The Investigation of Ice Shelf Englacial Properties from Borehole Seismic Data in the Ross Ice Shelf

Abstract

Ice deformation is a ected by the presence of crystallographic preferred orientation (CPO) fabrics, causing anisotropic deformation, and temperature, which is a key determinant on viscosity. Seismic wave propagation in ice has been shown to be sensitive to both the presence of CPO and temperature and is able to map these englacial properties spatially beyond individual ice coring sites which are traditional information sources of these properties. This study utilises borehole and surface seismic observations from two ice shelf sites to investigate the e ect of CPO and temperature on seismic properties in eld scale experiments. A focus is put on the data analysis from borehole seismometers at di erent depths inside the ice shelf and therefore o ering a wider range of raypath geometries than what conventional, surface-based seismic acquisition can achieve. The P-wave velocity structure of the ice shelf is derived at one eld site in the central Ross Ice Shelf, showing strong seismic anisotropy both in polar and azimuthal direction with a maximum P-wave velocity variation of 6:8%. A discussion of the sensitivity of di erent acquisition geometries to obtain seismic velocities in the given seismic anisotropy is performed. It is shown that the e ect of anisotropy can be avoided by studying velocities from a crosshole geometry, allowing to resolve vertically changing seismic properties. The analysis of this dataset demonstrates that ice temperature is neither the pivotal determinant on seismic Pwave velocity nor attenuation inside the ice shelf, leaving the employability of seismic methods to inform ice temperature questionable at the scale that is studied in this research. A further investigation into ice shelf anisotropy is performed through the study of shear wave splitting observations in an active source multi-azimuth and multi-o set vertical seismic pro le dataset. A full work ow from seismic acquisition to analysis and CPO model creation is introduced. A CPO forward model is found to explain the seismic observations and to be consistent with the macroscopic deformation regime, adding to the sparse observations of CPO from ice shelves. Vertically changing seismic properties as information source on the ice shelf structure are discussed by comparison with visual images from inside a vertical borehole and ice-penetrating radar data.