Tag Archives: Mala

“The way I see it, if you want the rainbow, you gotta put up with the rain.”

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

As I start this entry of the blog, the rain is splashing against my windows as was predicted by the Met Office. Although we might question Dolly Parton’s grammar, the sentiment seems true enough.  Yesterday, however, was a superb day with all three techniques collecting data across the site.

After yesterday’s excellent results, the GPR crew had great expectations.  The only problem was a tree in the way under which the shepherdess had put hay when the grass in the field was dead from lack of rain.  Unfortunately, sheep mean sheep droppings (Figures 1 and 2).

Figure 1: Mike on sheep poo removal duty.

Figure 2: Check out those wheels!

Luckily for everyone concerned, I think the effort was worth it (see Figure 3)!

Figure 3: GPR time slices from Day 18.

I could misquote Dolly along the lines of putting-up with sheep poo if you want excellent GPR results but I might be pushing my luck…  The many buildings are quite obvious in this data set.

Figure 4 shows this grid in context of the other GPR grids in this area.

Figure 4: GPR results including the day 18 data (SW corner).

We have added a very large number of new buildings to the map of Verulamium.  As I was only just starting with GPR data when we started collecting it in 2015, the processing keeps changing a bit from block to block. One of my jobs is to start from scratch and reprocess the whole thing so that the maps are consistent.  Should keep me busy for a while.  Figure 5 is a crude mosaic of images just showing the entire area surveyed so far.

Figure 5: crude mosaic of GPR time slices at the end of the 2018 season.

This represents 19ha of GPR data collected at 0.5m transect intervals.  Just pushing the machine along the lines, not including getting to the block, setting-up, moving strings etc. is 380km.  It also means 380km of radargrams!  No wonder the data takes-up 33gb of my hard disk and consists of over 70,000 files.

The mag team completed nine 40x40m grid squares which is 1.44 hectares.  Excellent progress!

Figure 6: the mag team in the southern field.

Figure 7 shows the whole of the 2018 survey (along with a big chunk of Verulamium).

Figure 7: the mag survey after day 18.

Even though we have been using the machine for some years now, and it does have its frustrations, when all is going well we can really cover some ground.  The season was planned for 20 days: we lost 3 days to rain, and most of a day to testing the mag at the start.  Despite this, the team have managed to collect 17.7 hectares of mag data.  Without actually getting to the grids and back (which is quite a bit of walking in itself), the team have pushed the cart 88.5km over the past four weeks.

Figure 8 shows the southern area in more detail.

Figure 8: the southern area of mag data after day 18.

The blue arrows in Figure 8 indicate the lines of old field boundaries.  These can be seen on old maps such as the 1699 parish map.  The yellow arrows mark ferrous objects.  Some are very big, but there are a scatter of smaller ones too.  Last, but definitely not least, there are a few magnetic features which may be archaeological, such as pits.  I have picked a few out with red arrows.  Although they look small at this scale, they are probably 1m to 2m across, a quite respectable size for a pit.

Although large mainly  blank areas are disappointing to collect, they are important nonetheless. The immediate environs of Verulamium are extremely rich, archaeologically. The field lies:

  • 360m W of the busy area of buildings recorded by the GPR discussed above;
  • 600m NE of the major Iron Age settlement at Prae Wood;
  • 600m N of the fields at Windridge Farm where metal detecting rallies have taken place;
  • 500m NW of the major cemetery at King Harry Lane;
  • 1,100m SE of Gorhambury Roman villa;
  • 1,000m NE of the new villa found at Windridge Farm.

Also, the Fosse, which is preserved in the woodland along the NE edge of the field, is a really very impressive earthwork.   We just seem to have hit an empty bit of landscape between all these sites!

The res survey now covers some 6.58ha, that is about 263,200 earth resistance readings.  Not into the millions like the mag and GPR, but this is res after all!  Figure 9 shows the entire survey.

Figure 10: the entire Earth Resistance survey after day 18.

At this scale the roads show very nicely as do some of the more substantial buildings.  Figure 11 is the area surveyed in 2018.

Figure 11: Res survey after day 18.

Given that the fields were baked hard and the grass was dead at the start of the season, I am pleased we managed any Earth Resistance survey at all this season.  The team yesterday put-up with my geophysics OCD and completed right into the corner by the theatre. We then doubled-back and started filling-in between the top of the survey block and the drive.  We have picked-up some parts of buildings seen in grids to the south, but in general along the edge the deep colluvium, as shown by the sunken nature of the drive, is to some extent masking the archaeology.

Many thanks to everyone on the team who made the 2018 season such a success.  A especially big thanks to those who helped move the equipment about including Ellen, Mike, Jim and Ruth.

For those who haven’t been involved but would like to join future surveys, do get in touch.  We are a friendly group, and provide on-the-job training.

And finally… (as they used to say on the news)

 

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A busy day

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

We had a large team today and as a result we managed ten mag grids, two and a bit GPR grids and seven earth resistance grids. Good job everybody!

First to the mag.  The team extended their survey in the field to the south of Mobbs Hole.  Figure 1 shows the overall survey and Figure 2 zooms in on this field.

Figure 1: the 2018 magnetometry survey.

Figure 2: the survey in the field to the south after day 2.

I have annotated Figure 2.  The red arrows indicate the line of the ditch of the Fosse.  It is salutary to note that a feature as big as the Fosse barely shows in the mag data.  Clearly the upper fills of the ditch are largely the same soil as the surrounding topsoil.  We can normally see pits and ditches on archaeological sites because they are filled with more organic, and thus more magnetic, soils, the result of nearby human occupation.  The green arrow shows a “blob” of higher magnetic readings. The rather diffuse edges to this feature make me suspicious that this might be a “tree throw”, i.e., where a tree has blown down.  The yellow arrow marks two strongly magnetic parallel lines.  At first I thought these might be something metallic but checking their actual values shows they vary from -10 to +29 nT.  Certainly strong, but unlikely to be metal.  The blue indicates something which is definitely metal; it has values of -1543 to +680nT!  The dark pink arrows indicate a faint line, possibly an old fence line.

The res team consisting of Deborah, Tim, Julia and Anne completed seven squares.  Figure 3 shows the whole survey from 2016–2018.

Figure 3: the earth resistance survey 2016–2018.

We have now covered 6.3ha.  For a resistance survey at 0.5m spacing between readings, that is pretty impressive.  Res has always been a poor third to mag and GPR in this survey.  We didn’t get started until a year after the other techniques when UCL purchased a new RM85, and we have had problems with weather.  Hopefully we can fill in the top corner on Saturday.

Figure 4 shows a detail of the area completed this year.

Figure 4: the northern area completed so far this season.

The street shows very clearly in Figure 4 running SW-NE, and slightly more faintly we can see the buildings either side. One problem to tackle in processing data is that very high areas, like the road, can make the more subtle stuff harder to see.  If we “clip” the image to bring-up the details of the buildings, the road area becomes one big black blob!  One way to get around this is to use a high-pass filter.  Figure 5 shows the same area with the high-pass filter applied.

Figure 5: the 2018 survey area after the application of a high-pass filter.

As you can see, the buildings show much more clearly but the road much less so.  Especially with resistance data, it is worth looking at several versions of the data processing to get the most detail from the survey.

The GPR crew finished off the grid from yesterday and did another 40x80m block.  Figures 6 and 7 are the time slices from the two days.

Figure 6: time slices from day 16 of the GPR survey.

Figure 7: time slices from day 17 of the GPR survey.

As you can easy see, we have some sweet buildings showing.  Figure 8 is a rough composite of the sites in this area.

Figure 8: composite of slices in the area of the day 16 and 17 survey blocks.

I need to do some cleaning-up of the various blocks as they were processed at different times and with different software packages, but in general you can see the mass of buildings crowding along this section of road.  Very nice!

Signing off now so we can go and start day 18.  This may be our last day as the weather forecast for Sunday is dire…

 

Seven red kites, two fire engines and a microlight

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

It was an eventful day. At lunch seven red kites descended on some tasty tit-bit not far from where we were sitting, and in the afternoon two fire engines drove up the drive and we were “buzzed” by someone in a microlight. None of this has anything to do with the geophysics, however!

The mag team completed the last two grids in Mobbs Hole (for now), and have started on the field to the south.  The first six grids were all wheel-spinning partials too.  They have, however, only one partial left and then there are eleven whole grids laid-out and waiting.  Partials are not the Foerster’s strong point.  The lack of an “end line” function means hours are wasted spinning the wheel to fool the odometer into thinking we have completed the line.  Open fields, however, are its strength and the team will be glad to be out in the wilds again.  Figures 1 and 2 show the results from Mobbs Hole.

Figure 1: the Mobbs Hole survey in its entirety.

Figure 2: the southern area completed today and the start of the next field.

The GPR team had a partial around the water trough this afternoon and so they didn’t quite complete their usual 80x40m block (I knew I should have kept quiet yesterday).  The next two figures are nine time slices of the western and eastern halves of the block.

Figure 3: time slices from the day 16 GPR data, western block.

Figure 4: Day 16 GPR data, eastern block.

The western block seems to be yet-more blobby stuff, although with some very strong reflections.  The western block, however, has some clearly recognisable Roman-style corridor houses.  Yay! Finally some buildings we can recognise!

The last two images show slice 6 in context, firstly on the mag data, and then the mag data with an outline of the location of the GPR blocks.

Figure 5: GPR data from day 16, slice 6.

Figure 6: mag data with the location of the Day 16 GPR data indicated by the red box.

The huge black and white feature in the middle of the mag plot (Figure 6) is the water trough. As you can see, some of the walls of the buildings show in the mag data, but are much clearer in the GPR data.  Some only show in the GPR.  I know I am beginning to sound like a stuck record, but that is the strength of multi-method survey.

Tomorrow is our antepenultimate day (I had to get that in once again), so fingers crossed for dry weather.

Many thanks to the whole team for their wonderful effort and commitment.

The end is nigh?

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

In this case, two ends: we have just started the final week of the 2018 survey season and the mag team are within two partials of completing as much as we can of Mobbs Hole and moving into the field to the south.  First to the mag.

After the annoying plethora of frozen sensors, the mag team spent a good proportion of their day re-doing duff squares.  It was worth it, however, as today’s data looks fine (Figure 1).

Figure 1: the mag survey in Mobbs Hole at the end of Day 15.

Although we can be pleased with the area we have covered, surprisingly little apart from the Fosse itself and related features show.  We must keep in mind, as Isobel Thompson reminded me this morning, that “even such negative evidence is information”.  Negative information may be important, but at the end of a long day’s survey some tasty looking buildings would be nice.  Figure 2 shows one possibility, although we may be grasping at straws!

Figure 2: a possible building in Mobbs Hole?

The Earth Resistance survey takes fourth place in priority after surveying in pegs, mag and GPR.  Anne and I did, however, manage to extend the main block of res data by another three grids.  Figure 3 shows the results.

Figure 3: the Earth Resistance survey after day 15.

As you can see, we have picked-up some more of the building to the east, but also part of Street 25 running SW–NE.  There is quite a break in the line of the street which is curious.  Figure 4 shows the GPR data in this area.

Figure 4: the GPR data in the area of the res survey. The red box marks the outline of the 2018 survey after day 15.

It is useful to note that some parts of the buildings show more clearly in the res data, and some in the GPR thus making the extra effort of doing res as well worth while.  The GPR data also shows a break in the road.  Figure 5 shows the mag data.

Figure 5: the mag data. The red box shows the 2018 res survey area after day 15, and the blue line the course of the aqueduct.

Note how the buildings that show clearly in the res/GPR barely show in the mag data, but how the “burnt building” (assuming my interpretation is correct) only shows in the mag data.  Multiple techniques rule, OK?  I have roughly marked the line of the aqueduct in Figure 5.  Let’s now look at how that maps back onto the res data (Figure 6).

Figure 6: the Earth Resistance data with the line of the aqueduct indicated.

Not only does the aqueduct kink around the two buildings as we noted in an earlier post, but it goes through the break in the road.  I guess there could be a wooden bridge (which we would not detect) or maybe a culvert where the roof has collapsed or has been robbed. Fascinating stuff.

The GPR crew in their machine-like fashion completed yet another 80x40m block.  Figure 7 shows six time slices.

Figure 7: GPR survey, day 15, six time slices.

Most of the action, so to speak, is in the NE corner.  There is a particularly clear corner in the fourth time slice indicated with a red arrow (Figure 7, top-right slice).  This might be a surviving floor. There also appears to be a long linear negative feature, as shown in the fifth time slice by three red arrows.  Figures 8 and 9 show slices 4 and 5 in context with the day 14 data.

Figure 8: GPR data from days 14 and 15, slice 4.

Figure 9: GPR data from days 14 and 15, slice 5.

Three things caught my eye.  The squarish “floor” which crossed over the boundary between the two days data, the sub-circular white “blob” which also lies across the boundary, and the long linear low-reflection feature (shown in white) which runs diagonally SW–NE across the lower half. I traced the square and the blob and had a look at the mag data (Figure 10, click on it to see full-sized).

Figure 10: the mag data with the “square” and the “blob” outlined.

The white blob corresponds with a faint “blob” of higher readings in the mag data.  On its own, I would have been tempted to ignore this, but it does look like a feature about 6m across.  The square is harder to assess.  There are magnetic features parallel to it and close by.  We are probably looking at parts of a building.  I had a quick look at the radargrams and the square high-reflectance feature in the GPR data looks like a solid layer, probably a floor.  I also noticed the long linear ditch-like feature running across the mag data, so I traced that and went back to the GPR data (Figure 11).

Figure 11: GPR data with the linear feature seen in the mag data highlighted.

The linear feature in the mag data fits the linear feature in the GPR data perfectly.  Lovely result.

It was a busy day surveying today, and so I didn’t have time to goof off and take photos of people or the views.  Maybe tomorrow!

Thanks to everyone who helped today.

 

 

Is that a magnetometer on the horizon?

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

Figure 1: Ruth and the magnetometer on the horizon.

The mag team were in wheel spinning mode today. In other words, it was partial madness.  For logistical reasons I don’t have the mag this evening to download the data, so it’ll be a magfest on Sunday.

The GPR team completed its usual 80 x 40m block today, although the steep slope made it harder work than usual.  Figure 2 shows the time slices.

Figure 2: GPR time slices from day 12.

Although we don’t have any clear exciting buildings we have picked-up the line of two roads running at right angles.  We’ll get the junction tomorrow.  the NW–SE road shows best in slice 6 (Figure 2, second row, first slice), and the SW–NE road in slices 4 and 5.   Figures 3 and 4 show these two slices in context.

Figure 3: GPR time slice 4 with the line of Street 26 (as numbered by Niblett and Thompson) indicated.

Figure 4: GPR time slice 6 with the line of Street 11 (as numbered by Niblett and Thompson) indicated.

The 1955 ditch also seems to show in some slices, e.g., in Figure 3.  The western end of the aqueduct as seen in Wednesday’s slice clearly has a complicated relationship with the 1955 ditch.  At some point I need to look at the radargrams too.

Tomorrow’s block across the road junction will be interesting to see.  Junctions are usually prime locations for structures, but there is nothing much showing in the mag data.  Fingers crossed!

The next update will be on Sunday as Ellen and I will be at a family wedding tomorrow.

Figure 5: One of these things is not like the others.

 

Back to the fire

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

Week three started well with all three machines collecting data.  The Earth Resistance survey was the poor cousin as regards person-power but Ellen and I, helped by Rhian, completed three grid squares after lunch.

Figure 1: Ellen and an earth resistance meter.

The grids are over the fascinating burnt building seen in the mag.  Figure 2 shows the mag data in this area.

Figure 2: the mag data in the area of the res survey.

The black line snaking across Figure 2 from top-left to bottom-right is the aqueduct.  The very bright black-and-white area in the NW corner of that figure is probably a burnt building which was never replaced in stone.  Figure 3 shows the Earth Resistance data.

Figure 2: the Earth Resistance data.

Figure 3 is a crude composite of the data collected in 2016, 2017 and 2018.  The three squares at the north edge are this year’s grids.  We have clearly picked-up a long wall running NW-SE, and some square areas of higher resistance (?floors, maybe).  This survey makes an interesting comparison to the GPR in this area (Figure 4).

Figure 4: the GPR survey in the same area as Figures 1 and 2.

There is a lot of work to do tracing off walls and features from the three surveys.

The GPR crew completed another 80x40m block, although the slope was quite a challenge.  Figure 5 shows the time slices.

Figure 5: day 11 GPR time slices.

Slices 4 and 5 (top-right and middle left) seem the most interesting.  No stunningly clear buildings but lots going on.  Figures 6 and 7 compare the fifth slice with the mag data.

Figure 6: Fifth time slice from day 11 (indicated by the purple line).

Figure 7: the mag data in the area of the day 11 GPR data (shown by the purple box).

Notice how the square of higher mag response shown in darker tones towards the bottom of the purple rectangle are an area of light “low reflections” in the GPR data.  It is possibly something like an earth floor?  Off the west corner of that square in the mag data is a lighter coloured line running to the SW which is matched by a black line of high reflections in the GPR data.  That is clearly a wall, probably made of flint. The very narrow section of the aqueduct which runs east-west across the plot shows very clearly in the GPR data whereas the broader sections do not.  Something odd happens with the aqueduct at the eastern edge of the GPR plot. A  great deal more to tease out.

One problem we have had this year is the sheep.  In general they keep away from us.  The main issue is that some of them think the flags are tasty… (Figure 8).

Figure 8: Tasty! A nibbled flag in Mobbs Hole with the mag crew in the background.

The mag crew consisting of Jim West, Peter Alley and Dave Minty had three annoying partials to do before marching eastwards across the field.  I’m afraid I have not finished processing those annoying squares but I have added in the complete ones to Figure 9 so you can see progress.

Figure 9: mag data in Mobbs Hole after day 11.

Unfortunately, today was a bust as it rained 8.5mm.  The forecast for tomorrow is looking good though.

End of week two, part 2

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

Just a quick update as week 3 will be starting in about eleven hours and I’d like some sleep!

The GPR crew on day 10 completed three areas of “sawtooth”.  Well done all for putting up with such an annoying, fiddly job, but it does look good along the edge of the survey.  It took a bit of setting-up, processing-wise, but all was well.  Sadly, not much showing (Figure 1).

Figure 1: the GPR survey in the northern area after day 10.

Starting from tomorrow, the crew will be working their way slowly southwards, back up the hill.  The downside is the hill, the upside is that they will be covering areas which clearly have buildings in them!

The earth resistance meter, operated by myself and Ellen, managed a modest two grids once we had set-up the other two machines.  The results were good, however, and clearly show many of the details of this building in the top-corner of the Theatre field.  The next three images show the mag, GPR and earth resistance results for this area.

Figure 2: mag data in the top corner. the building shows as white lines of low magnetism.

Figure 3: the GPR data showing this building very clearly as black lines of strong radar reflections.

Figure 4: the earth resistance data for the same building.

Although the GPR data appears very clear, the Earth Resistance and mag data appear to show more walls between the main range and the road.  There is a suggestion, also, that the “corridor” to the SW of the main range is in fact another phase.  It would be odd for a corridor to have subdivisions.  Plenty of room for debate over the details of this building.

Many thanks to all for your excellent work in the first two weeks.