Comprehensive monitoring of the subglacial stream of Glacier d'Otemma using seismic nodes and distributed acoustic sensing data

Subglacial processes are difficult to monitor due to their inaccessibility with conventional hydrological probes. We know relatively little about when and at what rate the products of subglacial erosion are evacuated, especially for coarse sediment (bedload). Environmental seismology is contributing to close this knowledge gap, providing some of the first, seasonal-scale datasets on bedload evacuation by subglacial streams. The advantage of seismic monitoring of subglacial sediment transport is that it does not need to be installed directly into the water.

 

The location of a static subglacial channel can be found using techniques such as GPR surveys, but rapid changes in the subglacial channel system require continuous data sets on channel location. Monitoring seismic amplitudes and applying beamforming methods to seismic array records, one can locate noise sources and thus identify variations in activity and location of subglacial streams and bedload transport. This may be done using arrays of seismic nodes and/or distributed acoustic sensing (DAS) along an optical fiber. To identify the best use of such methods for monitoring the subglacial stream, the present study compares conventional seismic sensors and fiber optic cables for beamforming source location.

 

The field site of this study is Glacier d’Otemma in Valais, Switzerland. Given two data sets of seismic nodes and DAS, as well as ancillary observations, we can identify the location of the subglacial river and track changes in its discharge and bedload transport rate. These findings mainly relate to variations in seismic noise throughout the diurnal cycle of glacier melt. Depending on frequency band and daytime, the location of the most intense seismic noise, averaged over two hours, varies. These variations relate to processes such as surface melt, which stops during night, and subglacial flow, which continues but is less intense. Seismology proves to be a temporally and spacially rich tool to monitor this constantly changing activity of glaciers.