Tag Archives: GPR survey

Hogshaw Redux

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

Archaeologists often have skeletons in their cupboards. Sometimes they are real skeletons. Sometimes, as here, they are unfinished jobs that they haven’t quite got around to completing.  There are a few surveys we have undertaken that never quite got finished, and for which there are no blog posts (shock! horror!). Way back when we got together with the Buckinghamshire Archaeological Society’s Active Archaeology Group and CVAHS to undertake some surveys at Hogshaw in Buckinghamshire.  The AAG had undertaken an interesting research project on this site including topographic survey.  We managed some mag (even though the mag was down to three probes) and some Earth Resistance survey (using our old system).  The results were posted at the time.

The following year, in 2016, we returned and expanded the mag survey and undertook some Ground Penetrating Radar survey.  We had only just started using GPR and I was still learning how to process the data.  The following year, Mike and I returned with the GPR to survey another two areas.  Due to problems with that data (we were distracted by lunch), that I couldn’t solve at the time, the results were put on the back burner.  Fast forward two years and I am now a little more confident and have a better handle on the software.  Having finished processing the awkward survey at Bovenay, I thought I would have a go at re-processing the Hogshaw data.  As you might guess from the fact you are reading this post, I had some luck and so, two years late, here are the results! (See the older post for the previous results and the background to the site.)

The magnetometry survey was mainly aimed at finishing the awkward bits around the edges, and an area to the south where the landowner kindly took down his fence so we could survey across it.  The results are shown in Figure 1.

Figure 1: the magnetometry data after the 2016 survey.

At first sight the magnetic survey is rather busy and hard to interpret.  This is not unusual in historic period sites where iron artefacts and fired bricks are relatively common.  I have labelled the plot with some basic interpretative points (Fig. 2).

Figure 2: magnetic data interpretation.

The fence line is where the farmer kindly removed the fence so we could survey.  It is fascinating to see that even when the fence has gone, we still detect the line of it.  Iron rust etc. washes down and permeates the soil, I guess.  The platform is a large flat area in the NW corner of the site.  We do not know what it is for, and the mag does not help a great deal (neither did the Earth Resistance last time).

Perhaps the most interesting feature that we detected last time is the four squares inside a square.  This was quite a surprise.  It looks very much like a formal garden.  If it is a garden, there appears to be a line heading out westwards to an area of magnetic noise.  I rather ignored that last time, but now I wonder if that is where the remains of the manor house were?  It was abandoned in the 18th century.

There are two lines of very noisy magnetic readings, one along the current road and one along the northern edge.  I’d like to see how these relate to the topographic features.  I think they line-up with the banks, and could be lines of brick rubble.  Unfortunately, the LiDAR data for this area does not cover the site, ending just under half a mile to the north (Fig. 3).  Typical!

Figure 3: the LiDAR data overlain on a Google Earth satellite image.

Three blocks of radar data were collected.  We used SEAHA’s Mala GPR, and we thank them for the loan.  The location of the three blocks are shown in Figure 4.

Figure 4: Location of GPR blocks.

The southern block was surveyed in 2016 because an excavation had found a couple of stone walls in this area, and it was suggested this might be the location of the lost chapel. Figure 5 shows the top nine time slices (note that north is downwards in these images).

Figure 5: time slices from the southern block.

The first time slice shows the road nicely.  Also helps build confidence when the method detects the absolutely obvious! By about the fourth slice (second row, leftmost image) the road is largely gone but there are two parallel lines running north south.  Could these be our missing walls?  Perhaps, but I suspect they are compacted earth either side of the fence which the farmer took down for us.  The area of high amplitude reflections in the bottom-right corner (north-west) is the area of wet mud around the various temporary structures that were moved.  All in all, a rather disappointing result.

The platform block was an attempt to see if we could work out the function of the platform in the NW corner of the site.  Figure 6 shows nine time slices.

Figure 6: time slices from the platform block.

Again, note north!  There is a vague hint of something in slice 7 (third row, first image) that might be rectangular, but it is quite low down in the sequence, and a bit amorphous.  Looking at the radargrams (the original vertical slices), I cannot see anything particularly wall-like.  I suspect that what little radar energy has been reflected has been greatly emphasised in these plots creating the illusion of something.  Figure 7 shows slice 7 in context.

Figure 7: platform block, slice 7, in context.

Last, but not least, is the “garden” block (Fig. 8).

Figure 8: six GPR time slices over the “garden” block.

In slice 1 (top left), the results just reflect the uneven surface. In slice 2 we can start to see something, but it is in slices 3 and 4 that we can see the “garden” feature quite clearly.  The whole feature is about 36m across with the internal square about 12m by 12m.  To the south there appears another strong linear reflection.  Maybe a road to the house?

Figure 9 shows slice 3 in context.  I’m glad to say that the mag and GPR data match very closely.  The edging around the features must be something both magnetic and that reflects radar data.  Brick is one possibility, and some form of igneous rock is another.

Figure 9: the “garden” block, slice 6 in context.

One might ask why I am so keen on it being a garden feature.  Looking at another much grander garden, we can see many similar features (Fig. 10).  The part I have outlined in red is approximately the same size as ours.  The inner squares of that garden at Hatfield are 11m across, the enclosing hedge 28m by 42m, the outer edges 37m by 57m.  As always, the only real way to tell is to dig a hole…

Figure 10: the gardens at Hatfield House.

Many thanks to everyone who helped on the four days of survey, especially to the very helpful landowner.  Also many thanks to Anne Rowe for commenting on the “garden” feature and sending me some very useful information. Hogshaw still has some secrets to give up!

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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.