Seismic wave propagation in anisotropic ice – Part 1: Elasticity tensor and derived quantities from ice-core properties

Abstract

Abstract. A preferred orientation of the anisotropic ice crystals influences the viscosity of the ice bulk and the dynamic behaviour of glaciers and ice sheets. Knowledge about the distribution of crystal anisotropy, to understand its contribution to ice dynamics, is mainly provided by crystal orientation fabric (COF) data from ice cores. However, the developed anisotropic fabric does not only influence the flow behaviour of ice, but also the propagation of seismic waves. Two effects are important: (i) sudden changes in COF lead to englacial reflections and (ii) the anisotropic fabric induces an angle dependency on the seismic velocities and, thus, also recorded traveltimes. A framework is presented here to connect COF data with the elasticity tensor to determine seismic velocities and reflection coefficients for cone and girdle fabrics from ice-core data. We connect the microscopic anisotropy of the crystals with the macroscopic anisotropy of the ice mass, observable with seismic methods. Elasticity tensors for different fabrics are calculated and used to investigate the influence of the anisotropic ice fabric on seismic velocities and reflection coefficients, englacially as well as for the ice-bed contact. Our work, therefore, provides a contribution to remotely determine the state of bulk ice anisotropy.

Ice fabric in an Antarctic ice stream interpreted from seismic anisotropy

“…SWS parameters expected for different ice fabrics. (left column) Schematic for each fabric type (adapted from Maurel et al [] and Diez and Eisen []) with the envelope of c axes (blue area) and the projection of these c axes on an upper hemisphere plot (blue dots on the horizontal plane). The angles ξ and θ are used to describe the opening angle of the c axes envelopes in the X 1 and X 2 directions, respectively.…”

“…The ice fabric model that provides the best fit to the observed SWS measurements is composed of a mixture of 47% horizontal partial girdle (HPG), an orthorhombic fabric with a narrow opening angle of θ = 22°, orientated near orthogonal to the ice flow direction (Figure a) and 53% cluster fabric with an opening angle of θ = 73° (Figure b). While partial girdle fabrics have been commonly hypothesized in the literature [e.g., Nanthikesan and Shyam Sunder , ; Maurel et al , ; Diez and Eisen , ] the HPG ice fabric has not been observed in glacial ice before; henceforth, we refer to the mixed HPG and cluster fabric as “diffuse HPG.”…”

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

“…The widely used nomenclature of COF in ice, such as cluster or girdle fabric, relates to the geometry of the envelopes which encapsulate the c axes of the fabric (e.g., Diez & Eisen, ). In a VTI, or cluster, fabric the c axes are orientated in vertical cone shapes.…”

“…Fabric strength may be described by the relative tightness of the cluster or girdle and quantified by the opening angle of the envelope (Diez & Eisen, ). The strength of the fabric may also be described by the eigenvalues of an ellipsoid fit to the envelope geometry (Diez & Eisen, ).…”

Cryoseismology

“…Structural anisotropy influences both the elastic [Diez and Eisen, 2015] and viscous [Placidi et al, 2006] properties of glacier ice. Seismic signals can therefore be used to study differences in viscous deformation of different ice layers.…”