Anja Diez

Ross ice shelf vibrations

Broadband seismic stations were deployed across the Ross Ice Shelf (RIS) in November 2014 to study ocean gravity wave-induced vibrations. Initial data from three stations 100 km from the RIS front and within 10 km of each other show both dispersed infragravity (IG) wave and ocean swell-generated signals resulting from waves that originate in the North Pacific. Spectral levels from 0.001 to 10 Hz have the highest accelerations in the IG band (0.0025–0.03 Hz). Polarization analyses indicate complex frequency-dependent particle motions, with energy in several frequency bands having distinctly different propagation characteristics. The dominant IG band signals exhibit predominantly horizontal propagation from the north. Particle motion analyses indicate retrograde elliptical particle motions in the IG band, consistent with these signals propagating as Rayleigh-Lamb (flexural) waves in the ice shelf/water cavity system that are excited by ocean wave interactions nearer the shelf front.

Investigating englacial reflections with vibro- and explosive-seismic surveys at Halvfarryggen ice dome, Antarctica

Abstract Explosive seismic reflection data from Halvfarryggen, a 910 m thick local ice dome of the Antarctic ice sheet, show numerous laterally continuous reflections within the ice between 300 and 870 m depth. We compare the quality of data obtained with explosive sources with that obtained using a vibroseis source for detecting englacial reflections with a snowstreamer, and investigate the origin of englacial reflections. We find vibroseis in combination with a snowstreamer is ten times more productive than explosive seismics. However, englacial reflections are more clearly visible with explosives, which have a broader bandwidth signature, than the vibroseis, which is band-limited at the high-frequency end to 100 Hz. Only the strongest and deepest englacial reflection is detected with vibroseis. We interpret the majority of englacial reflections to originate from changes in the crystal orientation fabric in closely spaced layers, less than the vibro-seismic tuning thickness of 13.5 m. Phase analysis of the lowermost englacial reflector, 40 m above the bed, indicates a sharp increase in seismic wave speed. We interpret this reflector as a transition to a vertical single-maximum fabric. Our findings support current results from anisotropic ice-flow models, that crystal fabric is highly anisotropic at ice domes, both laterally and vertically.

Influence of ice crystal anisotropy on seismic velocity analysis

Abstract In 2010 a reflection seismic survey was carried out on the Alpine glacier Colle Gnifetti. The processed and depth-converted data could be compared to a nearby ice core, drilled almost to the bed. Comparisons showed that the depth of the P-wave bed reflection was too shallow, while the depth of the SH-wave bed reflection fitted the ice-core length well. We are now able to explain the major part of these differences using the existing crystal orientations of the ice at Colle Gnifetti. We calculate anisotropic velocities for P- and SH-waves that are usually picked for stacking and compare them with zero-offset velocities needed for the depth conversion. Here we take the firn pack at Colle Gnifetti into account for P- and S-wave analysis. To incorporate the S-wave analysis we first derive a new equation for the relationship between density and S-wave velocity from diving waves. We show that anisotropic fabrics observed at Colle Gnifetti introduce a difference of only 1% between stacking and depth-conversion velocities for the SH-wave, but 7% for the P-wave. We suggest that this difference in stacking and depth-conversion velocity for the P-wave can be used to derive information about the existing anisotropy by combining our seismic data with, for example, radar data.

Tsunami and infragravity waves impacting Antarctic ice shelves

The responses of the Ross Ice Shelf (RIS) to the September 16, 2015 8.3 (Mw) Chilean earthquake tsunami (> 75 s period) and to oceanic infragravity (IG) waves (50–300 s period) were recorded by a broadband seismic array deployed on the RIS from November 2014 to November 2016. Here we show that tsunami and IG-generated signals within the RIS propagate at gravity-wave speeds (∼70 m/s) as water-ice coupled flexural-gravity waves. IG band signals show measureable attenuation away from the shelf front. The response of the RIS to Chilean tsunami arrivals is compared with modeled tsunami forcing to assess ice shelf flexural-gravity wave excitation by very long period (VLP; > 300 s) gravity waves. Displacements across the RIS are affected by gravity-wave incident direction, bathymetry under and north of the shelf, and water layer and ice shelf thicknesses. Horizontal displacements are typically about 10 times larger than vertical displacements, producing dynamical extensional motions that may facilitate expansion of existing fractures. VLP excitation is continuously observed throughout the year, with horizontal displacements highest during the austral winter with amplitudes exceeding 20 cm. Because VLP flexural-gravity waves exhibit no discernable attenuation, this energy must propagate to the grounding zone. Both IG and VLP band flexural-gravity waves excite mechanical perturbations of the RIS that likely promote tabular iceberg calving, consequently affecting ice shelf evolution. Understanding these ocean-excited mechanical interactions is important to determine their effect on ice shelf stability to reduce uncertainty in the magnitude and rate of global sea level rise.

First glacier-vibroseismic experiment - results from cold firn of Colle Gnifetti

In the summer of 2010, a small shallow reflection seismic experiment was carried out on the firncovered cold glacier of Colle Gnifetti, Monte Rosa group, Swiss/Italian Alps. At this site, the physical properties of ice are comparable to polar conditions, which is why this site is often used for methodological tests. The experiment at 4500 m elevation was designed to explore the scope of shallow vibroseis for seismic targets within and below the glacier. A small ELVIS vibrator system was used to generate shear waves and compression waves for SH- and P-wave receiver setups of two profiles. The resulting sections clearly show a boundary from ice to rock around 60 m and deeper structures below the glacier. The deepest features are estimated to be 150 m for the SH-waves and 220 m for the P-waves. Reflections could be detected also within the ice overburden, which are preliminarily interpreted as a change of density in the upper 30 m and possibly crystal orientation fabric in the ice column. Furthermore, elastic parameters could be derived from seismic velocities, due to clear basement reflections. The results of this unique experiment enable new insights into the internal structure of ice masses and open a promising new investigation method for sub-ice structures and properties, such as basal sediments.

Reassembling Gondwana: A new high quality constraint from vibroseis exploration of the sub-ice shelf geology of the East Antarctic continental margin

The breakup of Gondwana is manifested by coeval early Jurassic Karoo magmatism in South Africa and East Antarctica. In South Africa, the large volumes of volcanic rocks of the adjoining Lebombo and Mwenetzi-Save monoclines represent a volcanic rift margin, and in East Antarctica, a corresponding feature, the Explora Wedge is buried below sediments and floating ice shelves on the continental margin of Dronning Maud Land. We use the seismic vibrator source to explore the sub-ice geology in Antarctica, and the new seismic reflection and available regional aeromagnetic data enable us to outline a dogleg landward extent of the Explora Wedge in Dronning Maud Land. The congruent inboard wedge geometries on the two continents define a high quality constraint, which facilitate for the first time, a geologically consistent and tight reconstruction of Africa relative to East Antarctica within Gondwana. The uncertainties in correlations of major geological features (mobile belts) from one continent to the other may now be of the order of ten’s of kilometers rather than hundreds of kilometers.

Joint interpretation of explosive and vibroseismic surveys on cold firn for the investigation of ice properties

Abstract Two seismic surveys were carried out on the high-altitude glacier saddle, Colle Gnifetti, Monte Rosa, Italy/Switzerland. Explosive and vibroseismic sources were tested to explore the best way to generate seismic waves to deduce shallow and intermediate properties (<100 m) of firn and ice. The explosive source (SISSY) excites strong surface and diving waves, degrading data quality for processing; no englacial reflections besides the noisy bed reflector are visible. However, the strong diving waves are analyzed to derive the density distribution of the firn pack, yielding results similar to a nearby ice core. The vibrator source (ElViS), used in both P- and SH-wave modes, produces detectable laterally coherent reflections within the firn and ice column. We compare these with ice-core and radar data. The SH-wave data are particularly useful in providing detailed, high-resolution information on firn and ice stratigraphy. Our analyses demonstrate the potential of seismic methods to determine physical properties of firn and ice, particularly density and potentially also crystal-orientation fabric.

Shallow Shear Wave Reflection Surveying on Alpine Glaciers – Comparison of Results from Gepatschferner and Colle Gnifetti

In the summer season 2010, a shallow reflection seismic experiment using the small vibrator source ELVIS was carried out on the overburden firn and ice cover of the Colle Gnifetti, Monte Rosa group, Swiss/Italian Alps. This site is widely used for method testing, since the physical properties of ice are similar to those of polar regions. The unique experiment approved for the first time the shallow high-resolution vibroseis method using P- and S-waves for seismic targets on firn and ice masses at least to nearly 60 m depth. As a consequence of this successful experiment, the method was subsequently applied in April 2012 on the Gepatschferner, a glacier in the Austrian Alps, using S-waves only. In contrast to the commonly planted receivers at Colle Gnifetti, a land streamer modified for snow application was used as receiver system. The source was slightly modified by a ski to support operation on soft snow. The recorded data at Gepatschferner achieved clear reflections from the ice base boundary in nearly 150 m depth and from the glacier bed below.

Shallow Shear Wave Reflection Seismic on Firn and Ice – Insights, Challanges, and Opportunities From a Small Scale Experiment at Colle Gnifetti

In the summer season 2010, a small shallow reflection seismic experiment was carried out on the overburden firn and ice cover of the Colle Gnifetti, Monte Rosa group, Swiss/Italian Alps. This site is widely used for method testing, since the physical properties of ice are similar to polar regions. The whole experiment was designed to explore the scope of the shallow high-resolution vibroseis method for seismic targets of the firn and ice mass of nearly 60 m thickness. A special part of the experiment was the exploration of vibratory shear wave reflection seismic capabilities in such an environment, which was never done before. The small ELVIS vibrator system was used to generate seismic shear waves received by common planted horizontal geophones in a SH-SH (source and receivers arranged both perpendicular to the profiling direction) configuration on two profiles, arranged as a cross setup. The resulting seismic sections of both profiles clearly show the boundary from ice to rock (evaluated by ice coring in 2005), the structure dip and also deeper events within the underlaying rock up to 150 m in depth. Seismic velocities within the ice overburden were used to derive elastic parameters combined with the density function derived from the ice coring. The results of this experiment show promising new prospects for the vibratory shear wave reflection method on glaciers, firn, and ice.