The Relationship of Ultrasonic Velocities to c-axis Fabrics and Relaxation Characteristics of Ice Cores from Byrd Station, Antarctica

Abstract

Deep cores from Byrd Station were used to calibrate an ultrasonic technique of evaluating crystal anisotropy in the Antarctic ice sheet. Velocities measured parallel (V_p ↓) and perpendicular (V_p →) to the vertical axis of the cores yielded data in excellent agreement with the observed c-axis fabric profile and with the in-situ P-wave velocity profile measured parallel to the bore-hole axis by Bentley. Velocity differences ΔV (ΔV = V_p ↓ – V_p→) in excess of 140 m s⁻¹ for cores from below 1300 m attest to the tight clustering of c-axes of crystals about the vertical, especially in the zone 1 300-1800 m. A small but significant decline in V_p ↓ with ageing of the core, as deduced from Bentley’s down-hole data, is attributed to the formation of oriented cracks that occur in the ice cores as they relax from environmental stresses. This investigation of cores from the 2164 m thick ice sheet at Byrd Station establishes the ultrasonic technique as a viable method of monitoring relaxation characteristics of drilled cores and for determining the gross trends of c-axis orientation in ice sheets. The Byrd Station data, in conjunction with Barkov’s investigation of deep cores from Vostok, East Antarctica, also indicate that crystal anisotropy in the Antarctic ice sheet is dominated by a clustering of c-axes about a vertical symmetry axis.

Résumé

On a utilisé des carottes de glace profonde issues de la station Byrd pour essayer une technique ultra-sonique d’estimation de l’anisotropie des cristaux dans la calotte glaciaire antarctique. Les vitesses mesurées paralléles (V_p ↓) et perpendiculaires (V_p→) à l’axe vertical des carottes ont donné des résultats en excellent accord avec les profils de disposition cristalline des c-axes et avec le profil de vitesse in situ des ondes P mesuré parallélement à l’axe du trou de forage par Bentley. Des différences de vitesses ΔV (Δ V = V_p ↓ — V_p→) dépassant 140 m s⁻¹ pour des échantillons provenant d’en dessous de 1300 m confirment le groupement serré des axes-c des cristaux autour de la verticalc, spécialement dans la zone des 1 300-1800 m. Une légére mais significative diminution de V_p ↓ avec l’âge de l’échantillon, déduite des résultats de forages de Bentley est attribuée à la formation de fissures orientées dans les échantillons de glace lorsqu’elles échappent aux contraintes de leur milieu d’origine. Cette exploration de carottes provenant d’une calotte de 2164 m d’épaisseur à la station Byrd montre que la technique des ultra-sons est une méthode convenable pour repérer les caractéristiques de la relaxation des carottes de forage et pour déterminer les tendances principales de l’orientation des axes-c dans les calottes glaciaires. Les résultats obtenus à la station Byrd, comparés avec les recherches de Barkov sur des carottes profondes provenant de Vostok, Est Antarctique, indiquent également que l’anisotropie dans la structure de la glace de la calotte antarctique est diminuée par un regroupement des axes-c autour d’un axe de symétrie vertical.

Zusammenfassung

Für die Kalibricrung einer Ultraschalltechnik zur Feststellung der Kristall-Anisotropie im antarktischen Eisschild wurden tiefreichende Bohrkerne von der Byrd-Station herangezogen. Geschwindigkeitsmessungen parallel (V_p ↓) und senkrecht (V_p →) zur Längsachse der Kerne lieferten Daten, die ausgezeichnet mit dem beobachteten Profil des c-Achsen-Gefüges und mit dem Profil der Geschwindigkeit der P-Welle, in situ parallel zur Bohrlochachse gemessen von Bentley, überein-stimmen. Geschwindigkeitsunterschiede ΔV (ΔV = V_p ↓ — V_p ↓) von mehr als 140 m s⁻¹ für Kerne aus einer Tiefe über 1300 m bestätigen die dichte Häufung der c-Achsen der Kristalle um das Lot, besonders in der Zone von 1300 m bis 1800 m. Eine kleine aber signifikante Abnahme von V_p ↓ mit der Alterung des Kerns, wie sie gegenüber Bentley’s Bohrlochdaten festzustellen ist, wird der Bildung orientierter Risse zugeschrieben, die in den Eiskernen bei der Entspannung vom Umgebungsdruck auftreten. Diese Unter-suchung von Kernen aus dem 2164 m dicken Eisschild an der Byrd-Station erweist die Ultraschalltechnik als gangbare Methode zur Ermittlung der Relaxationscharakteristiken von Bohrkernen und zur Bestimmung der überschlägigen Tendenz der c-Achsen-Orientierung in Eisschilden. Die Daten an der Byrd-Station zeigen, zusammen mit Barkov’s Untersuchung tiefreichender Bohrkerne von Vostok, Ost-Antarktis, ausserdem, dass Kristall-Anisotropie im antarktischen Eisschild in erster Linie auf eine Häufung der c-Achsen um eine vertikale Symmetrieachse zurückzuführen ist.

Introduction

The importance of crystal anisotropy (oriented crystal structure) in determining the rheological behavior of polar ice sheets can no longer be ignored, especially in the light of recent observations of widespread crystal anisotropy in West Antarctica. This anisotropic state of the ice has been established both on the basis of direct examination of c-axis fabrics in ice cores from the 2164 m deep drill hole at Byrd Station and from seismic records. Fabric investigations of the Byrd Station cores by Gow and Williamson (1976) show that oriented crystal structure, involved principally with a clustering of crystallographic c-axes about a single (vertical) axis of symmetry, certainly exists to within 400 m of the surface of the ice sheet and thus represents about 80% of the ice column in the immediate vicinity of the drill hole. Major features of this fabric profile are illustrated in Figure 1. Maximum vertical orientation of crystal axes occurs in a zone from 1200 to 1800 m depth. Below about 1810 m this very strong axial orientation of crystals gives way to a fabric composed of several discrete maxima generally arranged in ring-like fashion about the vertical. Sonic logging of the drill hole by Bentley (1972) has also confirmed the existence of a strong vertical alignment of c-axes and Bentley (1971), on the basis of seismic records, has also demonstrated the existence of highly anisotropic structure throughout much of the West Antarctic ice sheet. Such structural anisotropy could involve as much as 90% of the ice column at some locations.

Fig. 1. Fig. 1 (left), c-axis fabrics, Byrd Station, Antarctica. Data from horizontal core sections. Contour range from ½% to 5% per 1% area for fabrics near top of the ice column to 25% per 1% area in deep ice fabrics. Symbols a-g are keyed to Figure 3.

Fabric studies (Gow and Williamson, 1976), in conjunction with other extensive investigations of the physical properties of the Byrd Station ice cores (Gow, 1970, 1971; Gow and Williamson, 1975; Gow and others, 1968), make them especially suitable for “calibrating” certain geophysical tools that might usefully be applied to investigations of crystal anisotropy in ice sheets. One such technique involves ultrasonic logging, a technique first applied to Greenland and Antarctic ice cores by Bennett (1972). More recent studies include measurements by Kohnen and Langway (1977) on ice cores from Milcent and Crête, Greenland, and measurements on cores from the Ross Ice Shelf by Kohnen and Bentley (1977).

In this report we wish to discuss some results of recent measurements of ultrasonic velocities performed on ice cores from Byrd Station. Sonic logging of these cores, in a sense, amplifies and extends the original bore-hole logging of Bentley (1972), but with the notable difference that Bentley’s measurements were restricted to P-wave-velocity determinations along the bore-hole axis, whereas our measurements were made in the diametral direction (V _p →), as well as along the vertical core axis (V _p ↓), the axis corresponding to the bore-hole axis. This dual velocity measurement permits immediate evaluation of the velocity difference (ΔV), a very important parameter whose magnitude depends entirely on the orientation of crystals in the ice core. ^* Our research served two principal objectives, namely, determination of the relationship between ultrasonic velocities and the c-axis fabrics, and evaluation of the relaxation characteristics of the drilled cores, especially the directional aspects of relaxation.

Experimental Techniques

A Krautkrämer USIP II System, utilizing barium titanate transducers, was used for measuring the transverse and axial velocities of cores ranging in length from 70 to 15 mm. The equipment was operated at a frequency of 2 MHz to ensure an optimal relation between time resolution, sample dimensions, and energy attenuation. All measurements were performed in a cold room operating at a nominal temperature of —10_°C. Sample dimensions were measured with a micrometer to ±0.1 mm. Errors in the velocity measurement are estimated not to exceed ±20 m s⁻¹. Core densities were measured to an accuracy of 0.0003 Mg m⁻³ by hydrostatic weighing in iso-octane. Most of the cores examined ultrasonically were sampled earlier for thin-section fabric studies. Re-examination of these thin sections together with observations on several new thin sections of the same cores revealed no detectable changes in either the textural properties or the c-axis orientations of the ice.

Results and Discussion

Profiles obtained from measurements of ultrasonic velocity on Byrd Station ice cores are presented in Figure 2. Both sets of P-wave velocity measurements (V _p↓ and V _p →) have been corrected to in-situ temperatures and densities to facilitate comparisons with Bentley’s (1972) down-hole velocity log. Axial (V _p ↓) velocities show very good agreement with Bentley’s velocity profile down to about 1200 m before beginning to diverge appreciably. The apparent reduction in V _p ↓ of the nine-year-old cores is real and can be attributed substantially to the existence of cleavage cracks that are propagated along the basal planes of ice crystals during relaxation of the cores. This propagation of cracks is concentrated in the transverse plane of cores from the zone of strong axial fabrics below 1200 m, giving rise to a distinctive crack fabric, that causes a small but measurable reduction of velocity in the direction normal to the plane of the cracks, that is in the direction of V _p↓. These data point up the significance of the directional nature of relaxation and the importance of oriented crystal structure in determining the orientation of cleavage cracks that form in cores during relaxation from environmental stresses. A similar depression of seismic velocities, normal to the plane of oriented cracks, has also been observed in rock cores even in rocks where the volume fraction of oriented cracks is only a small part of the total porosity (for example, see Anderson and others (1974)).

Fig. 2. Axial (Vp t) and diametral (Vp→) ultrasonic P-wave-velocity profiles for ice coresfrom Byrd Station deep drill hole. Bentley (1972) velocity profile was obtained from measurements along bore-hole axis. Note that Vp t is parallel to the bore-hole axis.

Figure 3 illustrates how well changes in ΔV (V _p ↓ — V _p →) can be correlated with changes in the c-axis fabric as defined by, α the half-apex angle of the cone containing all but the 10% most divergent c-axes in a given fabric. As noted above, ΔV is determined solely by the fabric, so that any significant change in the ice fabric should be reflected in a change in ΔV. In order to quantify the fabric, we selected α as the relevant parameter mainly on the assumption that the dominantly axial fabric patterns observed at Byrd Station can be reasonably approximated by conical distributions of the c-axes about the vertical, with α decreasing as the concentration of c-axes increases. The mirror-image pattern obtained with ΔV and α further establishes the strong relationship between P-wave velocity and fabric. In the zone 1 200-1800 m, where the magnitude of α is generally less than 23^°, ΔV values may exceed 140 m s⁻¹ which is within 30—40 m s⁻¹ of the total velocity difference for P-waves propagated parallel and perpendicular to the c-axis of a single crystal. This analysis was not extended below 1800 m, mainly because of the very large size of crystal encountered in this region. Samples used for ultrasonic velocity measurements generally contained too few crystals for any satisfactory verification of the ΔV/α relationship.

Fig. 3. Relationship of velocity difference (∆V) and fabric parameter (α) versus ice depth at Byrd Station. See text for explanation of a—g Symbols a-g refer to locations of c-axis fabric plots shown in Figure 1.

Conclusions

Based on results obtained at Byrd Station, we would advocate that all future studies of cores from deep drill holes include measurements of ultrasonic velocities, both as a means of monitoring the relaxation characteristics of the drilled cores and for determining the gross trends of c-axis orientation in the ice sheet. However, these measurements must be supplemented by optical thin-section studies in order to verify (I) the exact nature of the fabric at any given depth and (2) any inclination of the fabric symmetry axis with respect to the direction of propagation of the P-wave velocity. Bentley (1971), for example, has speculated from seismic records in West Antarctica that the symmetry axis for single-pole fabrics may deviate appreciably from the vertical; at some locations the symmetry axis may even approach the horizontal. However, Barkov’s (1973) orientation data from Vostok, East Antarctica, indica te that fabrics in this part of the ice sheet are also dominated by a vertical symmetry axis. The situation in Greenland cannot be ascertained because of the absence of published data on the internal structure and fabrics of the ice.

At Byrd Station, some slipping of ice along the bedrock must occur since the basal ice is at the pressure-melting point and liquid water is known to exist at the glacier bed. However, the textures and fabrics of the ice cores indicate that plastic deformation in the zone of strong single-maximum fabrics, and actual displacement of ice along horizontal shear planes situated well above bedrock, are also major contributors to the flow of the ice sheet at Byrd Station. These findings of highly anisotropic crystal structure at Byrd Station are especially significant with respect to widely held theories of ice flow that tacitly assume an isotropic condition of the ice. Indeed, the common practice of approximating the age of ice cores on the basis of arbitrary and simplistic models of ice flow, that either ignore or gloss over the anisotropic state of the ice, could lead to very serious errors in dating ice cores. If the situation demonstrated for the ice cores at Byrd Station is any guideline, then the need for comprehensive analysis of the structure and fabric of ice-sheet cores by any means available cannot be overemphasized.