Seismology is one of the standard pieces of the glaciological toolkit. Glaciologists have long known that seismic energy – vibrations travelling through the ice – can be used for actively imaging glacier structure, or for listening out for the natural rumblings as a glacier scrapes across its bed and water drains through networks of channels. Seismic energy is usually recorded by geophones – essentially, “microphones” that records seismic vibrations. Our trusty geophones have served us well for decades, however fibre optic technologies now offer a valuable alternative – and we’ve been applying them on Store Glacier in the RESPONDER project.
We typically think of fibre optic cable when it comes to internet and broadband solutions. In these systems, information travels along the broadband network as light pulses, which are interpreted by your home computer as your latest tweet, news bulletin or steaming service. Imagine if you were trying to send that tweet as someone was stretching the fibre optic cable… you could imagine that the information would become scrambled. This idea is at the heart of fibre optic seismic surveying: if a seismic wave vibrates ice in which a fibre optic cable is installed, those vibrations will slightly stretch and squeeze the cable, distorting laser pulses that are transmitted into it
Animation of fibre optic signals. Figure courtesy of Silixa.
To interpret the distorted light pulses, a sophisticated computer – called an interrogator – reconstructs the seismic disturbance, informing the user what it must have looked like. The fibre-optic cable therefore does the same job as our trusty geophones, although with the important difference that it records seismic energy along its entire length, metre by metre! The technical name for this approach is ‘distributed acoustic sensing’. A 1 km long cable can therefore provide the same information as a thousand geophones, which would be both expensive and time consuming to set up.
In the RESPONDER project, we deployed a fibre-optic cable in a 1,043-m-deep borehole drilled to the bed of Store Glacier. In doing so, we demonstrated – for the first time – the first successful application of distributed acoustic sensing in a glaciological borehole drilled on an ice sheet
Seismic data collection: creating a source signal (left) and watching the data come in (right).
We recorded the natural seismic heartbeat of the glacier, and also made our own ‘seismic source’ at various surface locations around the borehole. While this sounds sophisticated, it actually just involves whacking the ice surface with a sledgehammer! But this brute-force approach, combined with the fibre optic technology, has provided the most detailed seismic description of physical properties on outlet glacier in Greenland to date. Measurements of seismic velocity reveal a transition from isotropic to anisotropic ice crystal orientation fabric at 870 m depth. This boundary is known as the last glacial-interglacial transition, which means that ice below 870 m formed during the last ice age (the ice is older than 12,000 years) while ice above 870 m formed in the most recent interglacial period (younger than 12,000). Not only does this show that the ice has different ages, it also shows that the ice has vastly different physical properties and behaves differently.
The fibre-optic experiment also showed that the ice contains liquid water in the veins beyond 900 m, and we were furthermore able to retrieve physical properties from the subglacial sediments below the ice, likely a >20-m-thick layer separating the glacier from hard bedrock. Identifying these sediments has really important implications for the way water is draining beneath the glacier. Some of the water will for instance be stored in the pores of the sediment, making it weaker and therefore contribute to the glaciers fast flow. Did we mention that the glacier moves by several metres per day in the location where we drilled the borehole? This motion is often referred to as ‘fast glacier flow’. And in Greenland, fast flowing glaciers like Store cause half of the ice sheets annual mass loss, which has been increasing year by year over the last 20-30 years due to climate change.
A small example of our seismic data.
These are still early days, and we have a massive volume of data to analyse! To this end, we are developing machine-learning algorithms to automatically detect seismic waves within our dataset, and build an ever-more comprehensive picture of the seismic properties of Store Glacier.
We also used the fibre-optic cable to measure the temperature of the glacier, again metre by metre. But that is another story for us to write later.
Keep watching this space – the fibre optic future has come to glaciology!
Link: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2020GL088148
Booth, A., P. Christoffersen, C. Schoonman et al. (2020), Distributed Acoustic Sensing of Seismic Properties in a Borehole Drilled on a Fast‐Flowing Greenlandic Outlet Glacier, Geophysical Research Letters, 47(13), e2020GL088148.