A little damp

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 a little changeable. For a while at lunchtime we thought it was going to get the better of us, but it improved a little and we managed to complete five 20x20m res squares and a 80x40m block of GPR with a hole in the middle!  I was less lucky on my way back from Welwyn later in the evening.

Fig. 1: a little light diffraction.

The Earth Resistance survey continues to find the line of buildings along the road to the east of the Insula XVI temple.  The next three images are the data plotted normally (Fig. 2), then with a high pass filter (Fig. 3), and finally the mag data for the same area (Fig. 4).

Fig. 2: Earth Resistance data, day 11.

Fig. 3: Earth Resistance data, day 11, high-pass filtered to show the buildings more clearly.

Fig. 4: Mag data in the area of the Earth Resistance data, up to day 11.

The line in the data to the east of the “house on the hill” marks the edge of the survey before the huge deluge we had a week and a half ago.  Slightly annoying, but inevitable on a large survey like this.  Hopefully, I can process it out.

The GPR was filling-in a gap today which has annoyed me all year long!

Fig. 5: filling in the gap. Today’s block is marked in red.

Here is a composite image of the time slices from today’s survey.

Fig. 6: GPR time slices of today’s block.

Although Fig. 6 shows bits of wall, and areas of high reflection, nothing resolves itself into a clear building.  Partly this is because of the hedgerow: above it there is build-up of soil, below it is likely to be some erosion. However, the mag data does not show much in this area either.  It does appear that the road has been robbed!  The next set of figures show some time slices and then the mag data for the same area.

Fig 7: GPR time slice 7 from day 11.

Fig 8: GPR time slice 8 from day 11.

Fig 9: GPR time slice 9 from day 11.

Fig. 10: mag data from the area of the day 11 GPR block shown in figures 7–9.

Tomorrow the Earth Resistance meter will be working its way westwards, and the GPR will be completing a block against the hedgerow.  Should be some buildings showing there!

Thanks to Ruth, Anne, Jim, Mike and Dave for braving the weather.

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A good day

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

Today was definitely a good day. Although it was raining early in the morning, the weather had started to cheer up by the time we were on site, and was glorious in the afternoon.  Ruth, Adrian and I worked with the Earth Resistance meter, now restored to its normal operation.  Jim, Mike and Dave were up on the hill with the GPR.

The Earth Resistance team managed an excellent eight grid squares today, and there is now a link to the 2016 survey. I wonder if it was a reaction to the go-slow of the last three working days?  Fig. 1 shows everything we have done at Gorhambury using the Earth Resistance meter.

Fig. 1: the whole area surveyed with the Earth Resistance meter as of the end of 17/08/2017.

The next image shows the mag data from the same area (Fig. 2).

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

Over the next few days we will be working our way west filling in the gap between the two surveys.  That annoying hedge line will slow us down a bit. The next image shows a high-pass filtered version of the area surveyed in 2017. The filter flattens out the background variation and thus makes the buildings stand out more.

Fig. 3: the 2017 survey with a high-pass filter to bring out the buildings.

Unfortunately, my trick of spreading the remote probes wide apart to remove the edge-matching problem didn’t work so well here, so I have to play with the data some more to get a good images.  Today’s survey picked-up the line of buildings running SSW-NNE at the top of the plot.  These buildings lie along a road which run alongside the temple.  In the next day or so we should pick-up the cross roads with the road which runs parallel and slightly to the north of the hedge line.

The area covered by the GPR in the last three seasons is quite impressive (Fig. 4).

Fig. 4: the complete GPR survey.

Fig. 4 is a nonsense mish-mash of images from multiple depths, dates and even software packages, but it at least gives one an idea of the amount we have done, and serves to remind me about that annoying hole in the data around the hedge line.

The next image (Fig. 5) shows everything we have done in 2017.

Fig. 5: the 2017 GPR survey so far.

Fig. 6 shows the area surveyed today (with the yellow border).

Fig. 6: the area surveyed on day 10.

There is a substantial building about one quarter of the way along the area surveyed. and another one parallel to the 1955 ditch.  It is useful to look at the mag data (Fig. 7).

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

There is a magnetic anomaly in the same place as the substantial building, but I am pretty sure I would not have interpreted it as a building.  Although the building plan in Fig. 6 is obviously incomplete, it is certainly there which is very interesting given its position high on the hill outside the 1955 ditch. The building which shows in the topright corner of the GPR plot, doesn’t show at all in the magnetic data.  We have seen this before when we have got towards the edges of the town.

Many thanks to everyone who helped today.  Tomorrow the GPR will fill the annoying gap and the Earth Resistance meter will work slowly westwards.

Worn down

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

After three days of using the beast, the Earth Resistance team are feeling worn down. Not with the effort, but with boredom! We have completed a 60x40m block over the “House on the Hill” and that is enough. Tomorrow we return to “normal res” hoping to cover the same size area in a single day, but with just a single depth.

Some of you may remember that the Earth Resistance meter was brand new last year.  The meter came with six steel probes to allow for various configurations, of which we have used three extensively, and three very occasionally.  Clearly we have been busy if Fig. 1 is anything to go by.

Fig. 1: the probes before and after some use.

A small problem with today’s Earth Resistance survey was resolved with some programming in the statistical programming language R.  Here are the six depth slices along with the Wenner array image.

Fig. 2: the multiple depth Earth Resistance survey.

I am very pleased with the result, and it will be interesting to compare the images with the GPR time slices for approximately the same depths.  I think we can safely say we have “done” the house on the hill.

The GPR completed another 40x80m block up on the hill to the south of us. Firstly, a composite of the time slices.

Fig. 3: The GPR time slices for day 9.

Despite some strong reflections, nothing much resolves itself into anything very intelligible.  Slice 1 to 3 are basically the topsoil, and slice 10–12 are nonsense from the bottoms of the profiles.  Slice 6 does show an empty space where the 1955 ditch is.  A composite of days 8 and 9 in Google Earth, plus the mag, makes this quite clear.

Fig. 4: GPR results from days 8 and 9 showing the 1955 ditch.

Fig. 5: the magnetometry data with the area of the day 8 and 9 surveys shown as a yellow box.

As we get up here on the heights, there is less in the way of obvious buildings showing on the mag survey, but there are some intriguing enclosures.  Elsewhere at Gorhambury was have found buildings in these areas not visible on the mag surveys.  There next few days could be very interesting, or very disappointing!

Many thanks to Anne, Pauline, Jim, Adrian, Dave and Mike for all their help today.

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.