Techniques in Glaciology – CT Scanning

In glaciology we’re often interested in sediments, and in particular we tend to be interested in the sediments that glaciers deposit called till.  Till can tell us a huge amount about how the ice has behaved.  For instance, when till is under the ice it gets squashed, deformed and the stones within it can become aligned with the direction its being deformed in.

Because it’s extremely difficult to access the environment where this is happening, e.g. under hundreds of meters of ice(!), we’re often reliant on finding out properties of till to understand a variety of glacial processes. We’re also interested in glaciers that existed thousands of years ago, and as they’re no longer around examining the till is one of the ways to find out what they were doing.

If the past, if we wanted to find out the orientations of the stones within the till we have two different options.  If they’re big enough we can just go to an section of till, pull them out and measure them. Usually that gives us pretty decent results, but its very open to human bias, especially if you’re like some of the 1st year undergrads I demonstrated for last year and you make up half the measurements…If we’re interested in smaller stones we can take a sample in the field, bring it back to the lab, inject a load of resin into it, and cut it into thin slices.  We can then look at it under a microscope and measure the orientations of the stones.  Personally though, I’m not a great fan of doing this as converting from 2D orientations to 3D orientations is an easy way to introduce loads of error into your data.  The technique also involves effectively destroying your sample which generally isn’t a great idea in science.

Fortunately, there’s a new technology on the block.  Well, ‘new’ might be pushing it as it’s been used in soil science for at least 20 years, but it’s new to glaciology anyway and handily there’s a specialist centre for it in Southampton University.  CT scanning, also known as X-ray Computed Tomography, involves X-raying a sample from loads of different angles and then reconstructing all of those X-rays into a 3D ‘volume’.  Its exactly the same technology as CT scanning used in hospitals, although we tend to X-ray our samples 1000+ times which wouldn’t do a human any good at all…The 3D volume we get displays the density of the material we’ve scanned, so when we look through it, the denser thematerial the brighter it is.  Here’s a slice through piece of icelandic till:

Once we’ve a 3D volume to play with we can set about doing some pretty cool stuff. For instance, we can use image processing techniques reduce the volume to just the stones we’re interested in, and then we can measure all the orientations of the stones.

Whilst this technique is still in its infancy in glaciology, the first paper on it came out last year, its got great potential.  Not only is it potentially more accurate than the previous methods, we can also do a host of other processing techniques to determine other properties of the till, and importantly the till is preserved for future glaciologist in case they need to re-examine it.  I do have some worries about the technique, for instance validation of the techniques can be tricky when you’re working at a resolution of 2 microns, but they will be dealt with in time.

I’ll be doing some more detailed blogging on CT scanning in the future, but in the meantime I’ll leave you with a screencast of a quick segmentation I’ve done of the Icelandic till featured above. Its not perfect, and I’ve ‘cherry picked’ the larger stones from the sample, but its quite cool nonetheless.

Alex Clayton – Follow me on twitter: @glaciologist


One comment

  1. […] using a range of techniques. Within my PhD I use sensor networks, time series statistical analysis, X-ray CT scanning and geotechnical lab experiments.  I could find enough material for a PhD in any one of those […]

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