Talking, for a change

Occasionally, I will stop walking around with a machine that goes beep for long enough to actually talk about what I have been doing. If you live in the South-East of England, you might like to hear me waffle on about geophysics and Romans at two events this year.

Sussex Archaeological Symposium 2013

Date: Saturday 27th April 2013
Venue: The Fulton Building, Lecture Theatre A, University of Sussex, Falmer
Programme: Here
Application Form: Here

This will be a catch up talk, from when I last talked at the Symposium, two years ago. I've discovered a lot since then. Obviously, they have put me in the graveyard slot, after lunch, to wake up the weary troops with my superb oratory prowess (no laughing). Many of the other talks are on the Roman period, my favourite, so I'm very much looking forward to the Symposium this year.

CBA SE: LANDSCAPES OF SOUTH-EAST BRITAIN DURING THE ROMAN PERIOD

Date: Saturday 16th November 2013

Venue: The Assembly Rooms, Preston Street, Faversham, Kent

Programme: Here

This conference is right up my street, as a lot of what I do is a combination of geophysics and landscape archaeology. A lot of what I will be talking about will be the same as at the Symposium, but it wont be a catch up, so I will be taking a more general view of things, plus of course talking about what I find over the course of this year.

Latest Results: Barcombe

This is the big one. I actually did the bulk of this survey a couple of years back, but it has been kept under wraps until the fields had been cleared of coins, to avoid the place being looted by nighthawks. Now, finally, I can talk about it. This survey was done as part of the Culver Project and I thank the members for their help. We originally started this survey on the site of Ivan Margary's section 14 on the London-Lewes road  (See his book, "Roman Ways in the Weald"). Margary had mentioned that he had found pottery in the trench, by the side of the road he had exposed. Occupation is is good for showing up the Roman road ditches using magnetometry on clay, which are invisible on that geology otherwise, so we thought it would be good to look at what was there, and try to find out what was going on with the road further to the south. What Margary didn't know was that he was only a few metres short of the end of the road, and in the middle of a large fortified settlement, making this road the Barcombe to London road instead.

The northern road stops at an east-west road, most likely the Greensand Way, which is somewhat further to the south than Margary anticipated, and also continues to the east into the weald, most likely towards the settlement at Arlington. The settlement itself is quite substantial, with a defensive enclosure 165m square, which seems to cut a lot of other features. Many settlements were enclosed in such a way in the late second century, against a threat not well understood, apparently cutting through existing buildings and roads in the process, suggesting the locals weren't responsible for their construction. In Sussex, the roadside settlements at Alfoldean and Halland have defences dated to this period, though Chichester is a notable exception in Britain, with its walls being built at a later date.

The settlement and road network are surrounded by a number of plots of land enclosed by ditches. They may be fields, or small-holdings, or purely used for occupation, for which there is plenty of evidence in many of them on the geophysics results. One of these enclosures next to the east-west road seems to have been partially replaced by the northern road up to London, suggesting that the east-west road and at least part of the settlement were established before the construction of the road up to London.

Part of this settlement is being excavated this year, and is open to all for excavation, so if you wish to dig on this fantastic site, you can. I shall certainly be spending some time there. It is always nice to see my geophysics excavated, as I learn so much each time. I hope to see you there.

Version 1.1 of Snuffler Released

The latest version of my geophysics software, Snuffler,  has made an appearance. What wonders are there to be seen this time around?

The main new feature is channel merging. This is where you can display multiple plots on the same image, for example a magnetometry and resistivity plot. The one restriction is that the grid layouts of the various surveys must be on the same alignment, but apart from that, the grids needn't be the same size, shape, be in the same place, or be at the same resolution. Here is an example. The survey is of a  medieval farmstead that was in use until quite recently, and is now in a woodland, under some trees. The magnetometry is in red, and the resistivity is in green. As you can see from the top-right corner of the image, there is a high resistance feature that has a large magnetic halo around it. This is the whole point of the channel merging idea, comparing the location of features on different plots.

Also added is manual destriping. If you have areas of particularly high magnetism in your magnetometry survey, the normal destriping tool may struggle to get it right. You could always use the Modify Selection tool, but that isn't particularly fast. Now you can simply select a line to modify using Ctrl  + mouse drag, and then use the two new buttons in the toolbar to add or subtract 0.5. The selection of the line will be hidden, though not de-selected, to aid visual comparison with adjacent lines.

Finally, I wanted to speed up the screen drawing a bit, as Snuffler can be a bit sluggish at times, especially with large images. One of the things that was slowing it down was having to support the dot density plot. I was thinking of how to speed up this part of the software, when I suddenly realised, 'Why on earth would you want to use the dot density plot?'. Let me give you a bit of background on that. When archaeological geophysics first started out, computer equipment was a lot less sophisticated. Everything was in monochrome, unlike today, where you can display and print in shades of grey. A 4x4 square of pixels, with various pixels black or white, will give you 16 'shades'. This restricts you to having a single reading a minimum of 4 pixels wide, and it still wont look anywhere near as good as a proper grey scale. The dot density plot is an anachronism, and needs to go. No-one should be using it any more, so the choice to me was clear, dot density had to go. But where I taketh away, I also giveth. I had been asked if I could provide a greater number of grey shades for the display. Snuffler had supported 16 and 32 shades of grey, now there is an option for 64. When I first wrote Snuffler, 16 shades of grey was 'browser safe', i.e. you could be sure that any browser could display all the shades. I'm sure browser technology has moved on since then. Not all surveys will benefit greater from 64 shades of grey, but here is part of an image which shows that off to an extent. The graduation of colours shows up nicely in the alluvium next to the Roman port.

The difference between the 16 shades at the top and the 32 shades in the middle is obvious, but I must admit that I struggle to see much difference between the 32 shades and the 64 shades at the bottom. I am assured that some people can. On the subject of shading, I found some interesting blog posts about why colour shading is a really bad idea, but don't worry, I wont be removing those.

You can of course download the new version of Snuffler at the usual place.

What to look for in a magnetometer

I have been looking at buying a GPR recently, and trying to get decent comparative information between makes and models is difficult to say the least. It occurred to me that other people must have the same problem when buying a magnetometer, so I thought I would write a quick guide detailing what knowledge I have gained over the years. I wont be discussing anything too expensive, like alkali-vapour magnetometers, as they are outside most peoples budgets. I will stick to fluxgate magnetometers. There are three makes I will discuss.

Geoscan

Geoscan make the FM256, a fluxgate gradiometer with 0.5 metre sensor spacing. I have personally used its predecessor, the FM36. I am not entirely sure of the differences between the two models, though I gather than the main points I will touch on have not changed.

Bartington

Bartington make the GRAD601, a fluxgate gradiometer with 1 metre sensor spacing. I personally have a GRAD601-2, the version with two sensor columns.

Foerster

Foerster make the Ferex. I personally have no experience of these devices, and what I have to say on the matter is merely what I have learned from other people and the internet.

With all that in mind, I will discuss the differences between the devices in various categories.


Sensitivity

One of the most important things to consider is the sensitivity of the instrument, and how good it is at picking up the slight changes we are looking for in archaeology. In my personal experience, the Bartington is more sensitive than the Geoscan, perhaps helped by the longer sensor columns. Apparently, the Foerster is not very sensitive. The Bartington wins here, not sure who comes second, but I would guess the Geoscan.

Setup & Stability

Fluxgate instruments, being directional in nature, need to be balanced before use. The Geoscan instrument has a manual process, where physical knobs are turned to align the sensors. The process is somewhat time consuming, and is not helped by the device suffering a lot from thermal drift, so you may find yourself realigning the sensors after each grid. The Bartington is much better here. It has an electronic balancing process, which calculates the differences in sensor alignments and compensates electronically. It is also helped by being very temperature stable. I only tend to balance it a couple of times in a day. The Foerster is apparently set up in such a way that it doesn't need balancing. I'm not quite sure how this works, but it seems to do so, thus the Foerster wins here, with Bartington second.

Array Options

The Geoscan instrument has an option to carry two separate devices on a carrying frame, with a single button to start recording. Each device has to be balanced and downloaded separately. The Bartington comes in the single column GRAD601-1 variety or the dual column GRAD601-2 variety. You do not need  separate balancing and downloading for each column with the GRAD601-2. Because of the lack of setup needed with the Foerster instrument, it is easy to have a large array of devices, perhaps towed behind a vehicle even. Foerster wins this one with the Bartington second.

GNSS Integration

The geoscan instrument has no GNSS integration. The Bartington has an option of a separate data recorder that uses GNSS, but that will not do normal gridded recording. The Foerster has full GNSS integration, which helps with its cart and vehicle towed setups. The Foerster wins this one with the Bartington second.

Reliability

The Geoscan has a very good reputation for reliability, these things never seem to go wrong. The Bartington has a poor reputation for reliability. Personally I've had to have my machine repaired twice. Once to replace the motherboard in the data recorder, and once to have a sensor column rebuilt after water got in, causing thermal drift. They can be damaged by rain, especially after seals have perished. Other people I have talked to have had a similar experience. I don't have any information on the reliability of the Foerster instruments, but I would guess somewhere between the other two, so Geoscan wins this one with Foerster second.

Cost

I am somewhat lacking in information here. All I can say that is concrete is that my GRAD601-2 cost me £10,500 a few years back. I gather that the GRAD601-1 and FM256 are roughly the same price, but for two sensor columns, the GRAD601-2 is much better value than buying two FM256's. I know nothing about the Foerster prices. I can't call who wins this one, get some quotes.

Conclusion

What I would recommend going for depends on how you will be using it. If you are surveying using a gridless GNSS technique, then the Foerster is probably your best bet. If you are doing a gridded survey, I would recommend the Bartington. If anyone out there has further information to contribute to this guide, especially regarding the Foerster instruments, please leave a comment below.

NSGG Conference 2012

I managed to get to the Near Surface Geophysics Group Conference this year. It was good to chat to a few old faces, such as swapping geophysics software notes with the author of Archaeosurveyor, a thoroughly nice chap. I also spoke to Erica Utsi about buying a GPR, but I have yet to hear from anyone who has used one yet. I was quite impressed by what I heard, but I haven't got the money to buy one quite yet. Sometime next year hopefully. So what about the talks. Many of them used the sort of equipment that you can buy when money is no object, as it seems to be for academic departments, but it is not all about the bling. It's not even about the pretty pictures, though that helps. It's about some of the new ideas and how people go about things differently. Here are some of the highlights for me. 

James Bonsall talked about a new EM instrument called the CMD Mini-Explorer. I hadn't been hugely impressed by the results from EM in the past, but the results shown in the talk were quite impressive. The instrument takes both In-Phase and Quadrature readings at three different depths, increasing the chance that you will find something. The speaker said it gave better results and was easier to use than the Geonics EM38, though someone from Geonics in the audience suggested that a lot of the problems that the speaker had described had been sorted in the EM38 MK2, which takes readings at 2 depths compared to the 3 of the CMD. It would be interesting to hear from someone who has used both of these.

Armin Schmidt talked about GPR. This time, he took data from a Roman era cemetery and converted the raster data to vector data. This allowed him to use various GIS functions to process that data into a more agreeable format for viewing in 3D. You don't get the fine detail, but it makes things much easier to see for the average person not used to staring at geophysics plots.

James Lyall, famous in geophysics circles for the giant survey in the Vale of Pickering, talked about a national archive for geophysics results, much in the same way as aerial photography achieved. It is actually quite hard to get your hands on the data for any given geophysics project, and it is rare for any geophysics practitioner to store the data in any readable form outside the survey report. It is certainly possible, but it takes time and is expensive, so most surveyors don't. James asked the audience to get their collective heads around the problem.

Robert Fry talked about the work of the DART Project, which is something I've had my eye on for a while. One of the things they are attempting to do is see how earth resistance changes over the course of a year. Robert explained how they found a ditch feature using magnetometry, then put a series of fixed resistance probes across it in order to find out how the contrast between the ditch and its surroundings changed over time. Not surprisingly, the very wet weather in 2012 made everything waterlogged and made the ditch all but invisible to earth resistance for most of the year.

James Bonsall (again) talked about ground truthing geophysics data by comparing geophysics survey results to excavation results in Ireland. Most of the work was done with magnetometry, and the results showed a big difference depending on which geology the survey was taken, with limestones suffering. The results were broken down into true-positives, where both geophysics and excavation found features, true-negatives where neither did, false-positives where the geophysics found something but the excavators didn't, and

false-negatives, where the excavators found something that the geophysics hadn't spotted. The speaker suggested that for certain geologies, alternative methods to magnetometry, such as EM, should be used. Some members of the audience didn't agree with this, and suggested that, in particular, there wasn't a problem with limestones in mainland UK. Some also suggested that many of the false-positives were down to the excavators machining through shallow features, which everyone seemed to agree with.

Just for the pretty pictures this one. Lieven Verdonck demonstrated the sort of results you could get when you perform a GPR survey on a Roman town in Portugal at an absurdly high resolution. Nice if you have the time for it, and the results were certainly worth it, with very clear high resolution wall lines.

Closer to home, Paul Cheetham has been doing a very similar thing in Dorset as I have been doing in Sussex, and examining Roman rural settlement on a grand scale. The sort of results he was getting was quite familiar to me, and made me feel right at home amongst all the speakers with their expensive bling machines.

Latest Results: Oaklands Park

Finally, here is a blog post about the big Independent Historical Research Group survey I have been working on this summer, which is at Oaklands Park, Sedlescombe. Oaklands Park is one of the 'Big Three' Roman iron-working sites by volume of their slag heap, and right next to the Roman road down from Bodiam, so we approached Pestalozzi, the children's charity, who own the land, and they very kindly agreed to let us survey it all.

Margary's line for the main Roman road from Bodiam is marked in green, but for reasons I wont go into just yet (we are still investigating), that is somewhat in doubt south of the river. The site wasn't quite as big as we expected it to be, but there are certainly some very interesting features. To give you an idea of the geography, the playing field you can see towards the northern end is the floodplain of the River Brede, and the site is on the side of a hill, rising to the south, with a paleochannel cutting through. The paleochannel is visible on the results running north-south towards the eastern end of the main survey area. Iron ore can be found on the top of the hill to the south.

The main iron-working area is pretty obvious, hugging the north end of the field, which would also have been the northern edge of the land, with water coming close to this point in Roman times. It has previously been supposed that there was a port here in Roman times, which makes sense. They used to bring coal up the River Brede to Sedlescombe into Victorian times. A couple of enclosures can be seen towards the eastern end, but apart from these, there is a lot less settlement that we expected. Much of the local settlement may be towards the west, under the trees and houses.

Tracks seem to lead everywhere. Here are a couple to note. A track leads south, joining the edge of the paleochannel. This seems to head towards the top of the channel, which also seems to have been dug out to exploit the easy access to iron ore that the channel provides. There seem to be two tracks leading out of the iron-working area to the west. They both appear on the other side of the road, in a field owned by Luff's Farm, where they can be seen to join just before heading around the hill to the south.

IHRG are far from done at this site, there is still more to investigate. Most importantly, the main Roman road to the south, which most likely, is not how Ivan Margary envisioned it.

Latest Results: Ringmer Again

As an update to This Post about Roman road hunting around Ringmer, I've been spending more time with the Roman Ringmer Study Group to track more of this road. We did another survey in the field to the east, and found more of the road there (see image below), which also showed the side ditches a bit better. They are roughly 20 metres apart. The group also did some test pits on features in the first field, the road is flint metalled, with the occasional bit of iron. The other features in the field turned out to be geological, most likely that annoying gley stuff again. This is probably down to the field being quite boggy. Annoying geology is annoying. The full report for this survey has now been written. You can find it here.

Further to this, the Roman Ringmer Study Group very kindly left their home parish to track the road a bit further east. The image below is just west of Laughton Place, which is owned by the Landmark Trust. The road was found to head along the northern edge of the moat, which could possibly mean that the road was still in existence in some form in medieval times. Hopefully the course of the road will be fully mapped out at some point, but the course, at least to the east, is still a bit of mystery.