Subsurface investigations on Virkisjökull

The landscape in the Virkisjökull catchment is changing rapidly, both on and in front of the glacier. To understand how these changes relate to processes going on beneath the surface, we are using Ground Penetrating Radar (GPR). The GPR studies are being carried out in combination with surface structural mapping, terrestrial LiDAR scanning, ice velocity measurements, groundwater investigations, passive seismic studies (Tromino), and geological mapping so that we can answer the following questions:

  • How do the structures inside the glacier (thrusts and faults) relate to the changes that we can measure on the ice surface?
  • How are the internal structures, nature of the bed, and glacier motion related?
  • Does the glacier show a 'structural' response to climate change?
  • How much ice exists underneath the outwash sediments in front of the 'clean' glacier margin, and how does this influence the hydrology and geomorphological evolution of the proglacial area?

Ground Penetrating Radar

GPR emits short pulses of electromagnetic energy, which propagate into the subsurface from a transmitter unit. That energy is partially reflected where it encounters changes in the dielectric properties of the subsurface. Reflected energy is then picked up by a receiver, where its amplitude and two-way travel time (TWTT) is measured. Since dielectric contrasts are produced by variations in water content, sediment content, and at the glacier bed, GPR is an ideal technique to investigate glacier structure. For our surveys, we are using a PulseEKKO Pro system with 100 MHz and 50 MHz antennas.

3D Models of the glacier

We can combine the data from our GPR surveys, with surface structural maps and elevation data from LiDAR scanning, to build a three-dimensional model of the glacier. This allows us to pick out the geometry of major structures and relate them to the changes we see at the surface. To do this, we are using the software package, GOCAD®.

New Findings

Immediately in front of the glacier, we now know that up to 30 m of ice exists underneath the outwash sediments. This karst-like buried ice influences the hydrogeology of the catchment, and is a principle cause of the large surface changes we see here. The ice under the thin outwash sediments is heavily fractured with numerous (and sometimes large) conduits. It is constantly changing – during the summer, new potholes open up almost every day!

On the glacier, we have identified englacial fractures and thrusts, which are closely related to changes that we see at the surface. Large amounts of glacier surface lowering in the lower ablation zone are related to down-faulting in the ice that occurs over the winter. On parts of the glacier snout, the surface lowering caused by faulting is more significant than lowering caused by surface melt. This has implications for glacier mass balance calculations based on remote sensing data alone.

Farther up the glacier, we see that structures still play an important role in the movement of the glacier. Ongoing studies are now using GPR data and surface structural mapping to see how the glacier is structurally adjusting to a warmer climate.

Contact

Contact Andrew Finlayson or Emrys Phillips for more information.