Tasman Glacier ice core stratigraphy and high resolution density calculations using x-ray scanning techniques and image analysis

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Kroger, C. 2005 Tasman Glacier ice core stratigraphy and high resolution density calculations using x-ray scanning techniques and image analysis . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2005/36 113 p.

Abstract: A 54 m long ice core from the Tasman Glacier in the Southern Alps of New Zealand, an area of relative warmth and which experiences annual precipitations of several meters, was investigated using a novel method based on x-ray scanning. The stratigraphy of refrozen melt water layers in the ice core was created by using digital x-ray attenuation images, obtained from a commercial x-ray scanner. This scanner allows images to be taken at speeds of approximately 0.3 m/s, and produces synchronously two images at different mean energy levels. The images were also the basis to calculate the ice core density with a spatial resolution of ~1 mm2, which relates to 1 measurement for every millimetre of ice core depth. Suitable calibrations bodies were developed and utilized for the density calculations. Dual energy x-ray analysis methods were used for the melt layer stratigraphies, while single energy analysis was used for the density calculations. Detected strata in the images were compared with manually derived stratigraphies using visual light. The x-ray images reveal significantly more detail than light stratigraphies in shallow ice cores down to about 35 m, while light apparently detects more detail further down. The mean density derived from the high resolution density images agreed with the volumetric density within 2.8%. A bias existed; where x-ray derived densities are generally higher than volumetric values. This bias may be the result of the scanning procedure employed, namely the replacement of the x-ray source between the scanning of the ice cores and the calibration bodies. Overall the method provides an unprecedented way of non-invasively measuring a highly resolved density without interfering with the ice core before, during, or after scanning. The density calculation may be considered complementing or even substituting the manual light stratigraphy, as it is exact and not prone to human subjectivity. (auth)