Ice Properties Derived from Reflection Seismics at a Cold Alpine Saddle and a Local Antarctic Dome

For paleoclimate reconstructions or to predict future changes in sea level rise the dynamics of ice need to be known. This knowledge can be gained by a better understanding about the physical properties of ice. To derive these ice properties reflection seismics can be carried out. Up to now it is not very common to investigate internal structures of glaciers with the help of seismic measurements. For this purpose data from Colle Gnifetti, Monte Rosa, Switzerland and Halvfarryggen, Dronning Maud land, Antarctica were analyzed. Colle Gnifetti, an alpine saddle, resembles very well polar ice conditions and is thus an excellent test area close by. The shallow glacier (60 m) and the thick firn pack (30 m) made it impossible to filter out internal reflections. Further Problems during filtering occurred due to parasitic resonances of the geophone. Nevertheless the diving wave could be used to derive a density profile of the firn and ice. At Halvfarryggen, a local dome of 900 m thickness, internal reflections could clearly be observed below the firn transition zone down to the bed. It was the first time that internal reflections could be observed at an ice dome. The changes in seismic velocity, which caused the reflections, could then be linked to changes in crystal orientation fabric (COF) by the analysis of the reflection coeficient and partly by an amplitude versus angle (AVA) analysis. The results thus support ice-dynamic modeling studies which indicate the presence of strongly oriented crystal fabric and their changes over depth.For paleoclimate reconstructions or to predict future changes in sea level rise the dynamics of ice need to be known. This knowledge can be gained by a better understanding about the physical properties of ice. To derive these ice properties reflection seismics can be carried out. Up to now it is not very common to investigate internal structures of glaciers with the help of seismic measurements. For this purpose data from Colle Gnifetti, Monte Rosa, Switzerland and Halvfarryggen, Dronning Maud land, Antarctica were analyzed. Colle Gnifetti, an alpine saddle, resembles very well polar ice conditions and is thus an excellent test area close by. The shallow glacier (60 m) and the thick firn pack (30 m) made it impossible to filter out internal reflections. Further problems during filtering occurred due to parasitic resonances of the geophone. Nevertheless the diving wave could be used to derive a density profile of the firn and ice. At Halvfarryggen, a local dome of 900 m thickness, internal reflections could clearly be observed below the firn transition zone down to the bed. It was the first time that internal reflections could be observed at an ice dome. The changes in seismic velocity, which caused the reflections, could then be linked to changes in crystal orientation fabric (COF) by the analysis of the reflection coeficient and partly by an amplitude versus angle (AVA) analysis. The results thus support ice-dynamic modeling studies which indicate the presence of strongly oriented crystal fabric and their changes over depth

Determination of crystal orientation fabric from seismic wideangle data

It is known from ice core analyses that the crystal orientation fabric (COF) of ice sheets is anisotropic and changes over depth. A better understanding of these anisotropies as well as their remote detection is important to optimize flow models for ice. Here we show how seismic wideangle measurements can be used to determine the COF remotely. We demonstrate the principle formalism how observed seismic traveltimes can be related to COF properties by a forward model and then apply the formalism to field data. The eigenvalues that describe the ice fabric of the ice core EDML (Dronning Maud Land, Antarctca) are set into a relationship with the elasticity tensor. From the elasticity tensor the expected seismic velocities and reflection coefficients are calculated. Additionally we calculate the value eta from the Thomsen-parameters epsilon and delta. The value eta gives a measure of the anisotropy of vertical transverse isotropic (VTI)-media and is an important tool for the NMO-correction of anisotropic data. The approximation of reflection horizons as hyperbolas is not valid anymore in anisotropic media. The calculation of the moveout is therefore performed by a 4th order NMO-correction with the rms-velocity and the effective eta value as variables. This approach is applied to data from a wideangle survey shot at Halvfarryggen, Dronning Maud Land, Antarctica. From this data we derived rms-velocities and effective eta values. These values were than recalculated to interval velocities and interval eta values to give a hint on the measure of anisotropy of the different layers. The results give first insight into the anisotropies at Halvfarryggen

Characterization of the GNSS RFI Threat to DFMC GBAS Signal Bands

This article presents analysis results from a long-term multi-site Global Navigation Satellite System (GNSS) Radio Frequency Interference (RFI) monitoring campaign in the context of Ground Based Augmentation System (GBAS) Dual Frequency Multi Constellation (DFMC) concept operation. GBAS resilience against unintentional RFI is an important area for investigation as the ground station receivers often must operate adjacent to high-traffic roads where chances of being affected by RFI are high. To be able to develop algorithms and reaction strategies necessary to ensure continuity and availability of service, knowledge of interference signal characteristics and frequency band/bands affected, as well as relative occurrence rates between the considered frequencies and frequency combinations, is necessary. The analysis presented in the article covers the prevalence and properties of the RFI events observed on the GPS L1 and L5 and the Galileo E1 and E5a frequency bands that are considered by the on-going DFMC GBAS concept development initiatives. Due to being spectrally adjacent, the observed event analysis is also carried out for the Galileo E5b and GLONASS G1 frequency bands. The article also addresses the issue of spectral occupancy distribution of the observed events and presents new interesting RFI event types captured during the considered monitoring period.publishedVersio

Automatic GNSS RFI Classification Challenges

This article describes the real-world challenges that are encountered when trying to automatically categorize and classify the radio frequency interference (RFI) events captured in the GNSS signal bands by an international network of monitoring stations covering all L-band navigation signals. While signals frequently fall into the often-discussed categories such as 'chirp', 'continuous wave', or 'wideband noise', there is a large and growing number of modulations encountered in reality, both intentional and unintentional. These bear varying degrees of resemblance to the aforementioned traditional categories. Work presented herein focuses on some of the main complications encountered when categorizing multiple years of GNSS RFI event data, and the algorithmic approaches used to proceed with classification in these conditions.publishedVersio

Pyintegrity: An Open-Source Toolbox for Processing Ultrasonic Pulse-EchoWell Integrity Log Data

Many companies offer similarly designed wireline tools using ultrasonic pulse-echo measurements toevaluate barrier integrity in cased-hole wells. While these tools provide very similar data, differentcompanies process their data using different algorithms, typically to estimate the pipe wall thickness andthe outer material's acoustic impedance. While the algorithms themselves are public, no openly availablesoftware implementations are available. Therefore, we have developed an open-source software toolboxcalled Pyintegrity implementing many of these algorithms. In this article, we demonstrate Pyintegrity byapplying its algorithm implementations to a well integrity log from the open Volve Data Village dataset. Ourresults demonstrate that it is quite possible to process data recorded by a particular tool using processingalgorithms developed for use with other similar tools, and we find a good correspondence between thedifferent processing algorithms. Comparing the results produced by the different processing algorithms letsus confidently identify certain features in some of the results as processing artifacts that do not reflect thephysical state of the well.acceptedVersio

"Basal conditions of Kongsvegen at the onset of surge - revealed using seismic vibroseis surveys" in the IASC Workshop on the dynamics and mass budget of Arctic glaciers - Abstracts and program booklet.

Kongsvegen is a well-studied surge-type glacier in the Kongsfjord area of northwest Svalbard. Long-term monitoring has shown that the ice surface velocity has been increasing for the past 4 years; presenting a unique opportunity to study the internal ice structure, basal conditions and thermal regime that play a crucial role in initiating glacier surges. In April 2019, three-component seismic vibroseis surveys were conducted at two sites on the glacier, using a small Electrodynamic Vibrator source (ElViS). Site 1 is in the ablation area and site 2 near the equilibrium line, where the greatest increase in ice surface velocity has been observed. Initial analysis indicates the conditions at the two sites are significantly different. At site 1 the ice is around 220 m thick, sitting on a relatively flat and uniform bed, with no clear change in the bed reflection along the profile. There is a horizontally layered sediment package ∼60 m thick underlaying the bed. The ice column shows no internal layering. By contrast at site 2 (Fig. 1), where the ice is around 390 m thick, there is much more complex internal ice structure. Clear internal ice reflections are visible between 150-250 m depth – where we expect a transition from cold to temperate ice. Further reflections in the 100 m above the bed indicate there could be shearing or sediment entrainment in this area. Below the bed, cross-cutting layers are clearly visible and the bed reflection itself shows changing reflection polarity – suggesting water or very wet sediment is present in some areas. This suggests ice movement by basal sliding and shearing is likely

Approximations to seismic AVA responses: Validity and potential in glaciological applications

Amplitude-variation-with-angle (AVA) methods establish the seismic properties of material either side of a reflective interface, and their use is growing in glaciology. The AVA response of an interface is defined by the complex Knott-Zoeppritz (K-Z) equations, numerous approximations to which we typically assume weak interface contrasts and isotropic propagation, inconsistent with the strong contrasts at glacier beds and the vertically transverse isotropic (VTI) fabrics were associated with englacial reflectivity. We considered the validity of a suite of approximate K-Z equations for the exact P-wave reflectivity RP of ice overlying bedrock, sediment and water, and englacial interfaces between isotropic and VTI ice.We found that the approximations of Aki-Richards, Shuey, and Fatti match exact glacier bed reflectivity to within RP ± 0.05, smaller than the uncertainty in typical glaciological AVA analyses, but only for maximum incident angle θi limited to 30°. A stricter limit of θi ≤ 20° offered comparable accuracy to a hydrocarbon benchmark case of shale overlying gas-charged sand. The VTI-compliant Rüger approximation accurately described englacial reflectivity, to within RP ± 0.01, and it can be modified to give a quadratic expression in sin2 (θi)suitable for curve-matching operations. Having shown the circumstances under which AVA approximations were valid for glaciological applications, we have suggested that their interpretative advantages can be exploited in the future AVA interpretations

Deriving micro- to macro-scale seismic velocities from ice-core c axis orientations

One of the great challenges in glaciology is the ability to estimate the bulk ice anisotropy in ice sheets and glaciers, which is needed to improve our understanding of ice-sheet dynamics. We investigate the effect of crystal anisotropy on seismic velocities in glacier ice and revisit the framework which is based on fabric eigenvalues to derive approximate seismic velocities by exploiting the assumed symmetry. In contrast to previous studies, we calculate the seismic velocities using the exact c axis angles describing the orientations of the crystal ensemble in an ice-core sample. We apply this approach to fabric data sets from an alpine and a polar ice core. Our results provide a quantitative evaluation of the earlier approximative eigenvalue framework. For near-vertical incidence our results differ by up to 135ms−1 for P-wave and 200ms−1 for S-wave velocity compared to the earlier framework (estimated 1% difference in average P-wave velocity at the bedrock for the short alpine ice core). We quantify the influence of shear-wave splitting at the bedrock as 45ms−1 for the alpine ice core and 59ms−1 for the polar ice core. At non-vertical incidence we obtain differences of up to 185ms−1 for P-wave and 280ms−1 for S-wave velocities. Additionally, our findings highlight the variation in seismic velocity at non-vertical incidence as a function of the horizontal azimuth of the seismic plane, which can be significant for non-symmetric orientation distributions and results in a strong azimuth-dependent shear-wave splitting of max. 281ms−1 at some depths. For a given incidence angle and depth we estimated changes in phase velocity of almost 200ms−1 for P wave and more than 200ms−1 for S wave and shear-wave splitting under a rotating seismic plane. We assess for the first time the change in seismic anisotropy that can be expected on a short spatial (vertical) scale in a glacier due to strong variability in crystal-orientation fabric (±50ms−1 per 10cm). Our investigation of seismic anisotropy based on ice-core data contributes to advancing the interpretation of seismic data, with respect to extracting bulk information about crystal anisotropy, without having to drill an ice core and with special regard to future applications employing ultrasonic sounding

On-ice vibroseis and snowstreamer systems for geoscientific research

We present implementations of vibroseis system configurations with a snowstreamer for over-ice long-distance seismic traverses (>100 km). The configurations have been evaluated in Antarctica on ice sheet and ice shelf areas in the period 2010–2014. We discuss results of two different vibroseis sources: Failing Y-1100 on skis with a peak force of 120 kN in the frequency range 10–110 Hz; IVI EnviroVibe with a nominal peak force of 66 kN in the nominal frequency range 10–300 Hz. All measurements used a well-established 60 channel 1.5 km snowstreamer for the recording. Employed forces during sweeps were limited to less than 80% of the peak force. Maximum sweep frequencies, with a typical duration of 10 s, were 100 and 250 Hz for the Failing and EnviroVibe, respectively. Three different concepts for source movement were employed: the Failing vibrator was mounted with wheels on skis and pulled by a Pistenbully snow tractor. The EnviroVibe was operated self-propelled on Mattracks on the Antarctic plateau. This lead to difficulties in soft snow. For later implementations the EnviroVibe with tracks was put on a polyethylene (PE) sled. The sled had a hole in the center to lower the vibrator baseplate directly onto the snow surface. With the latter setup, data production varied between 20 km/day for 6-fold and 40 km/day for single fold for 9 h/day of measurements. The combination of tracks with the PE-sled was especially advantageous on hard and rough surfaces because of the flexibility of each component and the relatively lose mounting. The systems presented here are suitable to obtain data of subglacial and sub-seabed sediment layers and englacial layering in comparable quality as obtained from marine geophysics and land-based explosive surveys. The large offset aperture of the streamer overcomes limitations of radar systems for imaging of steep along-track subglacial topography. With joint international scientific and logistic efforts, large-scale mapping of Antarctica's and Greenland's subglacial geology, ice-shelf cavity geometries and sea-bed strata, as well as englacial structures can be achieved

Ross ice shelf vibrations

Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 42 (2015): 7589–7597, doi:10.1002/2015GL065284.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.Bromirski, Diez, and Gerstoft were supported by NSF grant PLR 1246151. Stephen and Bolmer were supported by NSF grant PLR-1246416. Wiens, Aster, and Nyblade were supported under NSF grants PLR-1142518, 1141916, and 1142126, respectively. Bromirski also received support from the California Department of Parks and Recreation, Division of Boating and Waterways under contract 11-106-107. The NIB data were collected under NSF grant OPP-0229546 and were downloaded from the IRIS DMC archives.2016-03-1