surveyed today in more detail, we can see the beginnings of the corner of the “1955 ditch”.

The area surveyed on day 24.

The area surveyed on day 24.

There is surprisingly little of anything much showing inside or outside the ditch in this corner.  The ditch is, however, slightly narrowing and bowing.  How curious!  Tomorrow should, fingers crossed, see us pick up the rest of the corner.

The GPR team, way down the hill near the drive, completed another 40x80m block.  Here are four 3ns thick time-slices.

Day 24 GPR, time slice 2 (9.5 to 12.5ns).

Day 24 GPR, time slice 2 (9.5 to 12.5ns).

Day 24 GPR, time slice 3 (12.5 to 15.5ns).

Day 24 GPR, time slice 3 (12.5 to 15.5ns).

Day 24 GPR, time slice 4 (15.5 to 18.5ns).

Day 24 GPR, time slice 4 (15.5 to 18.5ns).

Day 24 GPR, time slice 5 (18.5 to 21.5ns).

Day 24 GPR, time slice 5 (18.5 to 21.5ns).

The top image (time slice 2) just shows the noise in the ploughsoil.  The second image, however, shows a lovely little building 16.5m by 10m in size, aligned with the “1955 ditch”.  I’m not certain what this building is, it seems an unusual plan for a domestic structure.  The third image (time slice 4) shows this building, but also a very strong reflection from a wall on the eastern side.  Presumably this is part of a building which has been robbed out more thoroughly.  In the last time slice the signal has “attenuated” and we are only getting the strongest reflections showing.

The overall image gives some idea of how much we have now covered.

The area covered by the GPR at the end of day 24.

The area covered by the GPR at the end of day 24.

This image is a bit of a mismatched mishmash as the data was collected at different times and the time slices are somewhat variable as I have learnt to process the data over the last year.  At some point all the data will need to be reworked systematically, but that is beyond me while we are out collecting yet more data every day!

Many thanks to everyone who helped, and welcome to the people who have recently joined the team.  Your efforts are producing spectacular results.

 

Apse mad

We had a very successful day. It was grey and a little drizzly first thing, but true to the forecast it warmed up and turned into a gloriously sunny, if a little windy, day.  The mag team got right into the deep south and have made the mag plot look very tidy.

The magnetometry survey at the end of day 23.

The magnetometry survey at the end of day 23.

The team were determined to finish off the last few partial grid squares.  I think they were fed up walking the 650m from the cars!  The next swathe of grid squares will head north completing the Theatre Field at last.  Let’s look at the area surveyed today.

The area surveyed on day 23.

The area surveyed on day 23.

The band of buildings running from the NE seems to meet the 1955 ditch at a point where the magnetic response from the ditch changes markedly.  Perhaps the ditch was more deliberately filled-in here?  To the SW of the 1955 ditch there are fewer clear indications of stone buildings, but there are many linear features (probably ditches), and “blobs”, probably mainly pits but some of them are very large.  Of particular note is the small square enclosure right up against the town wall towards the south (seen as a black square).  One wonders what this was so close to the third century town wall.

At the other end of the field, the GPR crew managed another 80x40m block of data.  Apparently, nothing much was showing.

The day 23 GPR block (fourth slice, 15.5 to 18.5 ns).

The day 23 GPR block (fourth slice, 15.5 to 18.5 ns).

I beg to disagree!  Another small apsidal building is showing in the data, overlying the line of pits.  There is a large, rectangular, building to the west of it.  Are apsidal buildings the geophysicist’s version of busses?  One interesting aspect of this building is that it does not show in the mag data at all.

The same area as the previous image showing the magnetometry data.

The same area as the previous image showing the magnetometry data.

This is a great example of why undertaking both the GPR and the magnetometry surveys is so useful in giving a fuller picture of the town.  The GPR surveys are getting quite extensive!

All the areas surveyed with the GPR on the Gorhambury side of the town.

All the areas surveyed with the GPR on the Gorhambury side of the town.

The earth resistance team completed another excellent five 20x20m squares.

The earth resistance survey after day 23.

The earth resistance survey after day 23.

We can see some more buildings and bits of road, although there are some very high resistance areas.  One weakness of resistance survey is that there can be underlying variations in the data related to factors such as slope or geology which mask the archaeological patterning.  We can, however, apply a “high pass filter” which attempts to remove the underlying background trend and show the sharper differences more clearly.

The earth resistance survey, high pass filtered.

The earth resistance survey, high pass filtered.

The technique can create some artefacts in the data, but is very useful for bringing out some of the buildings.

Tomorrow and Tuesday are our “weekend”.  Many thanks for everyone who joined in during the first week, and here is to the next three!

Yet more 1955

Today was very warm and sunny with white poofy clouds and a gentle breeze. Ideal for sitting in the garden with a large jug of Pimms and a good novel. We, however, were out on site pushing a mag or GPR, or prodding the ground with the resistance meter. It was, however, worth it!  Many thanks to everyone who turned out on such a hot and sunny day.

The mag survey went well with Jim, Dave and Ellen completing eight whole squares and re-doing part of one where the sensors had frozen on Thursday.  This image shows the whole survey of the town, so far.

The overall magnetometer survey at the end of day 22.

The overall magnetometer survey at the end of day 22.

There are times when the “theatre field” feels so huge that we will never finish.  The mag, however, is getting there.  Another day or two and we should complete that swathe, and then we can start working our way north once more until we have finished the whole of that field.

Looking at the area that was surveyed today, the most striking feature is the infamous 1955 ditch.

Detail showing the area surveyed on day 22.

Detail showing the area surveyed on day 22.

The ditch shows clearly as the broad black line with white edges running across the plot.  At right angles to the ditch is a road with buildings (shown as low-magnetic white lines).  Outside the ditch to the south-west are several smaller ditches (shown as thinner black lines) as well as some probable pits (the black blobs).  As we saw in the park, the 1955 ditch seems to mark a clear divide in the town between the busy built-up area inside it and the quieter area outside it, despite the fact that the ditch is supposed to go out of use 250 years before the “end” of the town.

The earth resistance survey continued eastwards and picked-up the building seen in the GPR data last year,  Remember than in our resistance plots the walls show as darker, high resistance, lines whereas in the mag they show as white lines.

The earth resistance results after day 4.

The earth resistance results after day 4.

It is interesting to compare the resistance results to the GPR data of the same area.

The resistance results overlain on the GPR results.

The resistance results overlain on the GPR results.

The same area as the previous image showing just the GPR results.

The same area as the previous image showing just the GPR results.

Last but not least the GPR results.  The team completed another 80x40m block.  There is surprisingly little that is very obvious in the time slices, but the line of pits does show as an area of low reflections (white) and there are hints of robbed walls.  Most fascinating, however, is a small apsidal building about 15m long and 5.5m wide.  Here are the fourth and fifth time-slices.

The fourth time slice (15.5 to 18.5 ns) showing the apsidal building.

The fourth time slice (15.5 to 18.5 ns) showing the apsidal building.

The fifth time-slice (15.5 to 18.5 ns).

The fifth time-slice (15.5 to 18.5 ns).

Tomorrow will see us out in the field for the last day of the first week.  Monday and Tuesday is our “weekend”.

 

Bank Holiday Monday

The August Bank Holiday lived up to its reputation.  It rained, pretty much all day.  According to our rain gauge we had another centimeter and the roads were all flooded once more.  So, folks, that it is for Gorhambury for the next 11 months.  Plenty to do, however, with data processing, analysis and interpretation.

Today Ellen and I went back to site to collect the last remaining pegs.  Ellen organised us into walking back and forth across the field systematically to see if we could find my missing survey book.  On our second pass…

A very damp notebook!

A very damp notebook!

It was very soggy, but thanks to being a “rite-in-the-rain” product it looks OK and is now drying in the airing cupboard.  Yay!  Inevitably, as we were walking around picking up pegs and finding the book, it started to rain and we were soaked by the time we left.

On Sunday, I had some fun making timelapse videos.  Here is a condensed version of surveying three squares.

I did a poor job of setting it up for the GPR, but here it is anyway:

I have quickly processed the GPR data from using the large 160mhz antenna. We surveyed a block 20x40m which went over the “sinuous ditch” in the hopes we might be able to see it more clearly. We have picked up the walls that we saw with the 450mhz antenna.  In the radargrams (the vertical slices which is how the data is collected) I think I can see the ditch… but I need to check with someone more experienced at reading them.  Here are the time slices.  Note that we used a 1m spacing between lines which leads to a cruder picture.

Time slices from a 20x40m block at 1m spacing using the 160mhz antenna.

Time slices from a 20x40m block at 1m spacing using the 160mhz antenna.

I thought it would be useful to show one radargram from the day 15/16 block.  This is line 1893 which was surveyed from north to south, i.e., from the top of a block (as I usually present them) to the bottom.  Here is a screen grab from “radexplorer” showing unprocessed data.

An unprocessed radargram of line 1893.

An unprocessed radargram of line 1893.

It is very difficult to see much.  This is because most of the signal is the loud “noise” near the surface.  The first thing to do is tell the software where ground level is, i.e., the start of the first ‘loud’ reflection shown as black band at the top of the radargram.  Then we need to apply ‘gain’.  This is simply amplifying the lower reflections which are much weaker than the ones near the surface.

Line 1893 after the ground level has been input and gaining (using the autogain function in radexplorer).

Line 1893 after the ground level has been input and gaining (using the autogain function in radexplorer).

We can see much more now, but there is still a great deal of banding.  The bands are, essentially, echos and can be removed with the “background removal” function.

The processed radargram of line 1893.

The processed radargram of line 1893.

There is a great deal going on in this line of data.  Note, however, how the vertical column of echos under where I have put the label “wall” starts down into the radargram where as the column of echos where I have put “ring” starts at the top.  The “ring” is a small incipient mushroom ring which I noticed as we pushed the GPR over it.

A small incipient mushroom ring.

A small incipient mushroom ring.

If we look at the upper time slices we can see the ring:

A time slice from near the surface showing line 1893. The dark blob near the centre of the line is the mushroom ring.

A time slice from near the surface showing line 1893. The dark blob near the centre of the line is the mushroom ring.

For the wall, however, we need to look at a deeper slice.

Slice 3, 14-18ns.

Slice 3, 14-18ns.

In this slice we can see a small rectangular building showing faintly in the data. This building explains some of the things we can see in the radargram.  Remember that north is to the left of this radargram.

The arc we can see at the bottom of the top slice starting at about 27m west and going to 34m west is the northern edge of a very big mushroom ring.

A large mushroom ring.

A large mushroom ring.

The interaction between the fungus and the grass is quite complex.  The rings are showing in the radargrams probably because they are retaining water.  This can make the grass grow lush, or in extreme circumstances kill the grass.

A mushroom ring killing the grass.

A mushroom ring killing the grass.

Some of the rings merge and become quite complex.

A complex mushroom ring.

A complex mushroom ring.

And some of them cause mad photographers to get to their knees.

yummy circle

Yet another mushroom ring.

Yes, I became a bit obsessed.

Although we have finished for this season, there are lots of other things on the horizon, as well as working through all this great data.

I know I have said it before, but it is worth saying again.  Many thanks to everyone who contributed.  We collected great data, and we have found some really intriguing things.