21 January 2013

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.

06 January 2013

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


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.


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.


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.


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.


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.