Crystal Orientation Fabric Research Articles

In situ estimation of ice crystal properties at the South Pole using LED calibration data from the IceCube Neutrino Observatory

Abstract. The IceCube Neutrino Observatory instruments about 1 km3 of deep, glacial ice at the geographic South Pole. It uses 5160 photomultipliers to detect Cherenkov light emitted by charged relativistic particles. An unexpected light propagation effect observed by the experiment is an anisotropic attenuation, which is aligned with the local flow direction of the ice. We examine birefringent light propagation through the polycrystalline ice microstructure as a possible explanation for this effect. The predictions of a first-principles model developed for this purpose, in particular curved light trajectories resulting from asymmetric diffusion, provide a qualitatively good match to the main features of the data. This in turn allows us to deduce ice crystal properties. Since the wavelength of the detected light is short compared to the crystal size, these crystal properties include not only the crystal orientation fabric, but also the average crystal size and shape, as a function of depth. By adding small empirical corrections to this first-principles model, a quantitatively accurate description of the optical properties of the IceCube glacial ice is obtained. In this paper, we present the experimental signature of ice optical anisotropy observed in IceCube light-emitting diode (LED) calibration data, the theory and parameterization of the birefringence effect, the fitting procedures of these parameterizations to experimental data, and the inferred crystal properties.

Borehole fibre-optic seismology inside the Northeast Greenland Ice Stream

SUMMARYIce streams are major contributors to ice sheet mass loss and sea level rise. Effects of their dynamic behaviour are imprinted into seismic properties, such as wave speeds and anisotropy. Here, we present results from a distributed acoustic sensing (DAS) experiment in a deep ice-core borehole in the onset region of the Northeast Greenland Ice Stream, with focus on phenomenological and methodological aspects. A series of active seismic surface sources produced clear recordings of the P and S wavefield, including internal reflections, along a 1500 m long fibre-optic cable that was placed into the borehole. The combination of nonlinear traveltime tomography with a firn model constrained by multimode surface wave data, allows us to invert for P and S wave speeds with depth-dependent uncertainties on the order of only 10 m s−1, and vertical resolution of 20–70 m. The wave speed model in conjunction with the regularly spaced DAS data enable a straightforward separation of internal upward reflections followed by a reverse-time migration that provides a detailed reflectivity image of the ice. While the differences between P and S wave speeds hint at anisotropy related to crystal orientation fabric, the reflectivity image seems to carry a pronounced climatic imprint caused by rapid variations in grain size. Further improvements in resolution do not seem to be limited by the DAS channel spacing. Instead, the maximum frequency of body waves below ∼200 Hz, low signal-to-noise ratio caused by poor coupling, and systematic errors produced by the ray approximation, appear to be the leading-order issues. Among these, only the latter has a simple existing solution in the form of full-waveform inversion. Improving signal bandwidth and quality, however, will likely require a significantly larger effort in terms of both sensing equipment and logistics.

Strongly Depth‐Dependent Ice Fabric in a Fast‐Flowing Antarctic Ice Stream Revealed With Icequake Observations

AbstractThe crystal orientation fabric of glacier ice impacts its strength and flow. Crystal fabric is therefore an important consideration when modeling ice flow. Here, we show that shear‐wave splitting (SWS) measured with glacial microseismicity can be used to invert seismic anisotropy and ice fabric, if represented in a statistical sense. Rutford Ice Stream (RIS) is a fast‐flowing Antarctic ice stream, a setting crucial for informing large‐scale ice sheet models. We present >200,000 SWS measurements from glacial microseismicity, registered at a 38‐station seismic network located ∼40 km upstream of the grounding line. A representative subset of these data is inverted for ice fabric. Due to the character of SWS, which accumulates along the raypath, we include information on the depth structure from radar measurements. We find that the following three‐layer configuration fits the data best: a broad vertical cone fabric near the base of RIS (500 m thick), a thick vertical girdle fabric, orientated perpendicular to flow, in the middle (1,200 m thick), and a tilted cone fabric in the uppermost 400 m. Such a variation of fabric implies a depth‐dependent strength profile of the ice with the middle layer being ∼3.5 times harder to deform along flow than across flow. At the same time, the middle layer is a factor ∼16 softer to shear than to compression or extension along flow. If such a configuration is representative for fast‐flowing ice streams, it would call for a more complex integration of viscosity in ice sheet models.

Development of crystal orientation fabric in the Dome Fuji ice core in East Antarctica: implications for the deformation regime in ice sheets

Abstract. The crystal orientation fabric (COF) of a polar ice sheet has a significant effect on the rheology of the ice sheet. With the aim of better understanding the deformation regime of ice sheets, the work presented here investigates the COF in the upper 80 % of the Dome Fuji Station ice core in East Antarctica. Dielectric anisotropy (Δε) data were acquired as a novel indicator of the vertical clustering of COF resulting from vertical compressional strain within the dome. The Δε values were found to exhibit a general increase with depth, but with fluctuations over distances in the order of 10–102 m. In addition, significant decreases in Δε were found to be associated with depths corresponding to three major glacial to interglacial transitions. These changes in Δε are ascribed to variations in the deformational history caused by dislocation motion occurring from near-surface depths to deeper layers. Fluctuations in Δε over distances of less than 0.5 m exhibited a strong inverse correlation with Δε at depths greater than approximately 1200 m, indicating that they were enhanced during the glacial-interglacial transitions. The Δε data also exhibited a positive correlation with the concentration of chloride ions and an inverse correlation with the amount of dust particles in the ice core at greater depths corresponding to decreases in the degree of c axis clustering. Finally, we found that fluctuations in Δε persisted to approximately 80 % of the total depth of the ice sheet. These data suggest that the factors determining the deformation of ice include the concentration of chloride ions and the amount of dust particles, and that the layered contrast associated with the COF is preserved all the way from the near-surface to a depth corresponding to approximately 80 % of the thickness of the ice sheet. These findings provide important implications regarding further development of the COF under the various stress-strain configurations that the ice will experience in the deepest region, approximately 20 % of the total depth from the ice/bed interface.

On the Limitations of Using Polarimetric Radar Sounding to Infer the Crystal Orientation Fabric of Ice Masses

AbstractWe introduce a transfer matrix model for radio‐wave propagation through layered anisotropic ice that permits an arbitrary dielectric permittivity tensor in each layer. The model is used to investigate how crystal orientation fabrics without a vertical principal direction affect polarimetric radar returns over glaciers and ice sheets. By expanding the c‐axis orientation distribution in terms of a spherical harmonic series, we find that radar returns from synthetic fabric profiles are relatively insensitive to the harmonic mode responsible for a nonvertical principal direction; however, only for normally incident waves. Consequently, the strength of this mode might be relatively difficult to infer in glaciers and ice sheets, which in turn has implications for the ability to determine the full second‐order structure tensor, needed to infer the local flow regime, flow history, or to represent the directional viscosity structure of glacier ice for ice‐flow modeling.

Rapid and accurate polarimetric radar measurements of ice crystal fabric orientation at the Western Antarctic Ice Sheet (WAIS) Divide ice core site

Abstract. The crystal orientation fabric (COF) of ice sheets records the past history of ice sheet deformation and influences present-day ice flow dynamics. Though not widely implemented, coherent ice-penetrating radar is able to detect bulk anisotropic fabric patterns by exploiting the birefringence of ice crystals at radar frequencies, with the assumption that one of the crystallographic axes is aligned in the vertical direction. In this study, we conduct a suite of quad-polarimetric measurements consisting of four orthogonal antenna orientation combinations near the Western Antarctic Ice Sheet (WAIS) Divide ice core site. From these measurements, we are able to quantify the azimuthal fabric asymmetry at this site to a depth of 1400 m at a bulk-averaged resolution of up to 15 m. Our estimates of fabric asymmetry closely match corresponding fabric estimates directly measured from the WAIS Divide ice core. While ice core studies are often unable to determine the absolute fabric orientation due to core rotation during extraction, we are able to identify and conclude that the fabric orientation is depth-invariant to at least 1400 m, equivalent to 6700 years BP (years before 1950) and aligns closely with the modern surface strain direction at WAIS Divide. Our results support the claim that the deformation regime at WAIS Divide has not changed substantially through the majority of the Holocene. Rapid polarimetric determination of bulk fabric asymmetry and orientation compares well with much more laborious sample-based COF measurements from thin ice sections. Because it is the bulk-averaged fabric that ultimately influences ice flow, polarimetric radar methods provide an opportunity for its accurate and widespread mapping and its incorporation into ice flow models.

Inferred basal friction and mass flux affected by crystal-orientation fabrics

We investigate the errors caused by neglecting the crystal-orientation fabric when inferring the basal friction coefficient field, and whether such errors can be alleviated by inferring an isotropic enhancement factor field to compensate for missing fabric information. We calculate the steady states that arise from ice flowing over a sticky spot and a bedrock bump using a vertical-slab numerical ice-flow model, consisting of a Weertman sliding law and the anisotropic Johnson flow law, coupled to a spectral fabric model of lattice rotation and dynamic recrystallisation. Given the steady or transient states as input for a canonical adjoint-based inversion, we find that Glen's isotropic flow law cannot necessarily be used to infer the true basal drag or friction coefficient field, which are obscured by the orientation fabric, thus potentially affecting vertically integrated mass fluxes. By inverting for an equivalent isotropic enhancement factor, a more accurate mass flux can be recovered, suggesting that joint inversions for basal friction and the isotropic flow-rate factor may be able to compensate for mechanical anisotropies caused by the fabric. Thus, in addition to other sources of rheological uncertainty, fabric might complicate attempts to relate subglacial conditions to basal properties inferred from an inversion relying on Glen's law.

Acoustic velocity measurements for detecting the crystal orientation fabrics of a temperate ice core

Abstract. The crystal orientation fabric (COF) in ice cores provides detailed information, such as grain size and distribution and the orientation of the crystals in relation to the large-scale glacier flow. These data are relevant for a profound understanding of the dynamics and deformation history of glaciers and ice sheets. The intrinsic, mechanical anisotropy of the ice crystals causes an anisotropy of the polycrystalline ice of glaciers and affects the velocity of acoustic waves propagating through the ice. Here, we employ such acoustic waves to obtain the seismic anisotropy of ice core samples and compare the results with calculated acoustic velocities derived from COF analyses. These samples originate from an ice core from Rhonegletscher (Rhone Glacier), a temperate glacier in the Swiss Alps. Point-contact transducers transmit ultrasonic P waves with a dominant frequency of 1 MHz into the ice core samples and measure variations in the travel times of these waves for a set of azimuthal angles. In addition, the elasticity tensor is obtained from laboratory-measured COF, and we calculate the associated seismic velocities. We compare these COF-derived velocity profiles with the measured ultrasonic profiles. Especially in the presence of large ice grains, these two methods show significantly different velocities since the ultrasonic measurements examine a limited volume of the ice core, whereas the COF-derived velocities are integrated over larger parts of the core. This discrepancy between the ultrasonic and COF-derived profiles decreases with an increasing number of grains that are available within the sampling volume, and both methods provide consistent results in the presence of a similar amount of grains. We also explore the limitations of ultrasonic measurements and provide suggestions for improving their results. These ultrasonic measurements could be employed continuously along the ice cores. They are suitable to support the COF analyses by bridging the gaps between discrete measurements since these ultrasonic measurements can be acquired within minutes and do not require an extensive preparation of ice samples when using point-contact transducers.

Dielectric anisotropy as indicator of crystal orientation fabric in Dome Fuji ice core: method and initial results

AbstractPolycrystalline ice is known to exhibit macroscopic anisotropy in relative permittivity (ɛ) depending on the crystal orientation fabric (COF). Using a new system designed to measure the tensorial components of ɛ, we investigated the dielectric anisotropy (Δɛ) of a deep ice core sample obtained from Dome Fuji, East Antarctica. This technique permits the continuous nondestructive assessment of the COF in thick ice sections. Measurements of vertical prism sections along the core showed that the Δɛ values in the vertical direction increased with increasing depth, supporting previous findings of c-axis clustering around the vertical direction. Analyses of horizontal disk sections demonstrated that the magnitude of Δɛ in the horizontal plane was 10–15% of that in the vertical plane. In addition, the directions of the principal axes of tensorial ɛ in the horizontal plane corresponded to the long or short axis of the elliptically elongated single-pole maximum COF. The data confirmed that Δɛ in the vertical and horizontal planes adequately indicated the preferred orientations of the c-axes, and that Δɛ can be considered to represent a direct substitute for the normalized COF eigenvalues. This new method could be extremely useful as a means of investigating continuous and depth-dependent variations in COF.

Seismic observations of a complex firn structure across the Amery Ice Shelf, East Antarctica

AbstractWe use seismic refraction data to investigate the firn structure across a suture zone on the Amery Ice Shelf, East Antarctica, and the possible role of glacier dynamics in firn evolution. In the downstream direction, the data reveal decreasing compressional-wave velocities and increasing penetration depth of the propagating wave in the firn layer, consistent with$\sim$1 m firn thickening every 6 km. The boundary between the Lambert Glacier unit to the west and a major suture zone and the Mawson Escarpment Ice Stream unit to the east, is marked by differences in firn thicknesses, compressional-wave velocities and seismic anisotropy in the across-flow direction. The latter does not contradict the presence of a single-maximum crystal orientation fabric oriented 45–$90^{\circ }$away from the flow direction. This is consistent with the presence of transverse simple shear governing the region's underlying ice flow regime, in association with elevated strain along the suture zone. The confirmation and quantification of the implied dynamic coupling between firn and the underlying ice requires integration of future seismic refraction, coring and modelling studies. Because firn is estimated to cover$\sim$98% of the Antarctic continent any such coupling may have widespread relevance to ice-sheet evolution and flow.

Crystallographic analysis of temperate ice on Rhonegletscher, Swiss Alps

Abstract. Crystal orientation fabric (COF) analysis provides information about the c-axis orientation of ice grains and the associated anisotropy and microstructural information about deformation and recrystallisation processes within the glacier. This information can be used to introduce modules that fully describe the microstructural anisotropy or at least direction-dependent enhancement factors for glacier modelling. The COF was studied at an ice core that was obtained from the temperate Rhonegletscher, located in the central Swiss Alps. Seven samples, extracted at depths between 2 and 79 m, were analysed with an automatic fabric analyser. The COF analysis revealed conspicuous four-maxima patterns of the c-axis orientations at all depths. Additional data, such as microstructural images, produced during the ice sample preparation process, were considered to interpret these patterns. Furthermore, repeated high-precision global navigation satellite system (GNSS) surveying allowed the local glacier flow direction to be determined. The relative movements of the individual surveying points indicated longitudinal compressive stresses parallel to the glacier flow. Finally, numerical modelling of the ice flow permitted estimation of the local stress distribution. An integrated analysis of all the data sets provided indications and suggestions for the development of the four-maxima patterns. The centroid of the four-maxima patterns of the individual core samples and the coinciding maximum eigenvector approximately align with the compressive stress directions obtained from numerical modelling with an exception for the deepest sample. The clustering of the c axes in four maxima surrounding the predominant compressive stress direction is most likely the result of a fast migration recrystallisation. This interpretation is supported by air bubble analysis of large-area scanning macroscope (LASM) images. Our results indicate that COF studies, which have so far predominantly been performed on cold ice samples from the polar regions, can also provide valuable insights into the stress and strain rate distribution within temperate glaciers.

Large-scale englacial folding and deep-ice stratigraphy within the West Antarctic Ice Sheet

Abstract. It has been hypothesized that complex englacial structures identified within the East Antarctic and Greenland ice sheets are generated by (i) water freezing to the ice sheet base and evolving under ice flow, (ii) deformation of ice of varying rheology, or (iii) entrainment of basal material. Using ice-penetrating radar, we identify a widespread complex of deep-ice facies in West Antarctica that exist in the absence of basal water. These deep-ice units are extensive, thick (>500 m), and incorporate multiple highly reflective englacial layers. At the lateral margin of an enhanced flow tributary of the Institute Ice Stream, these units are heavily deformed and folded by the action of lateral flow convergence. Radar reflectivity analysis demonstrates that the uppermost reflector of the deep-ice package is highly anisotropic, due to abrupt alternations in crystal orientation fabric, and consequently will have a different rheology to the ice above and below it. Deformation and folding of the deep-ice package is an englacial response to the combination of laterally-convergent ice flow and the physical properties of the ice column.

The impact of temperature and crystal orientation fabric on the dynamics of mountain glaciers and ice streams

AbstractStreaming ice accounts for a major fraction of global ice flux, yet we cannot yet fully explain the dominant controls on its kinematics. In this contribution, we use an anisotropic full-Stokes thermomechanical flow solver to characterize how mechanical anisotropy and temperature distribution affect ice flux. For the ice stream and glacier geometries we explored, we found that the ice flux increases 1–3% per °C temperature increase in the margin. Glaciers and ice streams with crystallographic fabric oriented approximately normal to the shear plane increase by comparable amounts: an otherwise isotropic ice stream containing a concentrated transverse single maximum fabric in the margin flows 15% faster than the reference case. Fabric and temperature variations independently impact ice flux, with slightly nonlinear interactions. We find that realistic variations in temperature and crystallographic fabric both affect ice flux to similar degrees, with the exact effect a function of the local fabric and temperature distributions. Given this sensitivity, direct field-based measurements and models incorporating additional factors, such as water content and temporal evolution, are essential for explaining and predicting streaming ice dynamics.

A Polarimetric Coherence Method to Determine Ice Crystal Orientation Fabric From Radar Sounding: Application to the NEEM Ice Core Region

Ice crystal orientation fabric (COF) records information about past ice-sheet deformation and influences the present-day flow of ice. Polarimetric radar sounding provides a means to infer anisotropic COF patterns due to the associated birefringence of polar ice. Here, we develop a polarimetric coherence (phase-based) method to determine horizontal properties of the COF. The method utilizes the azimuth and depth dependence of the vertical gradient of the hhvv coherence phase to infer the dielectric principal axes and birefringence, which are then related to the second-order fabric orientation tensor. Specifically, under the assumption that one of the orientational eigenvectors is vertical, we can determine the horizontal eigenvectors and the difference between the horizontal eigenvalues (a measure of horizontal fabric asymmetry). The method exploits single-polarized data acquired with varying antenna orientation. It applies to ground-based “multi-polarization” surveys and is demonstrated using data acquired by Center for Remote Sensing of Ice Sheets (CReSIS) using Multi-Channel Coherent Radar Depth Sounder (MCRDS) from the North Greenland Eemian Ice Drilling (NEEM) ice core region in Greenland. The analysis is validated using a combination of polarimetric matrix backscatter simulations and comparison with COF data from the NEEM ice core. The results are consistent with a conventional model of ice deformation at an ice divide where a lateral tension component is present, with minor horizontal COF asymmetry and the greatest horizontal concentration of crystallographic axes orientated near parallel to the ice divide.

Constraining Recent Ice Flow History at Korff Ice Rise, West Antarctica, Using Radar and Seismic Measurements of Ice Fabric

AbstractThe crystal orientation fabric of ice reflects its flow history, information which is required to better constrain projections of future ice sheet behavior. Here we present a novel combination of polarimetric phase‐sensitive radar and seismic anisotropy measurements to provide independent and consistent constraints on ice fabric at Korff Ice Rise, within the Weddell Sea sector of West Antarctica. The nature and depth distribution of fabric in the ice column is constrained using the azimuthal variation in (1) the received power anomaly and phase difference of polarimetric vertical radar soundings and (2) seismic velocities and shear wave splitting measurements. Radar and seismic observations are modeled separately to determine the nature and strength of fabric within the ice column. Both methods indicate ice fabric above 200‐m depth which is consistent with present‐day ice‐divide flow. However, both measurements also indicate an oblique girdle fabric below 230‐m depth within the ice column, inconsistent with steady state divide flow. Our interpretation is that this deeper fabric is a remnant fabric from a previous episode of flow, which is currently being overwritten by ongoing fabric development associated with the present‐day flow regime. The preexisting fabric is consistent with ice flow from the south prior to ice‐divide formation, in agreement with models of Holocene ice sheet evolution. These findings apply new constraints to the flow history at Korff Ice Rise prior to divide formation and demonstrate the capacity of radar and seismic measurements to map fabric and thus constrain past ice flow.

Method for Anisotropic Crystal-Orientation Fabrics Detection Using Radio-Wave Depolarization in Radar Sounding of Mars Polar Layered Deposits

The polar layered deposits (PLDs) provide a wealth of information about the past climate evolution of Mars. Surface mass fluxes and ice flow mainly governed topography and layering of the PLD. China’s Mars probe including an orbiter and a landing rover will be launched by 2020. A new type satellite-borne Mars penetrating radar instrument has been selected to be a part of the payloads on the orbiter. Its main scientific objectives are to map the distribution of water, water–ice and to detect the soil characteristics at global scale on the Martian crust. Compared with Mars Advanced Radar for Subsurface and Ionospheric Sounding and Shallow Radar, the biggest difference is that the antenna system of this Mars penetrating radar consists of two dipole antennas mutually perpendicular. This special configuration enables the investigation of the ice flow of PLD by detecting and analyzing the features of anisotropic crystal-orientation fabric (COF). Thus, relying on the fact that the radio waves are depolarized while passing through an anisotropic COF layer, in this paper, a method for anisotropic COF detection based on this radar system is proposed. The radar echo formulation of anisotropic COF is derived and the ratio of the signals measured by the two perpendicular antennas is used to analyze the anisotropy of COF. We demonstrate that the ratio is an ideal criterion for the detection and analysis of COF, since it contains all parameters about the anisotropy feature of COF and it is independent of the attenuation in the propagation path and the reflection coefficient. In order to verify the validity of the derived analytical expression of the ratio for the detection of COF, finite-different time-domain simulations are carried out based on a simple model of the subsurface of PLD which contains an anisotropic COF layer. The advantages of this method, the potential application scenarios, and the effects of the Martian environment are also discussed.

Multi-channel and multi-polarization radar measurements around the NEEM site

Abstract. Ice properties inferred from multi-polarization measurements, such as birefringence and crystal orientation fabric (COF), can provide insight into ice strain, viscosity, and ice flow. In 2008, the Center for Remote Sensing of Ice Sheets (CReSIS) used a ground-based VHF (very high frequency) radar to take multi-channel and multi-polarization measurements around the NEEM (North Greenland Eemian Ice Drilling) site. The system operated with 30 MHz bandwidth at a center frequency of 150 MHz. This paper describes the radar system, antenna configurations, data collection, and processing and analysis of this data set. Within the framework derived from uniaxial ice crystal model, we found that ice birefringence dominates the power variation patterns of co-polarization and cross-polarization measurements in the area of 100 km2 around the ice core site. The phase shift between ordinary and extraordinary waves increases nonlinearly with depth. The ice optic axis lies in planes that are close to the vertical plane and perpendicular or parallel to the ice divide depending on depth. The ice optic axis has an average tilt angle of about 11.6° vertically, and its plane may rotate either clockwise or counterclockwise by about 10° across the 100 km2 area, and at a specific location the plane may rotate slightly counterclockwise as depth increases. Comparisons between the radar observations, simulations, and ice core fabric data are in very good agreement. We calculated the effective colatitude at different depths by using azimuth and colatitude measurements of the c axis of ice crystals. We obtained an average effective c axis tilt angle of 9.6° from the vertical axis, very comparable to the average optic axis tilt angle estimated from radar polarization measurements. The comparisons give us confidence in applying this polarimetric radio echo sounding technique to infer profiles of ice fabric in locations where there are no ice core measurements.

Physical Conditions of Fast Glacier Flow: 2. Variable Extent of Anisotropic Ice and Soft Basal Sediment From Seismic Reflection Data Acquired on Store Glacier, West Greenland

Outlet glaciers of the Greenland Ice Sheet transport ice from the interior to the ocean and contribute directly to sea level rise because discharge and ablation often exceed the accumulation. To develop a better understanding of these fast‐flowing glaciers, we investigate the basal conditions of Store Glacier, a large outlet glacier flowing into Uummannaq Fjord in west Greenland. We use two crossing seismic profiles acquired near the centerline, 30 km upstream of the calving front, to interpret the physical nature of the ice and bed. We identify one notably englacial and two notably subglacial seismic reflections on both profiles. The englacial reflection represents a change in crystal orientation fabric, interpreted to be the Holocene‐Wisconsin transition. From Amplitude‐Versus‐Angle (AVA) analysis we infer that the deepest ∼80 m of ice of the parallel‐flow profile below this reflection is anisotropic with an enhancement of simple shear of ∼2. The ice is underlain by ∼45 m of unconsolidated sediments, below which there is a strong reflection caused by the transition to consolidated sediments. In the across‐flow profile subglacial properties vary over small scale and the polarity of the ice‐bed reflection switches from positive to negative. We interpret these as patches of different basal slipperiness associated with variable amounts of water. Our results illustrate variability in basal properties, and hence ice‐bed coupling, at a spatial scale of ∼100 m, highlighting the need for direct observations of the bed to improve the basal boundary conditions in ice‐dynamic models.

Sonic methods for measuring crystal orientation fabric in ice, and results from the West Antarctic ice sheet (WAIS) Divide

ABSTRACTWe describe methods for measuring crystal orientation fabric with sonic waves in an ice core borehole, with special attention paid to vertical-girdle fabrics that are prevalent at the WAIS Divide. The speed of vertically propagating compressional waves in ice is influenced by vertical clustering of the ice crystal c-axes. Shear-wave speeds – particularly the speed separation between fast and slow shear polarizations – are sensitive to azimuthal anisotropy. Sonic data from the WAIS Divide complement thin-section measurements of fabric. Thin sections show a steady transition to strong girdle fabrics in the upper 2000 m of ice, followed by a transition to vertical-pole fabrics below 2500 m depth. Compressional-wave sonic data are inconclusive in the upper ice, due to noise, as well as the method's inherent insensitivity to girdle fabrics. Compared with available thin sections, sonic data provide better resolution of the transition to pole fabrics below 2500 m, notably including an abrupt increase in vertical clustering near 3000 m. Our compressional-wave measurements resolve fabric changes occurring over depth ranges of a few meters that cannot be inferred from available thin sections, but are sensitive only to zenithal anisotropy. Future logging tools should be designed to measure shear waves in addition to compressional waves, especially for logging in regions where ice flow patterns favor the development of girdle fabrics.

Distribution of preferred ice crystal orientation determined from seismic anisotropy: Evidence from Jakobshavn Isbræ and the North Greenland Eemian Ice Drilling facility, Greenland

Preferred crystal orientation fabrics (COFs) within an ice sheet or glacier are typically found from ice cores. We conducted experiments at the North Greenland Eemian Ice Drilling (NEEM) facility ice core location, where COF data were available at Jakobshavn Isbræ west Greenland, to test if COF can be determined seismically. We used observations of anisotropic seismic wave propagation on multioffset gathers and englacial imaging from a 2D reflection profile. Anisotropy analysis of the NEEM data yielded mean c-axes distributed over a conical region of 30° to 32° from vertical. No internal ice seismic reflectors were imaged. Direct COF measurements collected in the ice core agreed with the seismic observations. At Jakobshavn Isbræ, we used a multioffset gather and a 2D reflection profile, but we lacked ice core data. Englacial reflectors allowed the determination of ice column interval properties. Anisotropy analysis found that the upper 1640 m of the ice column consisted of cold ([Formula: see text]) and mostly isotropic ice with c-axes distributed over a conical region of 80° from vertical. The lower 300 m of the ice column was characterized by warm ([Formula: see text]) ice with COF. These observations were consistent with complex ice fabric development and temperature estimations over the same region of Jakobshavn Isbræ. This study demonstrated that the ice sheet and glacier ice anisotropy information can be gained from seismic field observations.