12 July 2015

Digging Up The Geophysics: 2015

The new archaeology digging season is underway, but is far from over! If you fancy having a dig this summer, here are some sites in Sussex under excavation on which I have previously done geophysics. At the end of the season, I will do another post about what they have discovered.

Ovingdean

Brighton and Hove Archaeological Society are again digging at the medieval site at Ovingdean, where are are uncovering part of the enclosure around the manorial enlosure. What they have looks much more substantial than a simple retaining wall. Are there more buildings against the edge of the enclosure?




Plumpton

The Sussex School of Archaeology are running a training dig at Plumpton Roman Villa. The digs run during the week and are targetting the eastern part of the villa.



Barcombe

Culver Archaeology Project are digging again at Bridge Farm, where they are targetting the intersection ot two Roman roads and the defensive enclosure around the centre of the settlement. When I visited, they were exposing a possible cremation burial and had found an intaglio. Their site has a weekly blog to keep up with what is going on.







24 June 2015

Version 1.15 of Snuffler released

A small update to Snuffler this time around, though bigger things are already in the pipeline for the future. This time sees the addition of an import for files generated by the new Frobisher TAR-3 earth resistance meter. Low cost options such as this are the way that community archaeology groups get started with geophysics, so it is always nice to see new options on that front. I haven't used the new hardware myself, so I can't comment further on it, but I wish them all the best in their new endeavour.

You can download the new version at the usual place.

This blog has been fairly quiet as I have been working on some big new projects and finishing work already reported on here, which you will hear about later. In the mean time, here is a picture from the Severn Sisters, where the National Trust are running an archaeological project this year. This is from Bailey's Hill, where some possible Bronze Age remains are due to be excavated in August. If you would like to get involved, you can!


26 May 2015

New Toy: Survey Grade GPS

I have a new toy! Oh joy, a new toy! This isn't actually a new piece of geophysics equipment, but survey equipment. Having previously set out grids using a total station and recorded data on an arbitrary grid, I can now get absolute coordinates, set grids out much quicker and with only a single person. I went for net rover rather than rover plus base as I wanted decent coordinates in the field rather than post processed.

I eventually chose a Javad Triumph-LS. There were several things that attracted me to this particular model. It received all signals from all constellations (so futureproof and accurate), it was cheap (relatively!) and the company had made an effort to deal with interference due to the lightsquared debacle, which is good for me, because part of its job will be sitting on top of my radar antenna. The unit even gives you a relative quantification of interference it is receiving. I put it on top of my radar, turned off, and the value was 4. Turned on, it was 22. Rather than having the receiver on top and data collection half way down the pole, both are combined at the top of a mono-pole. It does seem a bit top heavy and difficult to keep steady, but it will correct itself for pole tilt, which is a nice feature. Control is via a number of hardware buttons and a capacitive touch screen like a smartphone, so the whole thing seems very modern. Getting started with the kit, the support on their forums was excellent. On the downside, I found a few things unintuitive, but most of those can be put down to me being a geophysicist rather than a land surveyor. The receiver has not long been released and the manual still needs a bit of work to deal with the likes of me.



So that's the hardware, which I'm very pleased with. I also needed a network RTK corrections subscription. In the UK, all of the correction services are based off of the base stations run by OSNet from the Ordnance Survey. Of the five different services that use OSNet hardware, two are aimed at farming. While these are cheaper, they only connect you to the single base station, which is fine if you are near one, but not very good if you are not. You can only expect to get 5cm accuracy with these services. The other three services, SmartNet from Leica GeoSystems, VRS Now from Trimble and TopNET live from Topcon do things differently. They create a model of the atmosphere based on a number of base stations and create a Virtual Reference Station based on your current location, giving you a potential accuracy of 1cm. Apparently all three services produce a similar level of accuracy, but they don't give any information on which constellations/signals are corrected or whether they provide any additional base stations over and above those provided by OSNet. Price wise, at the time of beginning to look at all this, their websites showed the Leica and Topcon services charged £1200 ex vat per anum for the limited (40 hour per month) service, while Trimble charged £1500 or £1300 sans SIM card.

Trying to sign up to one of these services turned out to be a tale of woe. I started out contacting Leica. The contact email address and price on the website was wrong (now fixed) and the price is now £1260. After speaking to someone by phone and then emailing, contact went dead and they stopped replying to my emails. Not very good if they don't even want to sell you something. I next tried Topcon. I filled in their web form and got an email saying that my registration was confirmed... then heard nothing for a week. Next I tried Trimble. I filled in their web form, which then demanded that I give them my VAT number. Not even having a company, I don't have one of those, so Trimble fell at the first hurdle. They were too expensive anyway. I went back to Topcon, having noticed another contact email in their confirmation email. Once I had the attention of a human, service was prompt and helpful. There were further problems, like the first SIM card they sent got lost in the post and the second one was the wrong size (mini instead of micro), so I cut that down to size, but I messed it up and cut it slightly wrong.  They sent me a SIM of the correct size and all was well in the end, so Topcon came out on top(con). Connecting to the service, I got my desired 1cm accuracy. According to the receiver, it was receiving correction for GPS + GLONASS. Hopefully, the Ordnance Survey will upgrade their base stations to cover Galileo now that constellation is coming online.

One thing I wanted to test is that the receiver supported the black abomination that is OSTN02, a modification of OSGB36. I surveyed a random point in ETRS89, converted it internally to OSGB36 and did the same conversion with the Ordnance Survey's own coordinate transformation tool. Initially, it seemed it didn't, as the difference was about 2.06m. It turned out that I also had to change the datum used by OSGB36 to the Newlyn datum, which made it produce the same values as the OS site, so all good there. I also wanted to test how accurate Google Earth imagery was locally, so I recorded a bunch of points in ETRS89, converted them to WGS84, as used by Google Earth and converted it to kml. Here is a picture of how that turned out. Looks like the Google Earth imagery is very good indeed!


How do I use this fantastic new toy in the field? With the total station, I used to set up the total station at one end of a baseline along a straight edge of a field, tell it that it was at 500E,500N, point it along the baseline, then tell it that it was facing something like an angle of 270 (west), even though it wasn't. Then I could just go and look for 460E,500N etc. For reestablishing the grid, I used to record two resection points that could be described to a few cm, the downside being that those points could disappear, which did happen in a couple of cases. For placing on maps, I recorded a bunch of points at the edge of the field to match to the edge of the field on maps or aerials. The process is described in more detail here.

With the GPS, I start by collecting the same point I originally would have occupied with the base station, then setting it to stake out to that point and walking 120m away along the baseline. I then do a Multiple Point Localization on those two point, using coordinates of 500E/500N and 380E,500N, which will create a new projection which I can use to walk to further points just as with the total station, only with a single person instead of two. The two points used for the localization are recorded in lieu of resection points, but the new projection is stored on the device, so they are only needed if other people are re-establishing the grid. Since I get absolute coordinates, there is no need to record points around the edge of the field to overlay the geophysics correctly, so everything becomes much faster and more reliable.

Now to get out and use it. I'm going to be doing a big radar survey of Chichester in July, so watch this space for some results from that.

26 December 2014

Version 1.14 of Snuffler released

My last post of the year will be for an update to my geophysics software, Snuffler. The new feature this update is an extension of the support for RM15/MPX15/RM85 multiplexing. As well as doing parallel readings, this update will now cope with readings on multiple levels. It is a bit of a nasty hack as far as the user interface is concerned, but it works. Here's how :

Normally, you download your data into an import file, then export from that into grids. Job done. With multiple levels of multiplexor data, there is an extra stage in the middle. In the initial download, you select the total number of readings recorded each time you put the probes in the ground. For example, if you record two readings with 0.5m probe spacing and one reading at 1m probe spacing, that would be a total of three readings at each point.

When you export from this import file, rather than getting grids, you will be asked which readings you will be exporting at this level, creating a new import file containg just the readings at that level, from which you export grids in the normal way. In the above example, you would create two new import files from the original import file. The first file would contain readings one and two and the second file would contain reading three. Full details on how to do it properly are listed at the bottom of the Import Files section of the help file.

I didn't have access to a machine when developing this, so it may not work perfectly. If you try this yourself and do have problems and the help file is not being helpful, please let me know. Many thanks to two of my users, Helen and Manuel, for helping me test this.

You can download the new version at the usual place.

08 December 2014

Latest Results: Ovingdean

Brighton and Hove Archaeological Society are a very active group fieldwork wise, with an insanely long digging season and a lovely supportive approach to new diggers. It was with BHAS that I started doing geophysics, with Bill Santer teaching me how to do earth resistance. Bill sadly passed away this year, so I would like to dedicate this survey to him, the fantastic bloke who started me on this path. He will be sorely missed.

The site that BHAS have been digging this year is at Ovingdean, where there is a medieval manor complex within an earthwork enclosure next to the church. They have done a few seasons of excavations here on one side of the enclosure, based on some earth resitance they did before I started doing archaeology. They have excavated the main manor house, which has very chunky walls and an undercroft, and this year they have been excavating what looks like a post built barn structure next to it. There is still the other half of the enclosure unexcavated, and BHAS wanted to know what was there.

First, the original earth resistance, flattened with a high pass filter so you can see the main manor building amongst the rubble near the churchyard wall to the south-east. A trackway snakes through the middle of the enclosure from an entrance in the south-east and possibly out the other side to the north-east. To the north-west, parts of the enclosure revetments and bits of masonry buildings up against the earthwork enclosure can be seen.

Earth Resistance. Click for a larger image.

This year's surveys include magnetometry and GPR. Being chalk, the magnetometry results are predictably rubbish, but do show hits of the enclosure to the south-west. Much of the north-west part of this survey is obscured by the magnetic halo of a large water unfortunately.

Magnetometry. Click for a larger image.

The GPR was a lot more productive, showing what looks like an open sided barn and an attached dovecote against the north-west enclosure earthwork. Signs of the outer enclosure revetment are visible in other layers further down.

A single GPR slice. Click for a larger image.


05 December 2014

NSGG Conference 2014

The time came once again to attend the Near Surface Geophysics Group conference, which is held every two years, the last being in 2012. As always, the chats in between lectures were the best bit. My ego was thoroughly strokes by three people coming up to me out of the blue and complimenting me on Snuffler. It's a wonder I got my head out of the door at the end of the day. I also spoke at length to Erica Utsi to enquire about attaching my forthcoming purchase of survey grade GNSS to my GPR.

The talks were many and varied. There were 12 of them! My favourites were :

Kris Lockyear talked about getting community archaeology groups involved in geophysics and some of the results they had. The survey of part of Verulamium. A grant bought a fancy mag cart, which seem to be everywhere these days, which the various groups share, with training days on how to use it. This sort of work is excellent for getting smaller groups who otherwise would not have access to geophysics. They have an excellent blog.

The talk I was most looking forward to was Armin Schmidt (of course) talking about inversion modelling for magnetometry. It seems the subject is a lot more subjective than I imagined, with various models of 3D interpretation potentially fitting the 2D data. I asked where I could find out more about how this was done, as I was thinking of incorporating this into Snuffler. The ripple of laughter that went around the room suggests that this is a black art best left to the damned.

Pope-Carter (I think) spoke about some open source software for geophysics being developed in python by students at Bradford called ArchaeoPY. It was explained that a lot of the work of writing display stuff was already done and available in easy to use python libraries. A lot of the functions already available in the software were shown. The whole thing is impressive and has a lot of potential. Being open source, if there is a feature you think is missing, you can go ahead and add it yourself!

There were two interesting talks from overseas. Someone from Italy talked about the sites found there, including GPR over a proper Roman road with kerbstones and all, which was very nice to see. I do like a bit of Roman. Someone from Canada produced some actually really good plots using EM of buildings, probably because the remains were shallow. Perhaps EM isn't so bad after all, or maybe you just have to do it at the insane resolution that they did. Both speakers lamented the lack of understanding of geophysics there by the relevant cultural authorities.

More Roman from the Canterbury Hinterland Project, both close to me and Roman, my favourite. It is run by institutions outside of Kent, since no-one in Kent seems to do geophysics. Some of the buildings they found using high res radar were quite impressive, and odd looking. There was also an interesting attempt to clean up the rather messy GPR data, using a different method than Armin suggested two years ago. I think this was my favourite talk of the day.

Then, of course, there was the usual posters, many more this time around. The judging was by popular vote rather than the usual panel, so I'm wondering if it will end up a bit like the Eurovision Song Contest with everyone voting for their mates. I certainly did :)

Hardware wise, mag seems to be heading for multi-sensor carts, some of which were evident in the talks, posters and commercial exhibitors. The greater speed and resolution can only be a good thing. I just don't understand how people afford these things.

All in all, another good year.

06 October 2014

Latest Results: Barcombe

This summer, despite the aweful August, I found time to return to my favourite Roman part of Sussex, Bridge Farm, Barcombe. I wanted both to extend the magnetometry survey even further east along the roadside settlement than last time. I also wanted to examine parts of the settlement with the radar, to see what I could find out about the roads and buildings.

We were also lucky enough to have some interesting crop marks making an appearance in the latest (2014) Google Earth imagery, alongside an aerial of this years excavation by the Culver Archaeological Project. Parts of the main road through the settlement are visible heading WNW-ESE. To the south, the edge of the massive pit(s) associated with the big industrial feature that showed so well on the magnetometry. This extended further to the north than the magnetometry suggested, which showed two conjoined pits, with a third phase showing on the aerial data attached to the north end of them both. This pit, taken as a whole, is about 40m across each way, so this is a massive industrial undertaking. It is likely that clay is being extracted, but whether it for making tiles, pottery or bloomery kiln linings is unclear. In the field to the south of the industry was a number of linear crop marks that didn't look very natural, so that is one of the areas we targetted with the magnetometry. Finally, a new trackway appeared as a crop mark in the field to the east, with a small enclosure attached to it, which is also an area we targetted with magnetometry.

Bridge Farm crop marks. Click for larger image.

Let's start with the field to the south of the industrial area. These crop marks didn't look very natural to me, but as you can see from the geophysics below, they are. There is a hint of something aligned to those crop marks, which is probably geological. Archaeology wise, there are a couple of pits to the north-west and the track coming through from the field to the north in the north-east part of the survey. No need to return for further work in this field, but it does confirm the southern extent of the settlement.

Field south of industrial area. Click for larger image.

The new area at the eastern end of the settlement was much more fruitful with a good number of settlement realted pits visible. The main road heading east towards Arlington and Pevensey shows nicely, with the outer ditches 22 metres apart and a slight remnant of the inner ditches showing at 10 metres apart. Also making an appearance are two new side tracks on the north side of the main road. They seem quite different in construction. The long thin track heading east is only 6 metres wide. This is the track that appeared on the aerial photographs. The small enclosure also appears, but unlike the aerials, where it appears continuous, the rather chunky ditches seem broken here. The second track, which meets the first, is 13 metres wide on the outer ditches and 3 metres wide on the inner ditches. These two tracks seem to disappear as they meet. It is unclear whether either of them continue, or what their function is. Slag metalling can be seen where the larger track meets the main road. It looks like slag has been used to repair the road here, where the road has become worn due to the presence of the junction here. The settlement surrounding the junction seems stronger too.

 The eastern end of the settlement. Click for larger image.

The iron slag is scattered liberally on the surface in the south east corner of the field too, along with a couple of other interesting finds, which you can see below. The item on the left is the iron slag. The item in the middle seems to be opus signinum, the distinctive pink concrete the Romans used for fancy flooring. This seems to have something accreted to it. The item on the right seems to be a small amount of opus accreted to a piece of dressed stonework. Fancy stuff. None of this is likely to have come from the Bridge Farm settlement though. The source will be one of the larger iron working sites somewhere up in the weald.

Slag & opus. Click for larger image.

Finally on the magnetometer front, we resurveyed the area where this year's Culver Archaeological Project excavation took place, at a higher resolution. As well as the posthole building showing up nicely, there are a variety of ditches and pits for the diggers to get their teeth into. The dig was a fantastic success, with the highlight being a number of waterlogged leather and carved wood finds. You can read some of the project's blog posts here, here, here, here, here, here, here, here, here and here.

Higher resolution magnetometry of 2014 excavation area. Click for larger image.

Ortho'd drone photo of the dig after machining. Click for larger image.


Now onto the radar. I have been waiting to do this for quite a while. The wet August weather stopped me getting this done earlier, so I was glad to get out and do this in September. As well as examining the roads, I wanted to look for buildings with masonry foundations. Unfortunately, scanning around the defended part of the settlement with the radar, I found none of the latter, and the former were somewhat less in evidence than expected. I have intended to survey more areas, but scanning across them found nothing worth surveying.

Three radar areas in the centre of the settlement. Click for larger image.

You can see the three radar areas in the above image.  The left most survey area was disappointing. Only a small hint of a surface showed of the main road coming in from the west. The other two roads I was expecting (more on these in a bit) did not show at all. The right most survey area was more interesting, with the main road through the settlement showing nicely. Attached to this was a metalled extention of the road from London, which originally stopped at the road coming in from the west. There are a couple of patches of repair to the north of this, but the other roads don't seem to have been metalled other than that. They don't seem to have been ploughed away, and robbing doesn't seem to be the answer with what is left, so it seems these road were not metalled in the area of the settlement, which is most odd. Most of the other tracks within the defended area of the settlement don't seem to be metalled either, even the main north-south track to the west of the London road. Going back to the extension to the Barcombe to London road, apart from being metalled, which the main road north wasn't, the ditches are narrower and the road is on a slightly different alignment, reinforcing the different date for construction. If you look closely, you can see Ivan Margary's 'section 14' through the road surface, unknowingly only metres from the end of the road. The pottery he found in the road ditches lead me to find this settlement in the first place.

Talking of dates, or at least sequence, the order of building seems to be as follows. First, the road came in from the west and stopped just to the east of the defended area of the settlement. Next, the settlement started, followed by the road heading north up to London. Next, the main road through the settlement was built, with metalling this time, and this headed east towards Pevensey. This would most likely have all taken place in the first century. Finally, the defences were dug in the late second century.

Back to the radar, I surveyed the final area of the three, in the centre, because I could see something going on south of the main east-west road by scanning around with the radar. You can see the metalling of the main east-west road, but scanning in this field and the field to the west showed that this did not extend west out of the defended area. People would most likely have continued on and up to the main road west, despite the lack of metalling, but a formal road never seems to have been built. Attached to, what seems to be, the very western end of the road, a further road has been built at a diagonal to the settlement layout, heading south-west. Metalled surfaces on roads were mainly intended for cart traffic, so why was it heading south rather than north to the main road west out of the settlement? The answer may lie in an area close to the water and just to the south of this year's excavation.

Possible port area. Click for larger image.

That diagonal road leads to another east-west track, which was found to be lightly metalled in last year's excavation. Now look at the radar on the left of the above image. Ignore the two linear features, they are modern cow tracks, but in the north-east corner of that radar suvey is the end of the metalled surface of the east-west track. Also visible is light metalling from the end of another track coming in from the north-east, which is one of the three expected tracks that don't appear in the first radar survey I talked about. So what are these tracks heading for? On the magnetometry in this area are a group of strong metal dipoles, out in what would have been the water in the Roman period. My guess is that there would have been a small port here, which those carts would have been heading for. It seems to be roughly where you would expect such a feature, being downstream from the bridge and close to the centre of the settlement.

Finally, jumping over the river to the west of the settlement. I'd had a theory that the settlement extending east-west in this field was actually roadside settlement along the edge of the Greensand Way, so I decided to test this with the radar. The results weren't exactly spectacular, but did explain what had been going on with this field. The magnetometry results in this field were patchy at best, compared to the excellent results in the field to the south. Because of the gravels in this area, the radar was able to show why. Plough lines were visible 90cm down in the survey area, which is a huge depth for ploughing. Was this field steam ploughed in the past? The surface of the north-south road was visible, but only really at the southern end of the survey area. Similarly, the east-west road was only visible at its western end, as it seemed to be diving down in this direction somewhat, protecting it from the plough above.

Radar west of the river. Click for larger image.

If this is the real course of the Greensand Way, then where does this leave the road network in the area? In the map below, you can see how the road network interacts with the settlements east and west of the river, plus the villa complex. Deprecated parts of Margary's road course are shown in blue. The new course of the road avoids the stream valley north of Barcombe plus the steepness of Crink Hill.

The Barcombe settlement area. Click for larger image.

Finally, a big thankyou to all of the people from the Culver Archaeology Project and the Roman Ringmer Study Group who helped with the surveys. It wouldn't have been possible without you.