Researchers interested in determining the depth of the firn layer within a glacier might spend six hours drilling, extracting, and analyzing a core in frigid temperatures. Recent research poses a new method for determining firn layer depth based on an exciting technology finding expanding applications in seismology.
At the bottom of a firn column, air becomes permanently confined within the ice matrix. As a result, the firn layer functions as a valve between air trapped within ice and air that flows freely through snow. Air within the firn layers can be used to determine more recent atmospheric compositions, specifically the recent history of trace gasses and their corresponding isotopic makeups. Ultimately, this study allows researchers to determine the depth of the firn layer with extraordinary accuracy, posing exciting new insight for climatologists.
A May 2023 study published in The Seismic Record details the work of a team led by Andreas Fichtner on the Northeast Greenland Ice Stream. The study uses fiber-optic cable Distributed Acoustic Sensing (DAS) technology to estimate the depth of the firn layer—the transition material between snow and ice that contains valuable climate information. In order to create an adequate seismic signal to measure, the team took advantage of a noisy airplane landing near the EastGRIP ice core drilling site on the Northeast Greenland Ice Stream. The research team then deployed 3,000 meters of fiber-optic cable near the skiway, buried about 50 centimeters beneath the snow.
The airplane landing produced clear signals, perfectly recording the pattern of waves through the snow. The team of researchers then used a Backus-Gilbert inversion method on the data to solve for wave speed given the wave mode; they identified the type of wave based on its characteristics, using this information to then determine wave speed and firn density. The study found 15 wave propagation modes that correspond to numerous Rayleigh, pseudoacoustic, and leaky waves; it’s essential to identify the different propagation modes to ensure that models are created with the proper inputs and equations. The research team converted wave speed measurements to density measurements, ultimately determining the firn-ice transition to be at depths between 65 and 71 meters. This calculation was cross-examined with firn core samples that found that the firn-density transition occurs around 65 meters–a good match with the DAS estimate.
Fiber-optic cable technology is not limited to studies similar to this one: rather, this technology has the potential to monitor regional seismic hazards in the future. Fiber-optic sensing functions by firing light pulses from a laser through a fiber optic cable, and the characteristics of the resulting light pulse indicate the degree of strain of the fiber. Additionally, this technology allows researchers to see where along the cable measurements occur, functioning like a long line of seismometers.
This study demonstrates that DAS is an accurate, useful tool that allows for measurements over areas much larger than a single drill core. DAS also has a number of advantages compared to standard seismic sensor arrays, including rapid sampling rates, high spatial resolution over hundreds of meters, and heightened sensitivity to ground strain throughout. Apart from costs associated with deploying the fiber-optic cable, DAS offers a highly cost-effective method of data acquisition. While polar science is never easy, laying out a kilometer of cable is likely preferable to drilling a hundred cores.