Today (24 November 2021) marks our one month anniversary at McMurdo station, and we have been busy preparing for our upcoming deployment to WAIS Divide, and then onward to the two TIME sites.
In between the many various trainings that we have to attend and complete, we managed to get our first taste of fieldwork by conducting bistatic radar surveys on the McMurdo Ice Shelf. While the primary aim for engaging in local fieldwork was to troubleshoot and familiarise ourselves with the experimental procedures, we wanted to situate our surveys at sites of scientific interest, in case our experiment yielded interesting results. In other words, we can kill two birds with one stone!
We decided to align our surveys with a 2015 radar profile transect conducted by Seth Campbell and colleagues at CRREL, that was in the vicinity of the 2006 ANDRILL borehole site. As well as having previous radar imagery of which we can compare our results to, this site produces an anomalously bright reflection seemingly in the middle of the ice column (the section of ice that the radar was imaging through), induced by the build-up of brine at that specific internal layer.
Paul and Danny testing the ApRES (yellow pelicases) and the software-defined radios (orange pelicase) on the McMurdo Ice Shelf.
But first: what is brine, and why are they present within ice? Essentially, brine is super-salty water (think: the contents of the pot that your pre-Thanksgiving turkey is swimming in currently). In the polar regions, such as Antarctica, brine forms through the rejection of salt ions as water freezes to ice. It is thought that englacial brine layers arise from the inwards infiltration of seawater through the ice, and settles on a "table" that prevents it from penetrating further. The buildup of brine then creates a highly reflective horizon due to the high concentration of salt ions, that shows up in our radar observations as an extremely "bright" layer.
And next: what is a bistatic radar survey? Unlike most radar surveys on ice, where the transmitting and receiving antennas usually trail immediately one after another behind a snow scooter, bistatic radars involve antennas that are separated by a distance at least the distance of the intended target. So, if we wanted to image the base of the ice shelf, where ice meets seawater (~80 metres, at our study site), our antennas would have to be separated by at least 80 metres to technically be called a bistatic survey. The standards for this exclusive club is set high! For our experiments, we use an autonomous phase-sensitive radar (ApRES) to send radar signals, and a software-defined radio to receive the transmitted signals.
The radar survey blueprint for our experiment on the McMurdo Ice Shelf.
Last: what can we learn from the results of the experiment? The reason why bistatic surveys require large separation distances is to leverage the radar waves propagating at oblique angles relative to the planes of the surface and target. Again, this is different to standard ice-penetrating radar surveys where the radar waves are propagating near-perpendicular to the ice surface due to the antennas being situated next to each other. Oblique wave propagation allows us to characterise and infer changes in the speed of the radar wave as it travels through different properties of the propagating medium (in our case: ice). For example, if the column of ice that we were surveying is warmer nearer the surface and colder at depth, we can infer these changes by comparing the strength of the measured radar waves at the receiving antenna (via reflecting off the bed of the ice shelf) to theoretical received strength assuming different ice temperatures. This is possible, because radar waves travel attenuate (or dim) faster in warmer ice than in colder ice. If we then conduct more measurements at different antenna separation distances and at different locations around the area of interest, we can then reconstruct a temperature profile of the ice column with depth. While not particularly interesting for an ice shelf, reconstructing a temperature profile of the ice column at a shear margin, such as the primary field sites that we will soon be visiting, will be extremely valuable, as it could potentially reveal the stress-strain dynamics that occur as ice shears against each other as it creates the boundaries of Thwaites Glacier.
As well as englacial temperature, we also hope to use the results of this experiment to (i) characterise the properties of the bed interface, as well as to (ii) calculate the strength of its molecular "fabric". In (i), we can leverage the specularity (how reflective a surface is with different angles of incidence) of the basal interface to characterise its nature, for example, whether bedrock, sediment, or liquid water underlies glacier ice. In (ii), we can measure the anisotropy (the degree of asymmetry) of ice crystals that make up glacier ice to then infer whether it enhances or restrains the flow of ice. But, more on this later!
Anyway, this is a glimpse of the type of radar experiments that we hope to be conducting this year. We hope to have some exciting results to share once we return from the field! Many thanks to our implementer Jenny Cunningham, as well as the US National Science Foundation's Office of Polar Programs for providing us the support for our preliminary work on the sea ice this week!
Although radar experiments involve lots of waiting, just being out on the windy ice shelf is very tiring!
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