12th September 2019

Fibre Optic Sensing

To understand the present and future behaviour of fast-flowing marine-terminating glaciers such as Store, it is vital to accurately constrain temperature profiles and ice structure with depth. Conventionally, temperature would be obtained using discrete sensing units connected to a surface logger-box via electrical cable. While this enables several different measurements to be bundled together (e.g. tilt with temperature and pressure) it has the large drawback of only a limited number of sensors, typically 10-20, being installed per borehole. Internal structures are often obtained from surface geophysical datasets, including seismic reflection surveying, but these seldom benefit from ground-truth records of the acoustic response deep within the glacier. Thus, data interpretations can be ambiguous.  

Standing next to a borehole DTS installation set up to run remotely and record temperature changes through the end of the summer melt season

Fibre optic sensing enables measurements to be taken in a distributed manner over the entire >1 km depth of the borehole with spatial accuracy ranging from 0.5 m (temperature measurement) to 10 m (acoustic measurement). Sections of the depth profile that would otherwise be unsampled can therefore be examined in extraordinary detail. The rapid development of fibre optic sensing over the last decade means that this increase in spatial resolution does not damage the accuracy of the measurement, in many cases it is improved.

For RESPONDER, we used fibre optic distributed temperature sensing (DTS) cables to look at the thermal nature of the base of Store Glacier in unprecedented detail through a borehole, and uncover a large temperate zone. As temperate ice (ice with some fractional water content) can be ten times less viscous than cold ice, the presence and thickness of a temperate zone has clear implications for ice dynamics. 

We also lowered DTS cables down a moulin. A moulin is a large conduit that transfers melt water from the surface of the ice sheet to the bed. Not only do moulins provide pathways for melt water to leave the ice sheet and enter the ocean, they also exert strong controls on the subglacial hydrological system which controls basal sliding rates. However, the internal structure and heat dissipation of a moulin remains obscure, something we hope our DTS data will be able to address. 

RESPONDER also undertook the first glaciological measurements with distributed acoustic sensing (DAS) cables, to give unprecedented insight into the englacial variation of seismic properties. We monitored the natural seismicity of Store glacier over 3 continuous days, and also made our own controlled seismic energy using ~1500 sledgehammer impacts. One of our seismic records is shown below; by comparing the travel-time of seismic arrivals along successive lengths of the cable, we can determine the velocity of seismic energy. In turn, this relates to the internal ice fabric in Store and can also reveal zones of temperate ice. It will be really exciting to compare responses in our novel DAS and DTS deployments in resulting publications.

An example seismic record from the Store glacier DAS cable. By monitoring changes in the arrival time of seismic energy, we derive a model of seismic velocity with depth. a) Full data, showing seismic arrivals to > 1 km depth. b-d) Enlarged sections of the data to derive initial velocity estimates.