Tag Archives: geophysics

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.

 

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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.

End of week two, part 1

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

Despite the unpromising weather forecast (and the unpromising weather at about 8.30am), we managed a whole day of survey including mag, GPR and a little bit of earth resistance. There was some drizzle around about lunchtime, but other than that is was an OK day, if a little windy at times.

The GPR team were working on “sawtooth” edges to the theatre field.  As we have a couple of days off, I will post the results of their efforts tomorrow.

The mag team completed twelve grids, including two partials and despite one case of “sensor freeze”; a super effort (Figure 1).

Figure 1: the mag crew, consisting of Ruth Halliwell, Jim West and Dave Minty, operate the Foerster in Mobbs Hole.

Figure 2 shows all the areas surveyed so far at Verulamium.

Figure 2: all the mag survey so far.

This really shows that the strength of the Foerster cart system is when one has large open fields.  So far, according to TerraSurveyor, we have completed 9.8ha of Mobbs Hole over ten days, but remember we only had part of an afternoon on day 1 and lost parts of two days this week to rain.  Compare that to surveying in Verulamium Park with all its trees, hedge lines, park benches and so forth.  There we surveyed just under 30ha in 45 days.

Figure 3 zooms into the area surveyed over the last few days.

Figure 3: mag survey after day 10.

So far the results are curious.  There are linear features associated with the line of the Fosse, plough scars running NW–SE down the slope, a few large “blobby” anomalies (pits, maybe?) and some bits of old iron.  Very little which could be interpreted as structures has been found.  Why build an enormous ditch and bank around nothing?  Ploughing may have removed some superficial features, but there is very little that could be seen as pits or ditches either.  The Fosse remains a mystery.

Figure 4 shows where the Fosse enters the wood.  The dip in the fence marks the ditch.  Once inside the wood the Fosse does a sharp left turn and heads southwards.

Figure 4: the dip in the line of the fence shows where the Fosse enters the woodland.

Tomorrow I will report on the earth resistance and GPR surveys in the theatre field.

Many thanks to everyone who has helped this season.  We are already half way through.  Doesn’t time fly?

 

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…

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.

 

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.

Just too claggy

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

A group of us headed out to Little Hadham today with the aim of extending both the Earth Resistance and magnetometry surveys.   By the time Nigel had pushed the mag across the field, the wheels had diameters several inches larger than they should.  Jim tried a line or two of data collection, but the odometer was over-running by two meters and the wheel needed trowelling clean every transect.  We decided, therefore, to abandon the mag survey, and concentrate on enlarging the area of the Earth Resistance survey.  We managed another six 20x20m squares giving us a total of 100m by 80m, some 32,000 readings in total.

Figure 1, below, shows the initial results.  As before, the data is dominated by striping caused by the cultivation pattern.  A 2D fast fourier transform (as implemented in TerraSurveyor), quickly removed these stripes.

Fig. 1: the Earth Resistance data overlain on the mag data.

In Figure 2 I have applied the filter to remove the striping.  To the right I have put the mag data for comparison.

Fig. 2: The resistance data after processing with the 2DFFT. The mag data of the same area is shown to the right.

Unusually, most of the features show in the res and mag data.  The res data has nicely picked-up many of the linear features more normally only clearly seen in mag data.  In Figure 3 I have labelled a few points.

Fig. 3: the res and mag data with labels.

Ditch features A and B show nicely in both the res and mag data.  What is clear from the res data, however, is that the ditch continues between the two and they are one distinct linear feature.  If one draws a straight line along A and B, it lines up perfectly with the linear feature C we found across the road in 2014.   Linear feature D shows equally well in both data sets.  At E, something complex is happening.  In the mag data it looks almost as if A is turning and runs alongside E, whereas in the res data is looks more like AB cuts across the linear to the west of E.  The parallel lines to the west of E show quite well in both, and are probably some form of trackway.

Many thanks to Jim (CVAHS; both for surveying and transporting the equipment and myself), Nigel (NHAS), Caroline, Peter, Amanda and Mark (BAG).  Hopefully we can get to do some more when the field is less claggy.