Ice Core Tomography with Laser Ultrasonics - Colorado School of ...

MIKESELL ET AL.: ICE CORE TOMOGRAPHY X - 33. ResultsFirst results are taken on man-made ice cores. This manufacturedtest core is composed of a pure ice section anda sand/ice mixture section. In the sand/ice section, it isunknown how close the sand grains are to each other. Thegrains are placed in the column while the water is still inliquid form. Figure 4 shows the reproducibility of the tomographicmeasurement: three transmission measurementswith the source and receiver in the same position, are takenover a span of two hours while leaving the ice in the coldchamber. This particular experiment is performed in thepure-ice section of the core. The similarity of the wave formsshow that the source laser and the cold chamber have no significanteffect on the elastic wave propagation in the sample:the first arrival time and phases match trace for trace, evenat late times. Maybe more surprisingly, the source laser doesnot visually damage the face of the core, either.Figure 5 is a comparison of the wave forms in the pureice and sand/ice mixture. The overall signatures of the wavefields vary in terms of frequency content and amplitude, becausethe thermoelastic expansion of the ice/sand mixturetime (ms)00.010.020.030.04Figure 5. Wave fields excited and detected in ice (left)and a sand/ice mixture (right) show that the speed ofthe elastic waves, frequency content and amplitude varysignificantly for these two samples.time (ms)00.030.050.08distance (mm)50 100 150 200 250Figure 6. Top panel contains the individual wave fieldsalong a 25 cm ice core from Antarctica. The gray scalebar is our interpretation of layering. After these recordingsand interpretation was made, layers in the ice wereobserved visually and are indicated by the arrows. Notethat not only the variations in the first arrival time matchthe layers, but that the entire wave field characteristicsare different from layer to layer.is quite different than that of the pure ice. The figure alsoshows that the sand/ice mixture has a later arrival time forthe first (P-)wave than does the pure ice section. Similarslowing effects have been observed for elastic waves whenice contains a large amount of salt, diminishing the totalYoung’s modulus [Rajan et al., 1993].An ultrasonic tomogram of a 250-mm long ice core froma depth of 124 m below the ice surface at the South Pole,Antarctica is shown in Figure 6. Overall, this section ofcore contains layers that cannot be easily identified visually.At first glance, this core looks homogeneous throughoutwith a very constant grain size and color. The transmissionmeasurements are taken every 5 mm along the core, and thedata is bandpass filtered between 20 and 50 kHz. It is seen inthis plot that the average arrival times for P-waves is around12 µs. With the ice core having an approximate width of3.9 cm in the location where the transmission measurementis made, the resulting apparent velocity is 3250 m/s. Thisis well within the range of acoustic velocities expected [e.g.,Gagnon et al., 1990]. We see variations up to a magnitudeof 5 µs in first arrival time along the core. This implies smalldifferences in the elastic properties, as expected from annuallayering. In addition, larger variations in the wave forms occurat later times: while the first 70 mm of the core presentsstrong scattering or resonances, between 70 and 110 mm thecore only shows a clean first arrival. This alternating patternpersists in the rest of the core. Since waves arriving at latertimes have effectively sampled more of the core, variationsin the late-time wave forms are enhanced [Snieder et al.,2002]. Alternatively, the larger variations can be the resultof different thermoelastic response of individual layers to ourlaser source. In either case, characteristics of the first-arrivaltime and overall wave forms are consistent in five distinctiveregions of the core. These regions are depicted in Figure 6with the gray scale bar. After this interpretation, a morecareful visual inspection of the core confirms three of thetransitions between layers. These transition layers are seenat 4 cm, 11 cm, and 21 cm and annotated with the blackarrows. A change in wave form is easily seen in Figure 6at 11 cm and 21 cm while the transition at 4 cm is harderto decipher from the wave forms. On the other hand, distinctchanges in the waveforms at 7 and 18 cm cannot beconfirmed visually. However, these layerings correlate wellto what is known about overall layering thicknesses at thisdepth in this area of Antarctica. This suggests that ourmethod can complement a visual inspection.4. Conclusions and discussionWith a relatively simple data acquisition system, we areable to detect annual layering in ice cores, using noncontactinglaser ultrasonics. This method explores small variationsin the elastic parameters caused by annual layeringin the ice. This method is nondestructive, and can readilycontribute to existing dating methods, and further studieswith laser ultrasonics might provide other pieces of informationfor ice core research, such as contamination and (shear)strength of the ice. The latter could more easily be obtainedby measuring the horizontal components of the wave field[Fukushima et al., 2003] in future experiments. The nextgeneration system will include a cold chamber that will beable to hold cores of the standard borehole dimensions (7.5-12 cm diameter x 1 m length). The cold chamber we usehere is smaller than normal cores, so the sample must be cutdown prior to being placed in the cold chamber. This cuttingresults in alteration of the core, affecting further testingby other methods. A computer automated translation stagefor the cold chamber will allow for sub-mm precise movementof the core and thus higher spatial resolution for thetomographic measurement. Finally, a more powerful laser