Bedload export from an Alpine glacier inferred from seismic methods

Environmental seismology is the discipline that uses ambient noise to detect and to measure geomorphic processes. The basic principle relies on the unique seismic signal, in terms of excited frequencies and amplitudes, generated by such processes which is then propagated and recorded to sensors (geophones). Recent developments of this technique are interesting for geomorphology because there is evidence that it can be used to study processes that are rare and difficult to measure, such as bedload transport in rivers. This is of particular importance for quantify bedload flux in proglacial streams. This paper focuses on using this method to quantify bedload export from an Alpine valley glacier. At present, there is no time-continuous measurement of bedload export in such settings. This is because objective measurement of bedload flux is a challenge task for both theoretical and practical reasons. Theoretically, the high variability of bedload transport means that measurements need to be continuous to identify when it happens and with which intensity. Practically, in order to know how to sample the signal through time, you need to know its variance a priori, which is commonly unsteady, and therefore needs to be sampled in order to know how to sample it. Direct manual sampling (e.g. with Helley-Smith samplers) have a serious disadvantage in this sense. Existing indirect methods (e.g. hydrophones and plate geophones) allow collection of continuous data but, at the same time, their installation can be difficult and expensive and they need to be carefully calibrated against manual samplings.

In the present study we used seismic data collected by out-of-stream geophones to infer bedload flux along an entire melt season (June - September) at the Glacier d’Otemma proglacial forefield, south-western Swiss Alps. Raw seismic data were inverted into sediment flux following the methodology developed in Dietze et al. (2019), to produce the first continuous dataset of bedload export from an Alpine glacier. The inversion model has been calibrated using statistical (i.e. sensitivity analysis) and direct (i.e. in situ active seismic survey) approaches.