What lies beneath our feet? Measuring the ice thickness and quantifying the amount of accumulation is essential for glaciological research. But how do we do that? A common method consists of pulling a radar behind a skidoo on the ice surface or hauling the system by foot.
Ground penetrating radar (GPR) is a useful tool for mapping of features hidden under the ice surface. As ice is very transparent to radar signals, the signal can penetrate deep from the transmitting antenna into floating glaciers and ice shelves. Layers within the ice then act like a mirror and reflect parts of the signal back to the receiving antenna on the surface. We then record how much time it took between sending it out and receiving the signal – the so-called ‘two-way travel time’. From the lab we know how quickly radar signals travel through ice, so we can easily convert the two-way travel time directly to depth of the reflecting layer below the surface. But here is an example:
From the gym we know that Usain Bolt travels with 10.44 meters per second through air. If we let him race for 4022 s, he would have run 42 kilometers (that’s a marathon in 67 minutes). And now you say that he will get tired and can’t keep his 10.44 m/s up for that long. So how long can he keep it up? His world record over 200 m is 19.19 seconds – that is only 2.003 times his time over 100 m. So he can keep it up for at least 200 meters ! Anyway, you get the point. If you know the travel velocity of Usain (or radar) through a medum (like ice) and you stop the time, you can calculate the overall distance. But there is one mistake that can happen even to the best radar-glaciologist… the measured ice is suddenly twice a thick as expected, why ? Because you still have to divide the two-way travel time by a factor of 2.
But as easy as it sounds, there is a bit more to it. Different radar waves can penetrate to different depths – with lower frequencies penetrating ice thicknesses up to several kilometers. With these low-frequency radar systems we can then see what lies beneath the ice. Is it grounded on bedrock or floating on the ocean? Are there basal channels at the ice base through which meltwater is discharged? Or are there any crevasses at the bottom of the ice? Higher frequency radar systems don’t penetrate all the way to the ice base as their signals are reflected from internal layers near the surface. With these systems we can measure how thick eventual snow bridges are over burried crevasses or if there are any spatial differences in snow accumulation. Sometimes we see very clear internal layers and need to have an even closer look. We can then either drill an icecore to retrieve a sample, or if it is closer to the surface we do it the old way and grab a shovel and start digging. If there is one thing I have learned in Antarctica, it is digging.
Unfortunately though, radar isn’t perfect… as useful as it is for measuring internal layers and ice thickness, radar doesn’t tell us anything about the ocean floor underneath. Other techniques, such as (a) gravimetry measuring spatial differences of the strength of the gravitational field, and (b) active seismology using seismic waves from controlled explosions, can be used to map the ocean floor.
Christian Wild 