29 September 2015

Green Waste, A Growing Problem

A relatively new, but increasing, bane for geophysicists and metal detectorists alike, is the practice of spreading green waste on fields. Part of the drive for increased recycling in society, which is good, this material comes from our gardens, but is rarely pure. Many households are not too fussed with what they will put in their bins, so plastic and metal will end up in the green waste. Farmers will buy this from the council and then use it to fertilise their fields. The metal component of this will cause problems for magnetometers, producing white noise from thousands of tiny dipoles. For example, this is a Roman settlement :

This is a Roman villa :

Once it's in there, it's there for good. Metal detectorists get it worse, as they are also picking up non-ferrous material. There is even a blog dedicated to the problem.

05 September 2015

Latest Results: Chichester

This year, my biggest project was spending a week within the Chichester town walls, looking for Romans with the GPR. The original purpose of the visit was to test the theory that Stane Street did not start at the east gate, but actually went through the Roman town and out the other side. It soon became clear that the archaeology was too shallow to be visible amongst the modern services. Instead, I ended up surveying some grassed areas to look for signs of the Roman town. This is just a quick summary, but you can read the full report here.

There were two main areas I surveyed. The first was in Priory Park, in the NE corner of the Roman town. The survey revealed two Roman buildings south of the Guildhall and part of the Roman road grid internal to the town to the east, with some low status settlement. The road had been cut in two places by the medieval motte and bailey ditches. You can see a video of the results here and of the two buildings here and here.

Priory Park Interpretation. Click for larger image.

The second area surveyed was the amphitheatre, just to the south-east of the Roman east gate. It isn't in particularly good condition, most of the retaining walls have been robbed, but some structure is still visible. You can see a video of the results here, but on that version, a block of data is shifted from where it should be.

Amphitheatre Interpretation. Click for larger image.

While these are excellent results, they are nothing compared to what is currently being found at Verulamium, where they are using mag and radar to reveal the structure of the town. I'd like to give a shout out to their most excellent blog, which shows the results in great detail. Well worth a read.

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.


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?


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.


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.