Tag Archives: geophysical survey

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.

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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…

Deluge

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

We may be creating our own digital data deluge, but today was the more traditional sort consisting of water falling from the sky. At midday the heavens opened and that was that. Another 10mm or so of rain. That makes over 20mm (or about an inch in old money) in the last 36 hours.

The GPR crew managed one 40x40m block before midday.  Figure 1 shows the time slices.

Figure 1: GPR time slices from day 8.

Sadly, nothing hugely exciting jumps out.  There are a few straight lines towards the NW corner which might be surviving walls, and the big blob just left of centre.  The blob is very curious.  It is quite persistent through the top slices.

Figure 2 shows one of the slices in context.

Figure 2: slice 5 in context.

This area is proving quite frustrating.  Lots of blobs and odd bits of straight line, but no clear buildings as we have had elsewhere.  Very curious.

I told the mag team to take everything with them, so I’ll report on their progress tomorrow.

Figure 3: the mag team work under a cloud.

Figure 4: the GPR crew at work.

Thankfully, the forecast is better for tomorrow, but damp again on Sunday. Many thanks to everyone who got a little wet today in the name of archaeological survey!  Your efforts are much appreciated.

A light wind swept over the corn, and all nature laughed in the sunshine.

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

Not much sign of corn, but the wind was blowing over the parched grass.  It was a welcome relief to be working in cooler conditions, although the weather was still beautiful.

Figure 1: St Albans Abbey.

The mag team completed an excellent eight grids in Mobbs Hole (Fig. 2).

Figure 2: the mag survey after day 6.

As before I have overlain the survey on the 2006 imagery in Google Earth which shows the Fosse most clearly.  The hints of a line along the inner edge.  Maybe this is the remains of a palisade trench?  Disappointingly little otherwise.  One thing to note is that the “noise” from random ferrous trash is more prevalent to the south of the old fence line than to the north.  I wonder if the NW corner of this field was pasture previously?

The GPR crew completed their two grids.  Figure 3 shows some time slices.

Figure 3: Day 6 GPR results.

Not a great deal showing apart from in slice 4 (in the top-right corner of Figure 3) which clearly shows the aqueduct.  This is about as clear as I have ever seen it in GPR data.  Awkwardly, the direction of the transects is close to the direction of the aqueduct.  Figure 4 shows the slice in context with the others we have surveyed this year.

Figure 4: GPR day 6, slice 4.

The edges do not match because I keep playing with the settings in the software.  One day, I’ll slowly process the whole lot so that we get a nice final result.  One day (more like several months…).

Barney and Becca came and helped with the GPR in the morning.  After lunch, we blew the dust off our Bartington and did a couple of squares.  I rather liked this image of Barney.

Figure 5: Barney and a Bartington.

The survey moves on and we cover more ground.  Many thanks to everyone who comes and helps expand the area we have covered.  Luckily, most of the area you can see in the last inage has already been done!

Figure 6: Ellen and Mike operate the GPR.

 

Back and forth

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

Today was like most days, back and forth along the strings. We had the added delight of temperatures into the 30s centigrade. Just a little toasty. It is (hopefully) going to be cooler tomorrow.

The mag team consisting of Jim West, Ruth Halliwell, Dave Minty and Rhian Morgan continued extending the block of mag data southwards.  The results are shown in Figure 1.

Figure 1: the magnetometry data at day 3.

As you can see, there are a lot of linear features, some very faint. In the theatre field I was clipping the image to +/- 5nT.  Here I have clipped it to +/- 2nT.  They look like field boundaries, and the one to the west heads for the corner where the gate is.  They don’t, however, match the boundaries shown on the 19th century OS maps.  I’m probably going to have to shell-out for the tithe map of St Michaels.  Tomorrow the crew are going to keep heading south and up slope, and soon they should be hitting the Fosse.

Figure 2 shows quite how dry the site is.

Figure 2: John Ridge and Anne Petrie using the GPR.

The GPR team consisting of Mike Smith, John Ridge and Anne Petrie, occasionally hindered by yours truly, managed our standard 80x40m block, although in the heat in felt like a great deal more.  Time slices 3 to 11 are given in Figure 3.

Figure 3: time slices 3 to 11.

As with yesterday’s data, it is annoyingly blobby.  In a way this is probably a good thing as it means that not everything has been ploughed right down to its foundations.  The fourth slice (top row, middle slice) seems to show a wall running SW–NE across the middle part of the plot.  There is something going on here, but it is very unclear.  The next three Figures show slices 4, 5 and 6 from today and yesterday superimposed on the Google Earth image.

Figure 4: days 2 and 3, slice 4.

Figure 5: days 2 and 3, slice 5.

Figure 6: days 2 and 3, slice 6.

Hopefully, as we get closer to Watling Street to the north, we will start picking-up some clearer structures.

I think everyone was grateful for the end of the day.

Figure 7: at the end of the day.

Tomorrow, however, is sawtooth Saturday…

Figure 8: a plethora of survey flags.

Many thanks to everyone who helped today.  It was hard work in the sun!

The final results from Alba Iulia (Apulum)

We managed the final ten squares on the last day completing the area I hoped we would cover. We managed 2.6ha in seven days of survey.  The final four grid squares had an old excavation trench in the middle of them and a sea of mud from the spoilheap being bulldozed back into the hole.

Fig. 1: Stefan and Wyatt surveying in the mud.

I have left the grid on the next image (Fig. 2) which gives you some sense of scale.  The grid squares are 20x20m, and site north is to the right.

Fig. 2: the surveyed area with grid for scale.

In the next image (Fig. 3) I have labelled a few of the features.  One can easily see many of the buildings.  Because of the large differences between areas of low readings and areas of high readings I have applied a log-transform to the data to make some of the features more visible.

Fig. 3: the 0.5m survey with log-transformed data.

An alternative (and very common) method of evening-out the differences is to apply a high-pass filter (Fig. 4).  The downside to this is that areas of very high readings will often end-up with a area of very low readings next to it which are purely an artefact of the filter.  It is essential, therefore, to have several images and compare them so that you do not create features where there are none.

Fig. 4: the data after the application of a high-pass filter.

As well as the main survey with a 0.5m probe spacing (thus ‘looking’ 0.5 to 0.7m below the surface, approximately), we were simultaneously collecting 1.0m probe spacing data (which ‘looks’ about 1.0m into the ground), but at only two readings per square meter rather than four.  This extra data did not add that much more to the survey (Fig. 5), but does make the north-south road show more clearly.

Fig. 5: the 1m probe spacing data, high pass filtered.

There is quite a bit to do in terms of interpretation and drawing-up maps of the results, but I am very happy with the new results which have greatly improved an already excellent set of results from 2002/3.

The journey home seemed to go on for ever, but at least I had some entertainingly-named beer on the way!

Fig. 6: Munich Hell.

Many thanks to the whole team, and especially to Stefan, Wyatt, Doru and Ian.

Of snow and sheep

Yesterday started off well. The weather was a little cold, but other than that it wasn’t too bad.  Then it started to snow.  The snow was OK, not too much.  Then it snowed some more.  And a bit more…

Fig. 1: surveying in the snow.

Just to add to the delights, we had a visitation from some sheep.  They charged past me where I was laying in the grid for the next square, and surrounded the team using the res.

Fig. 2: The sheep visit the survey.

Fig. 3: The sheep visit the survey (heavily cropped photo).

By lunchtime, it was getting silly, and we packed-up.

Fig. 4: Getting silly.

We did, however, get some very nice results from both the 0.5m probe-spacing data and the 1m -probe spacing data (the latter “looks” a bit deeper into the soil).

Fig. 5: the 0.5m probe-spacing data.

Fig. 6: the 1m probe-spacing data.

The dark line running diagonally across the plot is a Roman road with a subsidiary road running off it at an angle.  The excavations were at the far left-hand side of the image.  The buildings along the road are pretty clear.  Both these images have been high-pass filtered to bring out the structures.

Although the landscape is covered in snow, it isn’t actively snowing at the moment, so I suspect we will be going out again today.