Tag Archives: earth resistance survey

Neat and tidy

Due to being rained off on our last day, a small team of us decided to go out and finish off some things on Bank Holiday Monday. Many thanks to Pauline, Judith, Ruth, Dave and Jim for turning out to do “just one more grid.”  I think it must be a geophysicists ailment that we always would like to be able to just a little bit more…

The mag team completed an impressive ten grids including two awkward partials.  Figure 1 shows the entire survey at the end of the 2018 season.

Figure 1: the mag survey after day 19.

The team have managed to add 19 ha to the survey in the last month.  Figure 2 shows the southern area that we have been surveying this week.  (This field is, confusingly, called “Prae Wood”.)

Figure 2: the southern area (Prae Wood) after day 19.

The team have picked-up an area of intense ferrous noise.  This looks like a small historic period site.  We will have to check out some old maps to see if we can work out what that might be.  The one hiccup in a brilliant last day of work is a single line of data where the sensor froze.  It is very annoying and I’ll have to find some way of fudging that until next summer!

The Earth Resistance survey had one last little block left to make the plot look all neat and tidy.  Many thanks to Pauline and Judith for helping me fill that in (Figures 3 and 4)!

Figure 3: Kris, Judith and Pauline (out of shot) extended the resistance survey. Image © Mike Smith.

Figure 4: the main block of Earth Resistance data collected 2016–2018.

The data collected shows some faint indications of buildings in that corner (Figure 5).

Figure 5: the northern area of the res survey. The NW corner was completed on day 19.

Although my trick of spreading the remote probes wide apart has worked on the whole, this year there is a bit of an edge.  This is because we started with a block in the SW corner, worked eastwards, and then when we had got to the corner, worked back along the hedge line westwards.  Between when we started this block and yesterday we have had in excess of 100mm of rain (or about 4 inches in old money) so it isn’t surprising this shows in the results.

We have now cleared away all the pegs and flags, packed-up the machines and left Gorhambury for another year.  It is a beautiful place to work and we are very grateful to Lord and Lady Verulam and their family for allowing us to extend the survey, to those who work the estate and put up with us getting in the way, and to the estate managers, especially Stuart Gray. Thanks to the Institute of Archaeology, UCL, for lending us the dGPS and the res meter, and SEAHA for the loan of the GPR.  I hope everyone involved thinks the results are worth all the effort. Most of all I would also like to thank all the volunteers who came this year, whether you only managed a day or two, or you came for the whole season.  You are what makes this project so much fun!

 

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“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)

 

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.

 

 

80 hectares and counting

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

At the end of week 3 of the 2018 season we have now collected over 80 hectares of magnetometry data, that’s about 16 million readings. It would be traditional to cite the area in terms of soccer pitches, but soccer pitches do not have a standard size! It is, give or take a bit of variation, roughly 106 pitches in size.  Figure 1 shows the entire area surveyed with the mag so far.

Figure 1: the total area surveyed with the mag at the end of Week 3.

The mag crew have had to deal with some awkward corners lately (Figure 2).

Figure 2: In the corner. The mag crew negotiate an awkward corner with the tower of St Alban’s Abbey in the distance.

The next two images show the whole of the survey in Mobbs Hole (Figure 3) and a zoomed-in view of the southern area completed in recent days (Figure 4).

Figure 3: the Mobbs Hole survey at the end of week 3.

Figure 4: the southern area of the Mobbs Hole survey.

The first thing that catches one’s eye are the blocks of stripes.  Those are the result of the dreaded “sensor freeze”.  Normally the team catch these in the field, but Friday and Saturday appears to have been bad days for it.  I wish Foerster would sort this problem out, it is such a monumental waste of our time to have to go back and redo those grids (and reprocess the data!).

In broad stroke terms, it is interesting to note that the “texture” of the survey is very different to the east than the west.  Looking at the superficial geology map there are gravel deposits a little way to the south, but the area of the survey has not been mapped.  The difference in texture is very similar to what we have seen elsewhere, e.g., between the “theatre” and macellum” fields, and I would argue that this represents some superficial gravel deposits in this area.  The large high-magnetic features towards the middle of Figure 4, along the edge of the texture change, could be quarries into this gravel?

Just to the west of the lone square of frozen data is a feature which looks like a semicircle on a square.  A building perhaps, or wishful thinking?

Running three teams, as well as the impact of flag-eating sheep, requires the GPS to travel widely (Figure 5). (My feet ache…!)

Figure 5: GPS and sheep (photo © Mike Smith).

We managed to extend the area surveyed by the Earth Resistance meter by another six 20x20m grid squares. Thanks to Ellen, Rhian and Anne for all their hard work (Figure 6).

Figure 6: the res team.

Figures 7 to 9 show the results of the Earth Resistance survey at the end of Sunday.  Remember this is only a day and half’s worth of survey.

Figure 7: the Earth Resistance survey overlain on the GPR survey.

Figure 8: a crude photo-mosaic of the GPR data in the same area as Figure 7.

Figure 9: the mag data in the same area as figures 7 and 8. The yellow square shows the location of the 2018 resistance survey.

There is a lot going on here.  The stone building which shows so clearly on the southern side of the resistance survey and in the GPR survey barely shows in the mag data, although to be fair it is partly obscured by the highly magnetic building. The aqueduct which shows clearly cutting across the mag data (the sinuous long line of high readings), barely shows in the resistance or GPR data.  The curious kink in the aqueduct does seem to have a high resistance feature along the southern edge, probably a wall.  Are we looking at the aqueduct avoiding some earlier feature, or is that some sort of water management feature like a sluice?

Comparison of the corner of the building in the right hand side of the resistance plot with the GPR data shows that each method has picked-up similar features, but some show better in the GPR and some show better in the resistance.

The GPR team faced the alpine challenge (Figure 10).

Figure 10: Nigel earns his Alpine GPR badge (photo © Mike Smith).

They completed 80x80m on Saturday and Sunday.  The land flattens off a little bit now!  Figure 11 shows the time slices from Saturday.

Figure 11: Six time slices from the GPR survey on Day 13.

In the fourth time slice (Figure 11, top-right corner), I have indicated a nice building showing with a red arrow. It lies short-end to Street 11 which is running east-west and is marked in slice five with a blue arrow.  Also in slice five is a curious curving feature marked with a red arrow.  I’ve no idea what that might be.

Figure 12 shows the time slices from day 14.

Figure 12: six time slices from Day 14.

I have started with slice 2 for a change.  This is essentially the ploughsoil.  I have marked in red (Figure 12, top-left hand corner) some curious circular features.  Long term readers of this blog will know that these are mushroom rings!  We’ve not had many mushrooms showing on the surface in the dry weather but after recent rains there are starting to appear.

In slice four I have marked an obvious building (red arrow) and a much more subtle one (purple arrow).  They are, again, lying short-axis to Street 11 (shown in slice five with a purple arrow).  Buildings like this are common for more modest dwellings in the Roman world.  We had similar buildings at Durobrivae along Ermine Street.

Figure 13 shows the GPR data and the mag data. (It is worth clicking on this and looking at it full-size.)

Figure 13: mag data (above) and GPR data (below). The yellow box indicates the location of the days 13 and 14’s GPR data.

Some of the buildings show in the mag data and the GPR data, but some do not show in the mag data at all.  This is not surprising as the flint used for the foundations is not magnetic.

I have spent the greater part of today processing data and writing this blog, so I am going to shut-up now, have a cup of tea and head for bed.  I dislike ending-up on Figure 13, however, and so Figure 14 is an entirely gratuitous photograph of the abbey taken on Sunday from the site.

Figure 14: St Alban’s Abbey.

We will back back on site on Wednesday for the last week of the 2018 season.  Many thanks to everyone who turned out even when faced with Mount Gorhambury and a GPR!

 

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.

One hundred and fifty

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

We managed a full day today, and I’m just about keeping up! The mag team completed two grids yesterday, half of one in the aforementioned deluge. Today they completed 11 grids: three partials and eight complete ones. Way to go! Well done everyone. Figure 1 shows the survey so far.

Figure 1: the mag survey after day 9.

One really does wonder if that break in the mag data is an entrance.  It doesn’t seem like it on the ground.  I have downloaded the LiDAR data but haven’t had a chance to process it yet.

The GPR crew finished their 80x40m block, and then did some of the next “sawtooth” section, another 14m worth.  Figure 2 shows the time slices.

Figure 2: day 9, time slices 3 to 6.

Nothing jumps out at one, although there are some curious “light” lines in the fourth slice (top-right) which are parallel to the aqueduct.  Figure 3 shows that slice in context.

Figure 3: GPR survey after day 9, slice 4.

After all the rain I thought it would be worth trying the Earth Resistance survey (Fig. 4).  I spent the morning laying in grids for the mag, but managed some survey in the afternoon.

Figure 4: Earth Resistance survey in action.

Although the rain has softened the surface, it won’t have penetrated 50cm yet, and I was concerned that there would be no contrast at that depth.  I decided to survey a grid where we knew there was a building.  Fig. 5 shows the comparison between the GPR survey and the two squares of res I managed to complete (thanks Anne!).

Figure 5: Earth resistance survey compared to GPR results.

Given the drought, the results are pretty good.  It would be interesting to compare these to results from a normal English summer!

Tomorrow isn’t looking great.  We might get some work done in the morning.  Fingers crossed.

Many thanks to everyone who helped out today.  Especially big thanks to Mike, Ellen, Jim and Ruth who take on the responsibility of shipping the equipment back and forth.

By the way, this is the 150th blog post…