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Institute of Physics Publishing Journal of Physics: Conference Series 41 (2006) 315 322 doi:10.1088/1742-6596/41/1/034 EPS Euroconference XIX Nuclear Physics Divisional Conference Ice core stratigraphy using dual energy x-ray absorptiometry (DEXA) Chris Kroger 1, Julian Thomson 2, Nancy Bertler 2, & Uwe Morgenstern 1 1 Institute of Geological and Nuclear Sciences, PO Box 31312, Wellington, New Zealand 2 Victoria University Wellington, Wellington, New Zealand c.kroger@gns.cri.nz Abstract. We are presenting a technique using x-rays to detect strata caused by density variation in 94 mm diameter ice cores. Moreover, high resolution density is determined. A 54 m long ice core retrieved from the Tasman Glacier of the Southern Alps in New Zealand has been x-ray scanned and the images were analysed. As a dual energy capable x-ray (DEXA) scanner was used, DEXA analysis techniques were used where appropriate, such as for the enhancement of strata visibility in the images. Density calculations though were based on a single energy model, using the fundamental law of x-ray attenuation. As the model does not precisely reflect realistic conditions, calibrations were made for the material properties and pixel scaling. Results of detected strata were compared to traditional visual light methods, where up to a depth of ~35 m better detail was achieved using x-rays. Density data was checked against the average volumetric density. Results compare well with the volumetric density, however a small bias exists, which at present requires further investigation. 1. Introduction Ice cores from glaciers and ice shelves, which are several thousand or more years old, play an important role for the determination of past climates. Such knowledge can contribute to the better understanding of present climates and its potential change due to human impact. Stratified structures due to the layering of precipitation events appear in an ice core like the layers of a tree. Layers are traditionally made visible by placing the ice core segment on an illuminated bench, and detected layers are transferred onto scaling paper by hand, inside a cold (sub-zero) chamber. Other anomalies, such as embedded gas bubbles, are also detected in this way. Further chemical analysis is used to find correlations with past climatic events, which generally are destructive of the ice core and often require relatively large amounts of ice, which can result in a coarse time resolution. After chemical analysis, the ice cannot be used for other investigations. Therefore non-invasive analysis techniques are desired to complement trace analysis. The variation in density along the depth of the ice core can point to precipitation rates and/or snow melting events. While visibly discernible layers are in the mm-range, density calculations are usually averaged over ~800 mm of ice core by weighing the entire core segment and estimating the volume. High resolution density data is therefore usually not available. Penetrating radiation techniques involving x- or γ-rays are well suited for layer detections and density determinations, yet they are expensive and not usually available to ice core researchers. Density measurements were performed successfully by Gerland et al. [1] using γ-rays, and by Hori et al. [2] using x-rays. Precise knowledge of the ice core thickness is necessary, and while Gerland et al. measured the thickness of the ice core at defined intervals, Hori et al. band sawed the ice core into a 2006 IOP Publishing Ltd 315
316 flat piece of constant thickness and low surface roughness. Both assumed that the ice core consisted of pure water and/or used calibrations. Here we used a dual energy x-ray scanner (DEXA) for the analysis, where the preparation of the ice core was minimal, and the scanning was fast. Although the analysis was based on single energies, calibrations on both energies were performed and the results compared with average volumetric densities. The ice cores were kept in their protective plastic sleeves during scanning and scanned at speeds of 0.3 m/sec. The DEXA method has been already successfully employed in a number of interdisciplinary applications [3, 4, 5]. 2. Fundamentals of dual energy techniques An industrial grade x-ray scanner was used operating at 160 kev and ~5 ma. It contains a single x-ray source and an array of dual energy capable detectors [1]. X-ray generator and detectors are stationary, while a conveyor belt transports the ice core through the fan shaped x-ray beam. Figures 1 and 2 show the x-ray scanner used, developed for applications in the food industry, and its principal operation, respectively. Figure 1. DEXA scanner as used in industry. X-ray Source Unit Ice core Detector Arrays Conveying Direction Figure 2. Principal parts of DEXA scanner The scanner is situated in the GNS DEXA laboratory, under ambient temperatures, but kept dry. Typically the conveyor speed is set to 0.3 m/sec, which guarantees the correct aspect ratio of the image, but can be adjusted to other speeds. Two images are taken simultaneously, which is achieved through the use of dual energy capable detectors as shown in Figure 3. X-rays exhibit a spectrum of energies, where the maximum is determined by the voltage settings of the x-ray generator. By filtering the low energies out of the original spectrum, a second spectrum, with a mean higher than that of the original spectrum, is created. The absorption of x-rays inside the material depends on the energy of the x-ray, where absorption patterns become complex with the application of an x-ray spectrum as used here. Therefore calibrations are used to determine absorption characteristics, which also allow the incorporation of the variation of absorption properties with transmission depth due to the so-called beam hardening.
317 LED HED Incoming radiation Filter plate Figure 3. Dual energy x-ray capable detector. LED low energy detector, HED high energy detector. The x-ray beam is partially absorbed by the ice, where for constant x-ray travel length the degree of absorption is a measure for either elemental composition or density. While it can be assumed that the elemental composition of ice cores is fairly constant with respect to the sensitivity of x-rays, changes in absorption are assumed to be attributable to the change in density as the ice core compacts due to pressure and melt-freezing. The fundamental law of x-ray attenuation is I = I 0 e μt. (1) I is the detected x-ray intensity, I 0 the undisturbed intensity, μ the linear attenuation coefficient, and t the thickness of the material. Equation 1 is valid for a pencil shaped, narrow, monoenergetic x-ray beam, but applied here to a fan-shaped x-ray beam with a spectrum of x-ray energies. Calibrations with a solid ice cylinder of the same diameter as the ice core segments were used to find an effective linear attenuation coefficient. The thickness of the travel path in Equation 1 was substituted with the effective thickness, which only considered the length of the path where the x-rays travel through ice and air inside the snow. It was then assumed that the ratio of the effective thickness to ice core thickness is the same as the ratio of ice core density to ice density [2], i.e., = ice t eff /t c, (2) with t eff and t c being the effective and ice core thickness, respectively, the local density and ice the density of water ice. 3. Experimental procedures The ice cores were transported from their storage place to the laboratory inside Styrofoam boxes containing also dry ice. The scanning of the 57 ice core segments took about 90 minutes, and each core was exposed to room temperatures for about 30 seconds. The ice core segments were kept in their plastic sleeves for scanning. The images were then analysed on a PC using Matlab. Using calibrations to determine the real size of one image pixel the thickness of the ice core was modelled for every point, where Figure 4 gives an example. The size of a pixel at the largest dimension, the diameter was used for this model.
318 Figure 4. Thickness model of for calculation of density using calibrated fundamental law of attenuation. Dimensions: x, y-direction mm, z-dimension cm. 4. Results 0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 core depth (m) Figure 5. Upper panel: Low energy x-ray image normalized on average intensity. Scale is in mm. Middle panel: Gradient of intensity along the ice core axis. Lower panel: Plot of detected strata in x- ray image (magenta) and in visual light analysis (black). The core depth was measured from top of the uppermost segment.
319 The upper panel of Figure 5 shows the single energy image of the upper most ice core segment and the strata detected in the image. Figure 5, middle panel, is a graphical enhancement to show increasing or decreasing attenuation. In the lower panel of Figure 5 the comparison between the x-ray image and the traditional light bench method is displayed. Clearly more detail was revealed in the x-ray image than in the ice core on a light bench, as the examples in Figure 5, and further below in Figure 6, show. Figure 6 describes the same analysis as in Figure 5, but investigates a lower laying ice core segment. Here only the x-ray was able to detect a layer of relatively high density, while it eluded the light bench analysis. Core 3, melt water (light) 1.76 1.86 1.96 2.06 2.16 2.26 2.36 2.46 2.56 2.66 core depth (m) Figure 6. Upper panel: Low energy x-ray image normalized on average intensity. Scale is in mm. Middle panel: Gradient of intensity along the ice core axis. Lower panel: Plot of detected strata in x- ray image (magenta) and in visual light analysis (black). The core depth was measured from top of the uppermost segment. Figures 7 and 8 give examples for the calculation of the local density. In Figure 7 again the uppermost ice core segment is shown, where density data is presented with a resolution of about 1 mm 2. Lighter pixels indicate denser areas, while the dark are in the middle section is an area of lower than average ice core. Figure 8 shows an ice core segment from greater depth, where the natural variation along the ice core axis can be shown.
320 Figure 6. High resolution density image for uppermost segment [Mg/m 3 ]. The curve in the ice core is a result of the movement of the ice core on the conveyor belt. Figure 7. High resolution density image for ice core segment at about 8 m depth [Mg/m 3 ]. Melt-layers are clearly visible. As a control measure the average density per ice core segment has been calculated and compared to the volumetric density. In Figure 8 both density values are plotted against each other. The values agree well, although a consistent offset exist, where x-ray calculated densities are higher than volumetric densities. It is assumed that the variation is caused by a) a larger error margin for the volumetric density for softer ice cores, and b) assuming the same attenuation coefficient for snow, firn and ice. The calibrations were performed using solidly frozen tap water. The assumptions are supported by the decreasing residuals with increasing density and/or increasing depth, expressed as relative and absolute errors, shown in Figure 9. More work on the calibration model will be carried out to understand and possibly correct this offset.
321 density [Mg/m3] 1 0.9 0.8 0.7 0.6 0.5 0.4 0 10 20 30 40 50 60 depth [m] Vol density DEXA Low DEXA High Density ice Figure 8. Comparison of volumetric with x-ray calculated densities over the average depth of the ice core segment. Relative difference (%) 16 12 Low High 8 4 0 0.4 0.5 0.6 0.7 0.8 0.9 1 avg density(dexa, volumetric) [Mg/m3] Absolute difference [Mg/m3] 0.02 0-0.02-0.04-0.06-0.08 Low High avg Low = -0.027 avg High = -0.018 0 10 20 30 40 50 60 depth [m] Figure 9. Relative difference of volumetric and x-ray densities as a function of average ice core density (top panel), and absolute difference over ice core depth (bottom panel). A decreasing variance can be seen with increasing density and/or depth. The error in the density calculation can be calculated from ρ ρ ρ Δρ = ΔI + Δμ + Δts 123 I μ t 123 s 123 i ii iii
322 with i) error due to x-ray variation, ii) error due to attenuation coefficient variation, and iii) error due to detector coverage (t s ). The detector has a finite size and in the image it will either be appear to be covered or not, even though only parts of it were covered by the scanned object. The error is only accumulative for the border pixels. Assuming that half of the detector had to be covered to register it in the image, the maximum error in t s is 1 pixel. The standard deviations, derived empirically and through assumption of a maximum possible error in t s, were used in this equation. The error in the density, mainly caused by a statistical variation in core diameter measurement, was calculated to be just under 2%. 5. Conclusion An complete ice core of 54 m has been scanned using x-rays at two different mean energies. The resulting images were analysed with respect to revealing existing strata local densities their differences to visual light analysis. Excellent detail is revealed at shallow depth up to about 35 m. At increasing depth, where density differences diminish, less detail can be seen. Local density calculations provide good results. The comparison of the average density with the volumetric density is favourable, but a consistent higher average density is shown, in particular for lower average x-ray energy. Improvements to the method may be made by more accurate thickness determinations of the ice core. Acknowledgments. This work has been supported with an internal grant by GNS. References [1] C.M. Bartle et al., New uses of x-ray transmission techniques in the animal based industries, Rad. Phys. Chem., 71, 2004. [2] A. Hori et al., A detailed density profile of the Dome Fuji (Antarctica) shallow ice core by x-ray transmission method, Ann. Glaciology, 29, 1999. [3] Kroger et al., Wool Base determination using dual energy x-ray absorptiometry (DEXA), Appl. Rad. Isotopes, in press. [4] C. Falcucci et al., X-ray differential absorptiometry for Pb content evaluation in airborne dust, Appl. Rad. Isotopes, 45, 1994. [5] Kroger, State-of-the-art x-ray scanning technologies in rough lumber mills, Client report 2004/05, 2004. [6] C.M. Bartle et al., New uses of x-ray transmission techniques in the animal based industries, Rad. Phys. Chem., 71, 2004.