The beast lives on

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

Yesterday saw Kris, Pauline and Jim wrestling with the beast once more. It really is something one only does once in a while simply because one would die of boredom to spend too much time on it. The tick-tick-tick-tick-tick-tick-cherble cannot go fast enough. We are going to do one more day so that we have covered the whole of the house on the hill. Here are the results so far:

Fig. 1: the multiplexed Earth Resistance survey, day 2.

As we saw yesterday, the downslope wall seems to have much deeper foundations than the upslope ones.  I’ll do a more detailed write-up when I have the last day’s worth of data and I can compare the images to the GPR slices.

The GPR team of Mike, Nigel, Adrian and Dave completed yet another 80x40m block. This time they headed south.  The block crosses the ‘1955 ditch’, the late first century boundary of the town. Figure 2 shows a composite of time slices.

Fig. 2: Composite of GPR slices (top row, then second row etc.).

As can be seen, there is a rectangular building in the top-middle of the plot. This seems to be a simple building with four downstairs rooms, the thin one of which is probably a staircase to an upper floor.  It lies parallel to the ‘1955 ditch’ which shows as a lighter band running across some of the slices from about 0m 30 to 40m 0.

The new software allows me to undertake an “overlay” analysis which produces a composite image from different time-slices.  This should help see the whole of a structure when different parts of it are at different depths.

Fig. 3: Overlay analysis of the GPR results from day 8.

The overlay shown in Fig. 3 certainly shows part of the 1955 ditch more clearly.

Fig. 4: the mag data in this area. The yellow box represents the area surveyed on day 8 shown in Fig. 3.

One curious thing about the 1955 ditch is how varied its response in the magnetic data is.  Here, the ditch shows as a strong black and white feature running across Fig. 4 from the NW corner.  There is, however, quite a sudden change just at the edge of the GPR grid shown in yellow. Clearly, the ditch must have had quite a complex history and that the story of “dug c.AD 80, out of use c.AD 125″ is probably too simple.  In the southern part of the town, we found a later building constructed over the line of the ditch, but here the ditch seems to have remained clear of buildings, at least for some of its length.  There are some quite large circular black blobs in the magnetic data (“circular magnetic anomalies” in the jargon) which are probably large pits.  A few of those show in the GPR data when one compares the two data sets carefully, but you wouldn’t notice them otherwise.  Although some things show in both data sets, such as the building we found in the latest GPR results, some only show in one or other.  It is definitely worth doing both.

On Wednesday we will be back at Gorhambury, one team using “the beast” and one using the GPR.  We’ll be back to “normal” Earth Resistance survey on Thursday.

Many thanks to everyone who helped.

The beast

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

I have a fondness for electrical survey techniques. It may be something to do with only having access to an Earth Resistance meter for the first twenty years of my surveying career. UCL’s new RM85, which we got last summer, has been a real treat, speeding-up survey times and giving excellent results. One technique I have seen in print that I wanted to try at Gorhambury, uses “the beast”…

Fig. 1: “the beast”, aka, a Geoscan RM85 with a 1.5m beam.

As you can see from Fig. 1, the beast is the RM85 can be fitted with a 1.5m beam and six probes (not Fergus the dog, CAGG’s mascot).  Why?, you may ask.  The depth that Earth Resistance survey is measuring is proportional to the width that the probes on the frame are spread.  Our usual 0.5m spread gives us a depth of around 50–70cm.  By using the multiplexer — basically a fancy programmable switching box — built into the meter, we can take six readings at six different spacings and thus get six depths: 25cm, 50cm, 75cm, 100cm, 125cm and 150cm.  Just for good measure I got the machine to also take a reading using a 50cm spacing Wenner array, i.e., the two outer probes are passing the current and the two inner ones are measuring the voltage potential.

The downside?  Take a slow survey method, and make it really slow…  We managed two 20x20m grids in a day, and it is unlikely one would manage three without starting very early and ending very late.  Fig. 2 gives one a sense of how “quick” the method is.

Fig. 2: the string movers under pressure [not].

Our aim is to survey the “House on the Hill” using this technique so we have some nice comparative data to compare with the two GPR surveys.  It will take three days to complete six grid squares.  Here are the interim results.

Fig. 3: the results of the first day of multiplexed survey.

I have done the minimum of data processing to each of the images in Fig. 3: despiking to remove high points caused by rocks and some clipping to show the image more clearly.  As can be seen, the northern wall of the corridor seems to have quite substantial foundations.  I wonder if the house was terraced into the hill a little way, and this wall was a retaining wall down slope?  This isn’t going to be something we do often, but it is an interesting test.

The GPR team continued north.

Fig. 4: the loneliness of a long distance GPR-pusher.

They managed an excellent 80x40m block, good going on the slope and with the long grass.  Here are a selection of slices:

Fig. 5: a montage of time slices of today’s block.

As can be seen, we have a large building in the bottom centre of the block. This matches-up with yesterday’s building to give quite a curious looking structure.

Fig. 6: GPR survey, day 7.

It seems to have a very long corridor running down the SW side, the the rest of the structure to the NE is hard to see. There is then more rooms running SW-NE with some substantial “blobs” in the middle.  Clearly this strip of buildings running NE-SW across the site is very busy.  The next image (Fig. 7) shows the underlying mag.

Fig. 7: the mag data in the same area as the GPR data from days 6 and 7 shown in Fig. 6.

It seems to be a good year for our old friends the fairy rings…

Fig. 8: fairy ring.

Many years ago my brother wrote in my autograph book: “how do you get four elephants in a mini?  Two in the front and two in the back…”  I think Mike knows the feeling…

Fig. 9: equipment transportation.

Many thanks to Adrian, Dave, Mike, Jim, Pauline and Ellen for an excellent day’s survey.  We’ll be back tomorrow, and then two day’s off for a well-earned rest.

An amazing effort

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

A quick post on yesterday’s results. The GPR crew consisting of Mike, Nigel and Dave managed an amazing 100x40m of GPR, 20m finishing off Thursday’s two grids and two more. Good job! The Earth Resistance team of Jim, Julia and Kris finished a series of fiddly partials and a couple of whole blocks. A quick look at the results.

Fig. 1: slice 4 of the GPR results.

Figure 1 shows the main 80x40m block.  A series of rooms can be seen running NW-SE across the SW corner.  The walls have been robbed and are showing a light lines, but the floors survive and are the dark block blobs. On the original image, there may be signs of a hypocaust showing.

Fig. 2: slice 5 of the GPR results.

The next slice down shows the courtyard building to the north of the previous one in better detail, and a long two-roomed building showing clearly lying along the line of the dry valley.

Fig. 3: slice 6 of the GPR results.

In Figure 3 we are already getting below the level of the buildings, but a few more walls are showing-up.  Today we will be looking at the block to the north, and so should pick up more of the courtyard building.

Fig. 4: the Earth Resistance survey.

The Earth Resistance survey shows another building quite clearly near the fence.  One of the dark black blobs is the result of the oak tree sucking water out of the ground.  The other one is the line of the Roman road.

Fig. 5: the Earth Resistance survey, high-pass filtered.

The high-pass filtered version (Fig. 5) shows the walls more clearly and gets rid of the some of the impact of the tree and the road.  Some more details can be seen, such as a double line along the edge of the road.  Drains, maybe?

Fig. 6: the mag data in the same area as the Earth Resistance survey.

Fig. 6 shows the mag data in the same area as Figs 4–5.  Some of the building lines can be seen, nut not as clearly as in the Earth Resistance data.

Fig. 7: total area covered by the Earth Resistance survey.

The last image just shows the area covered by the Resistance survey so far.

Many thanks to everyone who helped.

Today we are going to try the six-probe multiple depth resistance survey, otherwise known as “the beast”!

A postdiluvian survey

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

Yesterday’s survey was called off due to rain. Not the usual English wet weekend in September sort of rain, but real rain. We had over 20mm at home, and I guess St Albans was much the same.  Many thanks to Anne, Julia, Pauline, Mike, Grahame, Joe and Dave for braving the damp conditions. I think it says a great deal as to how dry the preceding months have been that it still didn’t seem particularly muddy underfoot.  We managed a good six 20×20 grids with the Earth Resistance meter, including one awkward one with a big oak tree in the middle, and a 40x60m block of GPR.

Fig. 1: the Earth Resistance survey after three days.

As can be seen in Fig. 1, we have picked-up a nice courtyard building to the east near the road.  This isn’t listed in Niblett and Thompson or is it on the UAD, so this is a nice addition to the corpus.  We can see it in the mag data as well.

Fig. 2: the magnetometry data for the same area as Fig. 1.

It is a little clearer in the resistance data. It shows in the GPR too, much some features more clearly and some less.  A combination of techniques should give us a nice result.  We have covered quite large area with the resistance meter now (Fig. 3).  The difference between last year’s survey and this is simply in the processing.  I have applied a high-pass filter to this year’s data.  Once we have joined the two areas up, I can apply the same processing to the whole survey.

Fig. 3: the total area surveyed with Earth Resistance.

The GPR undertook the two grids to the west of the “House on the Hill”.  Here are four time slices, each just over 3ns thick.

Fig. 4: time slices of the area surveyed today.

As can be seen, we have multiple bits of buildings, but nothing (yet) that forms a coherent plan.  The curious empty SW-NE band persists.  This is in a small dry valley running downhill to the Ver.  Was it empty in the Roman period?  Has the archaeology eroded away, or has the archaeology been buried by alluvium?  Impossible to say at the moment.

The next three figures give time slices 5 and 6 with the other GPR data from this field (all just one slice).  The greyer (is that a real word?) images to the SE are this year’s survey, the brighter images to the NE are surveys from 2015 and 2016.  The detached block is today’s survey.  (We will complete the block to join it to the House on the Hill tomorrow.)

Fig. 5: time slice 5 with previous survey results.

Fig. 6: time slice 6 with previous survey results.

Fig 7: the magnetometry data underlying the GPR surveys.

Tomorrow we will be back, extending both the Earth Resistance and GPR surveys yet further.

Sawtooth Sunday

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

Today saw the GPR team doing the sawtooth edges of the field. A bit fiddly to do, but very fiddly to survey-in and process.  I have only processed one block of the three, I’ll work on the others tomorrow and also start joining the various blocks together into one big survey.  The  Resistance team also managed a very respectable six grid squares, including some that covered the east wing of the “House on the Hill”, otherwise known as Insula XXVI Building 2 or even Niblett and Thompson Monument 445…  This building was first published by Corder in 1941 and has only been seen through aerial photographs until we came on the scene.

First the underlying magnetometry data from 2015.

Fig. 1: the magnetometry data underlying the 2017 Earth Resistance survey.

As can be seen from Fig. 1 we can see the building quite clearly in the mag data as a series of white (low magnetism) lines.  The east wing does not show very clearly.

Fig. 2: the Earth Resistance survey.

The results from the Earth Resistance survey show the building beautifully.  The image has been high-pass filtered.  The large room on the east wing shows very clearly, although the “apse” on the west wall of it is still a little unclear.  It looks like a large dining room.

Let’s look at the 2015 and the accidental 2017 GPR surveys.

Fig. 3: the 2015 survey of the “House on the Hill”.

The slice from the 2015 survey (Fig. 3) shows some of the detail more clearly than the Earth Resistance survey, and some less so.  The other details may show more clearly in other slices and I’ll reprocess the data in due course. It will be interesting to see what the Resistance survey makes of the building to the far east of the GPR data.

Fig. 4: The 2017 GPR survey overlain on the other data.

Just for completeness sake, here is one of the time slices from the 2017 survey (Fig. 4).

Further south (and further up the hill!), the team completed a block to the south of the nice building we saw yesterday.  The new software can create an image with all the slices on.

Fig. 5: slices 3 to 10 of the day 4, block 3, GPR data.

This is useful to be able to see how the details change as one goes deeper.  (Take the depths given with a pinch of salt as 0.09m/ns is just a guess at the moment.)

The next series of images shows slices 4-8 from the above plotted next to yesterday’s building (just yesterday’s slice 5). The joins between blocks will improve when I process it all as one big survey.

Fig. 6: slice 4 (see Fig. 5 for depths).

Fig. 7: slice 5 (see Fig. 5 for depths).

Fig. 8: slice 6 (see Fig. 5 for depths).

Fig. 9: slice 7 (see Fig. 5 for depths).

Fig. 10: slice 8 (see Fig. 5 for depths).

As can be seen there are various buildings in this area.  It looks like there might be another small town house just the SW of the one we saw yesterday.  There is also something quite large parallel to the SW-NE road which shows in the deeper slices.  The large ditch which runs parallel to that building, just to the west which can be seen in the mag data, shows as a whiter line of “no reflections” in the GPR data.  There is quite a lot going on in this little block of data.

Tomorrow I will work on the other blocks and try to integrate the first four days of survey.  We are not out on site again until Wednesday.

Many thanks to Ellen, Jim, Mike, Graham, Nigel, David and Pauline for all their hard work, and especially to Mike, Jim and Ellen for transporting all the equipment as well as myself!


Working in the rain

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

Today’s weather was “variable”, to put it mildly.  I ended-up with slightly sun burnt legs, but at lunch time we were shivering under a tree as the hailstones hammered down.  Nevertheless we got quite a bit done, and so many thanks to Dave, Anne, Ellen, Pauline, Mike, Jim, Nigel and Graham for persisting despite the dark clouds and the occasional flash of lightning and thunderclap (thankfully a long way off!).

Some surveys we get results where a great deal of squinting is required and some optimistic mental joining of dots is needed.  Today wasn’t one of those days!  Let us start with the Earth Resistance survey.  We decided it would be interesting to run the meter over the nice house on the hill for which we have mag and two GPR data sets.  The first image shows the magnetometry data from two years ago, the second today’s resistance data.

Fig. 1: the magnetometry data of the area with the “house on the hill”.

Fig. 2: the Earth Resistance data of the “house on the hill”.

The Earth Resistance data has been high-pass filtered to bring out the building nicely, but I think you’ll agree it is pretty clear.  It is interesting that the rooms on the western end of the house are showing much more clearly than they did in the GPR survey that we did two days ago (see this post).

Having wiped the GPR’s memory, we completed the grid square from yesterday and then managed another grid and three-quarters further south. Tomorrow we have to do the jaggedy bits along the edge of the field.  First of all, lets look at the mag data.

Fig. 3: the magnetometry data of the area covered by the GPR survey.

The strong linear feature running diagonally across the lower part of the image is the “1955” ditch, the 1st century boundary of the town which had a section cut across it by Sheppard Frere in 1955. The “house on the hill” can be seen in the middle towards the top.

Now for the GPR.

Fig. 4: the GPR survey at the end of day 3.

The grid below the house on the hill is mighty dull.  There is the faint building we saw yesterday, and one or two other possible features, but nothing too exciting.  The grid towards the bottom though shows as lovely town house facing SE.  The next image zooms in a little.

Fig. 5: the town house in the south of today’s GPR survey.

It looks like it lies on the road which runs along the inside of the 1955 ditch. Sweet!

The new software has a variety of palettes.  You have seen the BW one, and the default colour scheme No. 7.  The software can create a composite of all the palettes.

Fig. 6: a plethora of palettes.

Which do you prefer?  Answers on a postcard, please…

The weather forecast is better for tomorrow.  Fingers crossed!

9999 is the number…

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

….not of the beast, but of the number of transects we have surveyed with the GPR.

Fig. 1: Nigel completing transect 9999.

Fig. 2: line 9999 completed.

The completion of line 9999 is of interest because (9999 x 40) / 1000 = 399.96km.  That’s a lot of GPR data…!  Secondly, we were wondering what happens after line 9999.  Does the machine reset to line 0?  Does it go on to line 10,000? Or, millennium bug-like, does the machine die horribly?  Well, on the next line the machine went on to line 10,000.  Yay!  OK!

But…  yes, there had to be a but…  Sadly, the next line was also 10,000, as was the next one, and the next one!  We needed to reset the clock back to zero, but the only way to do that is to download the data, then delete all the files. Unfortunately, I hadn’t thought to bring a USB stick, so we had to head home early.  Well, Friday was always known as POETS day, wasn’t it?

The one good piece of news is that I managed to get the grid in the right place.  The next two images are the underlying mag data, and then yesterday and today’s GPR timeslices.

Fig. 3: the underlying magnetometry data.

Fig. 4: The GPR data.

As you can see, the lower block of GPR data, which we surveyed today, seems remarkable unexciting.  The slices largely reflected cultivation patterns.  I was playing with the new software and tried other colour palettes.  How about traditional black and white?

Fig 5: the less colourful version of the GPR data.

It is faint, but at the northern edge near the southernmost corner of the townhouse we surveyed yesterday, is a faint rectangular building.  In the colour image it is impossible to see it, but in the BW it is faint, but clear. Lesson to self, the “boring” BW palette actually works quite well!

Thanks to Mike and Nigel for coming today.  Tomorrow we soldier-on with the GPR’s memory freshly-wiped.