Dominik Gräff

Last Name

Gräff

First Name

Dominik

ORCID

0000-0003-1642-4783

Show all metadata

Search Results

Publications 1 - 10 of 21

Empirical investigations of the instrument response for distributed acoustic sensing (DAS) across 17 octaves
Paitz, Patrick; Edme, Pascal; Gräff, Dominik; et al. (2021)
Bulletin of the Seismological Society of America
With the potential of high temporal and spatial sampling and the capability of utilizing existing fiber‐optic infrastructure, distributed acoustic sensing (DAS) is in the process of revolutionizing geophysical ground‐motion measurements, especially in remote and urban areas, where conventional seismic networks may be difficult to deploy. Yet, for DAS to become an established method, we must ensure that accurate amplitude and phase information can be obtained. Furthermore, as DAS is spreading into many different application domains, we need to understand the extent to which the instrument response depends on the local environmental properties. Based on recent DAS response research, we present a general workflow to empirically quantify the quality of DAS measurements based on the transfer function between true ground motion and observed DAS waveforms. With a variety of DAS data and reference measurements, we adapt existing instrument‐response workflows typically in the frequency band from 0.01 to 10 Hz to different experiments, with signal frequencies ranging from 1/3000 to 60 Hz. These experiments include earthquake recordings in an underground rock laboratory, hydraulic injection experiments in granite, active seismics in agricultural soil, and icequake recordings in snow on a glacier. The results show that the average standard deviations of both amplitude and phase responses within the analyzed frequency ranges are in the order of 4 dB and 0.167π radians, respectively, among all experiments. Possible explanations for variations in the instrument responses include the violation of the assumption of constant phase velocities within the workflow due to dispersion and incorrect ground‐motion observations from reference measurements. The results encourage further integration of DAS‐based strain measurements into methods that exploit complete waveforms and not merely travel times, such as full‐waveform inversion. Ultimately, our developments are intended to provide a quantitative assessment of site‐ and frequency‐dependent DAS data that may help establish best practices for upcoming DAS surveys.
Calving-driven fjord dynamics resolved by seafloor fibre sensing
Gräff, Dominik; Lipovsky, Bradley Paul; Vieli, Andreas; et al. (2025)
Nature
Interactions between melting ice and a warming ocean drive the present-day retreat of tidewater glaciers of Greenland, with consequences for both sea level rise and the global climate system. Controlling glacier frontal ablation, these ice–ocean interactions involve chains of small-scale processes that link glacier calving—the detachment of icebergs—and submarine melt to the broader fjord dynamics. However, understanding these processes remains limited, in large part due to the challenge of making targeted observations in hazardous environments near calving fronts with sufficient temporal and spatial resolution. Here we show that iceberg calving can act as a submarine melt amplifier through excitation of transient internal waves. Our observations are based on front-proximal submarine fibre sensing of the iceberg calving process chain. In this chain, calving initiates with persistent ice fracturing that coalesces into iceberg detachment, which in turn excites local tsunamis, internal gravity waves and transient currents at the ice front before the icebergs eventually decay into fragments. Our observations show previously unknown pathways in which tidewater glaciers interact with a warming ocean and help close the ice front ablation budget, which current models struggle to do. These insights provide new process-scale understanding pertinent to retreating tidewater glaciers around the globe.
What do Subglacial Stick-Slip Asperities tell us about Stress Distributions and Dynamic Triggering on Natural Faults?
Gräff, Dominik; Walter, Fabian (2019)
IUGG 2019 Abstract Book
Analogous to tectonic strike-slip faults, where frictional processes build up stresses between plate boundaries eventually leading to earthquakes, icequakes caused by the buildup of gravitational driving stress occur at the interface between glaciers and the underlying bedrock. Similar to creeping and locked fault sections regulating the relative movement of tectonic plates, smooth sliding and stick-slip motion controls ice flow at the ice-bedrock interface. Under temperate alpine glaciers microseismic stick-slip events with regular interevent times cluster spatially in “asperities” and swarm temporally. Asperity fault sizes of less than ten meters are small compared to the entire ice-bedrock interface, but the slip distance of tens of micrometers to a few millimeters per event may contribute significantly to local basal sliding and thus release most local driving stress. Similar to earthquakes at tectonic plate boundaries, but on smaller temporal and spatial scales, this must lead to a re-distribution of stress between areas with enhanced friction. For asperities in a state close to failure, nearby stick-slip events may disturb their critical state by shifting static stresses and thus influencing their activity or even triggering contemporary failure. From multi-seasonal recordings of seismometers on the tongue of a Swiss alpine glacier, we systematically analyze microseismic basal stick-slip events. By comparing the spatial and temporal activity of asperities we evaluate their influence on neighboring ones. Given their regular and high event rate, basal stick-slip icequakes may give new insights not only into ice flow but also into the process of remote earthquake triggering.
In-situ Measurements of Stick-Slip Motion at the Bed of an Alpine Glacier
Gräff, Dominik; Walter, Fabian Thomas (2018)
Until recently, observations of sudden frictional processes at the bed of glaciers were limited to monitoring form the glacier surface. We study these processes causing stick-slip motion with a new approach that was applied the first time in a field campaign in summer 2018: We carried out the first in-situ measurements of an active seismogenic fault at a bi- material interface beneath a glacier. Enabled by guided hot water drilling, we targeted borehole experiments to specific glacier bed regions where spatially limited microseismic stick-slip sliding happens and combine them with the recordings of a high-densitiy network of seismometers at the glacier surface. From the various measurements we can determine the subglacial water- and thus pore pressure evolution and its effect on the fault stability. Futhermore the in-situ borehole measurements enable us to study material properties such as the till and ice characteristics within the stick-slip asperities and compare them to off-site reference measurements in seimically non-active regions of the glacier bed. Finally from acceleration, ice deformation measurements, and borehole camera videos from the glacier bed, we can estimate the amount of aseismic and co-seismic sliding, which cannot be obtained remotely from the ice surface. Summed up, with our in-situ measurements of an seismogenically active strike-slip fault beneath an alpine glacier, we open a unique posibility for studying seismogenic stick-slip motion at a bi-material interface in a natural environment.
Borehole Measurements and Basal Velocity of Rhonegletscher
Gräff, Dominik; Walter, Fabian; Bauder, Andreas (2017)
Basal sliding of glaciers and the ratio between the sliding velocity at the bed-ice interface and the glacial surface motion is of critical importance for ice flow models. Although the surface velocities of glaciers can be measured easily by satellite, GPS or classic geodetic methods, the sliding velocity at the ice- bedrock interface is difficult to determine experimentally. During a field campaign on the tounge of Rhonegletscher in August 2017, we drilled three boreholes from the glacier surface to the bed by using a hot water drill. For one borehole, fine-grained sediments that were swirled up by the turbulent outflow of the hot water drill, settled on the bottom of the borehole and thus made it possible to observe the glacier bed visually with a borehole camera. Pictures of the glacier bed taken with the borehole camera show a hard bedrock partially covered by a thin till layer of a few centemeters thickness. Isolated and repeated measurements within a ten-day period allow estimation of basal sliding velocity and sliding direction of the glacier. Here we present the deduced sliding velocity vector together with data from surface motion measurements at four differential GPS stations in the neighborhood of the borehole. We discuss these results in terms of general glacier dynamics and offer an outlook on future applications of our approach.
DAS to discharge: using distributed acoustic sensing (DAS) to infer glacier runoff
Manos, John-Morgan; Gräff, Dominik; Martin, Eileen Rose; et al. (2024)
Journal of Glaciology
Observations of glacier melt and runoff are of fundamental interest in the study of glaciers and their interactions with their environment. Considerable recent interest has developed around distributed acoustic sensing (DAS), a sensing technique which utilizes Rayleigh backscatter in fiber optic cables to measure the seismo-acoustic wavefield in high spatial and temporal resolution. Here, we present data from a month-long, 9 km DAS deployment extending through the ablation and accumulation zones on Rhonegletscher, Switzerland, during the 2020 melt season. While testing several types of machine learning (ML) models, we establish a regression problem, using the DAS data as the dependent variable, to infer the glacier discharge observed at a proglacial stream gauge. We also compare two predictive models that only depend on meteorological station data. We find that the seismo-acoustic wavefield recorded by DAS can be utilized to infer proglacial discharge. Models using DAS data outperform the two models trained on meteorological data with mean absolute errors of 0.64, 2.25 and 2.72 m3 s-1, respectively. This study demonstrates the ability of in situ glacier DAS to be used for quantifying proglacial discharge and points the way to a new approach to measuring glacier runoff.
Small-Scale Processes at the Glacier Bed: Stick-slip, Crack Waves and Sliding from Surface and Borehole Observations
Gräff, Dominik (2021)
VAW-Mitteilungen
Quantification of seasonal and diumal dynamics of subglacial channels using seismic observations on an Alpine glacier
Nanni, Ugo; Gimbert, Florent; Vincent, Christian; et al. (2020)
The Cryosphere
Water flowing below glaciers exerts a major control on glacier basal sliding. However, our knowledge of the physics of subglacial hydrology and its link with sliding is limited because of lacking observations. Here we use a 2-year-long dataset made of on-ice-measured seismic and in situ-measured glacier basal sliding speed on Glacier d'Argentière (French Alps) to investigate the physics of subglacial channels and its potential link with glacier basal sliding. Using dedicated theory and concomitant measurements of water discharge, we quantify temporal changes in channels' hydraulic radius and hydraulic pressure gradient. At seasonal timescales we find that hydraulic radius and hydraulic pressure gradient respectively exhibit a 2- and 6-fold increase from spring to summer, followed by comparable decrease towards autumn. At low discharge during the early and late melt season channels respond to changes in discharge mainly through changes in hydraulic radius, a regime that is consistent with predictions of channels' behaviour at equilibrium. In contrast, at high discharge and high short-term water-supply variability (summertime), channels undergo strong changes in hydraulic pressure gradient, a behaviour that is consistent with channels behaving out of equilibrium. This out-of-equilibrium regime is further supported by observations at the diurnal scale, which prove that channels pressurize in the morning and depressurize in the afternoon. During summer we also observe high and sustained basal sliding speed, which supports that the widespread inefficient drainage system (cavities) is likely pressurized concomitantly with the channel system. We propose that pressurized channels help sustain high pressure in cavities (and therefore high glacier sliding speed) through an efficient hydraulic connection between the two systems. The present findings provide an essential basis for testing the physics represented in subglacial hydrology and glacier sliding models.
Complex spectral line profiles resulting from cryogenic deformation of the SINFONI/SPIFFI diffraction gratings
George, Elizabeth M.; Gräff, Dominik; Hartl, Michael; et al. (2017)
Journal of Astronomical Telescopes, Instruments, and Systems
The integral field spectrograph, spectrometer for infrared faint field imaging (SPIFFI), has complex line profile shapes that vary with wavelength and pixel scale, the origins of which have been sought since the instrument construction. SPIFFI is currently operational as part of SINFONI at the Very Large Telescope (VLT) and will be upgraded and incorporated into the VLT instrument enhanced resolution imager and spectrograph (ERIS). We conducted an investigation of the line profiles based on the measurements we could take with the instrument calibration unit, as well as laboratory measurements of spare SPIFFI optical components. Cryogenic measurements of a spare SPIFFI diffraction grating showed significant periodic deformation. These measurements match the cryogenic deformation expected from bimetallic bending stress based on a finite element analysis of the lightweighted grating blank. The periodic deformation of the grating surface gives rise to satellite peaks in the diffraction pattern of the grating. An optical simulation including the cryogenic grating deformation reproduces the behavior of the SPIFFI line profiles with both wavelength and pixel scale as measured with the instrument calibration unit. The conclusion is that cryogenic deformation of the diffraction gratings is responsible for the nonideal line profiles, and that the diffraction gratings should be replaced during the upgrade for optimal instrument performance.
Crack wave resonances within the basal water layer
Gräff, Dominik; Walter, Fabian; Lipovsky, Bradley P. (2019)
Annals of Glaciology
Hydraulic processes within and beneath glacial bodies exert a far-reaching control on ice flow through their influence on basal sliding. Within the subglacial system, rapid changes in these processes may excite resonances whose interpretation requires an understanding of the underlying wave mechanics. Here, we explore these mechanics using observations from a kHz-sampled pressure sensor installed in a borehole directly above the hard granite bedrock of a temperate mountain glacier in Switzerland. We apply a previously established theory of wave propagation along thin, water-filled structures such as water-filled voids, basal water layers, or hydraulic fractures. Within such structures, short-wavelength waves experience restoring forces due to compressibility and are composed of sound waves. Long-wavelength resonances, in contrast, experience restoring forces due to elasticity and are composed of anomalously dispersed crack waves or Krauklis waves. Our borehole observations confirm the occurrence of both sound and crack waves within the basal water layer. Using both the resonance frequencies and attenuation of recorded crack waves we estimate thickness, aperture and length of the resonating basal water layer patch into which we drilled. We demonstrate that high-frequency observations of subglacial hydraulic processes provide new insights into this evolving dynamic system.

Publications 1 - 10 of 21

Dominik Gräff

Last Name

First Name

ORCID

Organisational unit

Filters

Search Results