Author Archives: hertsgeosurvey

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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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.

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…

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.

7.2mm

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

That is the quantity of rain which fell on Lamer Park today up to when I started writing this posting.  It is now at 10.3mm! No wonder we gave-up at lunch time all feeling a bit cold and damp.

The mag team did manage three grid squares before lunch (Fig. 1).

Figure 1: mag survey after day 7.

There is a interesting possibility that we have detected a break in the Fosse, maybe some form of entrance.  I have noticed that some of the maps of the Fosse show a break, and some do not.  Perhaps we may be able to resolve this.

The GPR crew managed their usual 40x40m block before the rain drove us away at lunch time.  Figure 2 shows four time slices.

Figure 2: GPR data from day 7.

There is a quite substantial feature showing in slices 3 and 4 (top row) to the west.  This “big blob” is on the edge of the aqueduct, but I am not sure what it is.  Finally, however, we have some clearer looking buildings, best seen on the east side of slice 4 (top-right of Figure 2).  Figures 3 and 4 show this slice in context.

Figure 3: GPR from day 7 in context, slice 4.

Figure 4: the mag data for the same area as Figure 3.

As we have seen before, the buildings in this part of town show in the GPR data but not in the mag data.

On an unrelated note, this week’s Herts Advertiser (dated August 9th) carries a short letter from me about the project.  It also carries the news that the school at Batford has been given the go-ahead (see https://wp.me/p3AAk9-gy for our survey there).

Getting from the Theatre Field to Mobbs Hole is a bit of a problem because some pesky Romans built a wall between them.  There is, however, a gate we can use which is moderately convenient, which we have dubbed “The Manikin Gate”.

Figure 5: the Manikin Gate.

The weather tomorrow morning looks OK according to the Met Office, but we may be rained off again in the afternoon.  On the bright side, it might be possible to break out the earth resistance meter soon!