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C. R. Bentley: Recent sonic logging experiments in a hole to bedrock in an ice cap on Ellesmere Island reveal a distinct increase in P-wave velocity in the Wisconsin ice. The velocities further suggest that the principal fabric change is shallower, and R. M. Koerner (personal communication) believes it reasonable by analogy to Devon Island to assume that there is no particular tightening of the c-axis concentration in the Wisconsin ice. Recognizing the difficulty in the Byrd Station core, where several changes occur in the same depth range, have you tried to correlate P-wave velocity with bubble content, bubble elongation, grain-size, dirt content, etc. ? The effect of these properties on the velocities would be small compared to the fabric changes but might nonetheless be significant, and useful in interpreting sonic velocity elsewhere.
A. J. Gow: Studies of the physical properties of the Ellesmere Island ice cores have not yet been performed and, until they are, I feel any objective discussion of the velocity increase you measured down-hole would be a bit premature at this time. I might hazard a guess that when the cores are analysed you will find that fabrics are responsible for the velocity increase.. Other effects, such as bubble concentrations, grain-size, dust content, etc., would be difficult to separate from fabric effects which, as you agree, over-ride all other effects with regard to velocities and velocity differences in the ice cores from Byrd Station. Dr Koerner informs me that cores from the same level that you observed your velocity increase contain no visible concentrations of debris so presumably this particular effect can be eliminated.
H. Kohnen: We looked for effects other than that caused by crystal anisotropy, which is certainly the dominating effect that over-rides all other effects. We have investigated the influence of preferred orientation of elongated bubbles in the Little America V ice cores. Our examination of several different samples in these cores indicates a minimal effect. However, an elongated, oriented bubble fabric is not fully developed in Little America ice and more data are needed to determine whether a preferred bubble fabric does exert a significant effect on P-wave velocities. At Byrd Station, crystal size is known to affect velocities and velocity differences in ice samples composed of only a few crystals. These investigations are not yet completed. As demonstrated in our paper, the effects of relaxation can lead to the formation of oriented cracks that can cause significant reduction in velocities for P-waves propagated normal to the crack fabric. In order to investigate some of these effects, it is absolutely essential to obtain velocity data on freshly-drilled ice cores.
J. W. Glen : Since the authors have used difference in velocity as the parameter of interest, I do not think we should expect grain-size as such to be important—grain shape might be if the grains were not equi-axed.
Gow: In general, I would agree with what you say. However, grain-size (and grain-shape) effects are known to be significant in ice below I 800 m at Byrd Station. In this ice the size of crystals is so large that samples used for ultrasonic velocity measurements usually contained fewer than ten crystals, often complexly interlocked. This situation can lead to significant velocity bias.