Interrogating the cryosphere using distributed acoustic sensing (DAS): examples of active and passive surveys in West Antarctica

Abstract:

Seismic methods provide insights in ice sheet dynamics through active and passive source surveys. They reveal fracture mechanisms, bed properties and ice fabric, all valuable constraints for modelling ice sheet responses to climate change. However, the deployment of seismic instrumentation in polar regions presents its own set of challenges, including instrument power, data coverage and survey repeatability. Here we investigate the use of fibre optic cables as distributed acoustic sensors (DAS) in glacial settings. In January 2020, we conducted a series of seismic experiments using DAS in West Antarctica.

One kilometre of cable was used to record icequakes from the base of the Rutford Ice Stream. We investigate different surface geometries and compare results with those from a local array of short-period seismometers. The DAS measurements are lower in signal-to-noise and limited in that only a single component of strain-rate is recorded. Nevertheless, we are able to detect and locate clusters of events from a region beneath the cable. Furthermore, the frequency response of the buried cable is superior to that of the geophones. We are also able to use full waveform inversion to determine source mechanisms. Using a hammer source and a combination of P-wave refraction and surface wave analysis (MASW), we use DAS to image the P- and S-wave velocity structure of the Rutford firn to a depth of nearly 60m. DAS presents some unique challenges, including the effects of gauge length on travel-time picks.

In 2019, a multi-mode fibre was deployed in a borehole at Skytrain Ice Rise for Distributed Temperature Sensing (DTS). We acquired DAS data using the same fibre, using a sledge-hammer source to collect Vertical Seismic Profiles through the ice column. Our results show that it is possible to image variations in seismic velocities and anisotropy due to ice fabric in such VSP surveys.

In summary, our results suggest the DAS will not replace conventional seismic instrumentation in glacial settings, but could play a complementary role or possibly create new opportunities.