Tag Archives: Geoscan RM85

St Mary Magdelene, Bovenay

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

The small chapel of St Mary Magdalene lies in the south of Buckinghamshire, not far from Windsor and Eton.  Now only used for occasional services, it is looked after by the Friends of Friendless Churches.  It is a lovely little chapel, and well worth a visit (Fig. 1).

Figure 1: the chapel of St Mary Magdelene.

We were contacted via the Buckinghamshire Archaeological Society’s Active Archaeology Group to see if we would undertake a geophysical survey around the chapel.  The question was deceptively simple: are there graves around the church?  The reason for the question is partly because, usually, ‘chapels of ease’ were not used for burial, and partly because the nearness of the water table makes digging deep holes problematic.  Although the site is a long way out of our usual area, we agreed to try and see what we could find.

Three things make the job difficult:

  1. burials are notoriously  difficult to detect at the best of times.  They aren’t very big, and usually the same soil that came out of the hole goes back in again pretty quickly.  There is, therefore, relatively little contrast between the grave fill and the surrounding soil.
  2. Small areas are difficult to interpret. The whole churchyard is only 0.07ha (less than two 20x20m grid squares), and you have to subtract the footprint of the church itself and the path.
  3. GPR surveys near standing buildings suffer from airwaves.  Although the antennae are shielded, some of the radar signal will ‘leak’ and will bounce off nearby buildings etc.  Airwaves can be seen in the data as having hyperbolas with a much flatter profile than the usual point sources in the ground such as rocks or walls.

Given the very small size of the area, and the surrounding metal fence, magnetometry survey was going to be pointless.  Often, the best method for finding graves is GPR, and so that was our primary method (Fig. 2).  Due to the building, and the odd shape of the church yard, we had to do the survey in six small blocks at 25cm intervals.  Although it would have been easier to do the survey east-west, if we are trying to find graves working north-south would be more effective allowing the transects to cut across the grave rather than along it.  Lastly, we decided to try the multi-depth Earth Resistance survey, aka ‘the beast‘ (Fig. 3).

Figure 2: the GPR in action at Bovenay. Photo: © Mike Smith.

Figure 3: the ‘beast’ in action at Bovenay.

We all headed off to Bucks on a cold and slightly damp Sunday at the end of January (yes, I know this posting is late!) and we were assisted by members of the Bucks ASAAG.  Both GPR and resistance surveys were awkward due to the small space we had available.  The site is also very busy with walkers, cyclists and people enjoying their Sunday.

The idea of “the beast” is that the depth to which an Earth Resistance survey will measure is proportional to the distance between the mobile probes on the frame.  The two remote probes have to be at least 45m away!  Each time the machine is moved, it takes seven readings: one between two probes 25cm apart, one at 50cm, 75cm, 100cm, 125cm and 150cm.  Yes, that makes six.  Just for comparison, the seventh measurement is taken using a “Wenner array”.  This simply means that instead of using the two remote probes at the end of the cable, it uses the two outer probes on the frame to pass the current, and the two inner probes to take the reading.  This is an older method for laying out probes that has generally been abandoned in archaeology, although it can be useful in circumstances when having remote probes at the end of the long cable is impossible.  The results for all seven readings are shown in Figure 4.

Figure 4: results from the multi-depth Earth Resistance survey. (Click for larger image.)

Figure 4 looks a little odd because the satellite that took that image was clearly passing overhead a little to the south-east.  It is, however, the best one available on Google Earth Pro.  Comparing the various surveys at different depths, there is very little difference between them.  Unsurprisingly, near the walls are areas of high resistance, possibly due to the foundations.  The path was a pain.  One problem was a number of ‘spikes’ in the data.  These were probably caused by the rabbit holes: a hole with air is going to be high resistance (in fact the current passes through the soil around the hole).  I worry slightly that processing those out may also have processed out the graves, but somehow I doubt it.  It doesn’t look like we detected any graves with this method.

The GPR survey was processed using the package GPR Slice.  Figure 5 shows all the slices from the survey.  These were processed using 2.92ns slices with a slight over-lap between them.

Figure 5: the time slices from Bovenay.

The bright red line in the first slice is the path to the north entrance of the chapel. If you look at the image on Google Earth, one can see the south entrance too as a lighter blue line (Fig. 6).  Not an Earth-shattering observation, but it is always encouraging when one can see the obvious!

Figure 6: GPR time slice 1.

What seems interesting, at first, at the strong reflections (shown in red) in the lower slices.  They are at a slight angle to the hole in the survey where the church is.  Looking at slice 11 in more detail (Fig. 7) we can see they are parallel to the wall… it is my grid that is at a slight angle. (The grid was set-up along the southern fence line.)

Figure 7: GPR time slice 11.

If we look at the radar data in 3D, we can see these strong reflections low down curving-up towards the edge of the survey (Figure 8). You can see those strong reflections intersecting with the red line in the time slice (labelled ‘air waves’).

Figure 8: 3D image of the radar data.

To understand what is happening, we need to go back to GPR basics.  What is happening when we do a survey?

  1. The transmitting antenna sends out a radar pulse.  Due to the shielding, most of this goes down into the ground, but some will leak out and bounce around like echos.
  2. The receiving antenna measures the returning radar waves.  It records two things: the strength of the signal (amplitude) and the time since the pulse was transmitted.
  3. The software plots the strength of the return signal in shades of grey.  Strong returns are plotted in black and white, and weaker returns in mid-greys. These are plotted as a single vertical band below the centre point of the antenna. The radargram one looks at on the screen are all these vertical bands added together to give the overall image.
  4. Because the longer the time between the pulse and the return, the weaker the signal will be, we apply a ‘gain curve’ to the data.  This is just a multiplication factor so that the deep returns are visible compared to the shallow ones.
  5. As we push the GPR towards a wall, some of the signal will bounce off that wall.  At first, the distance between us and the wall is relatively large, so the reflection will be plotted by the software low down the profile.  As we get closer, the time taken for the signal to bounce off the wall gets less, and so the reflection is plotted higher up the profile.  As a result, the signals bouncing off the wall will show as a gently rising curve.
  6. Radar waves travelling through the air travel at the speed of light.  Radar travelling through the soil is much slower.  As a result, curves in our data which are quite steep are the result of a reflection from something in the soil.  Very gentle curves are “air waves” and are the result of the radar bouncing off buildings, or even the underside of tree canopies.

To cut a long story short, the strong, deep reflections are airwaves caused by the radar bouncing off the walls of the chapel.  Figure 9 shows the southern radargram from Figure 8 with some of the relevant reflections indicated.

Figure 9: Radargram 7028 at 12m east with some reflections marked (see text).

In Figure 9 the airwave has been marked in red.  See how it is a gentle curve across the radargram.  A more normal hyperbola from a point source is indicated in purple.  The green line near the surface represents the compacted soil outside the south door.  Notice how there are bands of strong reflections below it.  These are like echos.  There is another, deeper, surface marked in yellow.  I’m not sure what this it, perhaps an earlier entrance path?  If we look at time slice 5, we can see this area of high reflections outside the south entrance to the chapel (Fig. 10).

Figure 10: GPR time slice 5.

Archaeologists, (me included!), dislike looking at radargrams as they find them confusing (they are).  Often one will see reports with only the time slices presented.  One thing I have learnt from Larry Conyers, however, is that it is vital to look at both the time slices and the radargrams if one wants to understand in detail what is happening.  Although in general I am not a fan of pseudo-3D representations of things (don’t get me started on the invention of the Devil, the 3D pie-chart!), the 3D plots in GPR Slice do help work out what is going on.

So the million dollar question is: have we found any graves?  The short answer is: none that I can see.  The long answer is, sadly, that that does not mean there are no graves.  Geophysics does not detect everything, as much as we would like it to.  Also, I need to spend some more time going through the radargrams and trying to see if there are graves which show in the vertical radargrams but do not show in the horizontal time slices.  Later this year I plan to spend some time with a friend in the US who does this sort of thing all the time, having a look at the data from this site and a couple of others in the hopes I have missed something vital. Watch this space!

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A chilly day at Little Hadham

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

Firstly, apologies to those waiting for the results of the GPR survey in the churchyard at Ashwell undertaken just before Christmas.  I need to do some more data processing to see if we have some graves or not.

Meanwhile, the site at Little Hadham has been one to which we have returned on-and-off almost since the beginning of the project.  (Use the drop-down box on the right of this page to see previous posts on this site.)  The site clearly extended beyond the area we had surveyed so far, and so CAGG once more teamed-up with members of the Braughing Archaeological Group (itself part of the East Herts Archaeological Society).  We could only access one of the fields, the one we first surveyed in April 2014.  Our aim was to extend the survey to the edges of the field, and to examine one group of features we had detected with the mag previously with an earth resistance survey.

The morning was cold and crisp with a hard frost.  The thermometer in Jim’s car registered minus 3 Celsius.  Brrrrrr…  Unfortunately, the frost melted quite quickly and muddy wheels on the mag became a problem (Fig. 1)!

Figure 1: Nigel wheels the mag across the field.

The mag team completed six grids, five of which were partials.  Given the amount of time spent trowelling the mud off the wheels, this was a good haul.  We have four partial grids left to complete the field.  The results are given in Figure 2.

Figure 2: the mag survey.

The areas completed yesterday were the block to the far east of the survey area, and the incomplete strip of partials on the southern edge.  In the new area we can see the ditch previously detected (shown by the red arrow) carrying on across the site.  It is fainter in Millfield to the west of the road, but is still evident.  Near the hedge is another clear line of a ditch (marked with the yellow arrow).  This one is worryingly straight and almost parallel to the  field boundary.  It maybe more modern than some of the other features.  The curvy, more irregular ditches (shown with the blue arrow) may be something like a farmstead with boundary ditches.  Picking apart the phasing of all these features is going to be difficult and would require some targeted excavation.

As we had a good sized team we also undertook some Earth Resistance survey (Figure 3).  We targeted one of the possible farmstead enclosures.

Figure 3: Katie Burgess and Peter Baigent (BAG) using the RM85 Earth Resistance meter.

The team completed five 20x20m grids at a 0.5m reading spacing.  The results are shown in Fig. 4, and the underlying mag in Fig. 5.

Figure 4: the Earth Resistance survey results.

Figure 5: the mag results in the area of the Earth Resistance survey.

I had been hoping — rather optimistically — that the resistance survey might pick-up some structures.  Sadly, it did not.  There are, however, correspondences between the mag and res results.  The ditch with the right-angle corner in the mag survey shows well, if slightly more diffuse, in the resistance data.  Similarly, the long curving ditch also shows well. I have indicated one end of it with a blue arrow in Figure 6.

Figure 6: Resistance survey results with arrows.

More curious, however, is the change from low to high resistance along a straight line indicated with a red arrow in Figure 6.  This corresponds exactly with the diagonal line in the mag data which cuts east-nor-east west-sou-west across the D-shaped enclosure. I’m at a loss to know what this represents.  It maybe a reflection of the various cut features in the underlying geology.

At the end of the day we were treated to a beautiful moonrise and sunset.  Not quite the blood wolf moon seen some 12 hours later (when I was tucked-up and asleep in bed!).

Figure 7: Moonrise. Shame about the electric cables!

Many thanks to everyone who turned-out on a freezing but beautiful day.  This site continues to repay our attention, and it worth the effort.  We should try and survey some of it in the summer, however!

Neat and tidy

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

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!

 

“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)

 

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…

 

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.

 

 

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.