Questions on the interpretation of ice radar data from ...

Questions on the interpretation of ice radar data from the Bonar Glacier, New Zealand Adrian McCallum University of the Sunshine Coast, Australia Matthew Rogerson Department of Defence, Australia Abstract

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Preliminary radio glaciological investigation of the Bonar Glacier, New Zealand occurred in May 2014. Data recorded from bespoke ice radar equipment is difficult to interpret and it is unknown whether the originally envisaged data, depth to bedrock, is recoverable from the discrete information available. This letter presents a brief overview of the work and provides details on preliminary analysis to this time. Advice is sought from the broader glaciological community on ways via which useful data may be extracted from the discrete data presented herein.

Introduction The Bonar Glacier is a high-level valley glacier situated in the shadow of Mt Aspiring, in the New Zealand Southern Alps (see Fig 1).

~ 2000 m

Figure 1: The Bonar Glacier in the Mt Aspiring National Park, New Zealand. Mt Aspiring is in the top left of the image. Approximate position of Colin Todd Hut and the access ridge (French Ridge) are noted. (Image courtesy of New Zealand National Institute of Water and Atmospheric Research (NIWA).) 13 14 15 16

Whilst glaciological studies have been undertaken on many New Zealand glaciers Chinn (2001) and mountaineers regularly access the area, no glaciological observations of the Bonar Glacier are known to have occurred (T. Chinn, personal communication, 18 June 2014). 1

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A small glaciological expedition to the Bonar Glacier was conducted over the period 22 May to 1 June 2014 to conduct a preliminary glaciological assessment of the glacier. The intent of the research was to confirm logistical arrangements necessary to access the glacier and to obtain preliminary surface ice movement and ice thickness information. Ongoing analysis of recorded radar data is proving challenging. This letter serves as a brief presentation of recorded data and analysis to this time; the intention being to draw productive discussion and advice from the broader glaciological community, to better allow for useful interpretation of gathered data.

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Method

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Access to the Bonar Glacier was made on foot, accessed via French Ridge from the road-head at Raspberry Flat. Testing occurred over the period 28-30 May 2015, based from Colin Todd Hut (see Figure 1). The original intent was to use available commercial Ground Penetrating Radar (GPR) equipment, a MALA ProEx with 25 MHz Rough Terrain Antenna (RTA). However, personal communication (L. Mingo, 14 Feb 2014) suggested that other parties had experienced difficulty in imaging to depths of ∼ 300 m in polythermal valley glaciers with this envisaged antenna, therefore it was decided to use a bespoke ice radar with a centre frequency of ∼ 8 MHz utilising 5 m resistively-loaded dipole antennae, based upon the Narod impulse transmitter (Narod and Clarke, 1994). Antennae were constructed in accordance with guidance provided within Icefield Instruments Inc. (2000); each antennae dipole arm consisted of five wire-linked resistors in series, giving a total resistance of 205 Ω per arm. The utilised ice profiling system comprised:

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1. Narod impulse transmitter,

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2. 5 m resistively loaded transmitting antenna,

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3. 5 m resistively loaded receiving antenna, with 50 Ohm feed-through adapter to reduce noise, and 4. Picoscope 5243A, 2-channel, 100 MHz, USB oscilloscope, for identification of received radar pulse and data recording.

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Use of Gecko data acquisition software (Pettersson, 2014) was envisaged. However, incompatibility with the 5243A Picoscope USB oscilloscope meant that continuous profiling was not possible and only discrete data were obtained at forty points over a two day period. Data were recorded by photographing the screen of the Panasonic Toughbook laptop. Selected individual digital photographs were later digitised using GraphClick (Arizona Software, 2010) software to enable data manipulation.

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Results

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Figure 2 shows an example of the radar data as photographically recorded and Figure 3 shows an example of digitised data, generated using GraphClick (Arizona Software, 2010); no useful scale is presented. Most recorded data were of this form or similar in nature.

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Figure 2: Photograph of typical ice radar return. In this image, each vertical increment represents 40 mV and each horizontal increment 1.0 µs. The yellow diamond shows the value (+ 80 mV) at which the oscilloscope trigger was set to commence data capture.

Figure 3: Digitised version of dominant data shown in Figure 2, created using GraphClick (Arizona Software, 2010). 56

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Discussion Upon initial examination of data such as that represented in Figure 2 we naively assessed that the second pulse of radar data may be a bed return. However, soon thereafter, discussion with colleagues suggested that such an assessment was most probably incorrect and that the observed second return may be an air return from the walls of the mountains surrounding the valley glacier. Cursory calculation suggests that such a suggestion was plausible as a delay of ∼ 6 to 7 µs (as evident within Figure 2) suggests a distance to reflective air boundaries of ∼ 1000 m; this is consistent with the geometry of the location. Continuing this line go investigation, further discussion with T. Chinn (personal communication 18 June 2014) and application of equation 1 (Chinn, 2001) D = 5.2 + 15.4A1/2

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(1)

where D is mean ice thickness (m) and A is total glacier area in km2 , suggested that for the Bonar Glacier, area ∼ 20 km2 , that estimated ice thickness may be of the 3

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order of ∼ 70 m. If the speed of sound in ice is estimated as 1.67 x 108 ms−1 (Hubbard and Glasser, 2005) then a bedrock radar return may be expected ∼ 0.4 µs after transmission. This is the zone highlighted by the rectangle in Figure 4.

Figure 4: Time zone (rectangle) in which a bed return may be expected if ice thickness was ∼ 70 m. This expected temporal range is ‘smothered’ by the transmitted radar pulse resulting in limited potential for bedrock return extraction, should it exist. 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89

The implications of this observation was that any bed return (if it existed) was expected to lie within the time range obscured by the extended transmitted pulse, rendering immediate observation very difficult. Therefore, digitisation of elected data as presented in Figure 3 was undertaken to enable subtraction of the normalised supposed air return from the original transmission, in the hope that the data remaining would show evidence of the ‘obscured’ bedrock return. However, this additional manipulation yielded no more conclusive results. The second, inferred ‘air wave’ appears to be predominantly a replicate of the originally transmitted pulse. This observation led to further collegiate discussion where it was suggested that the possible ‘air wave’ return may not be a return but may solely be an artefact or ‘ghosting’ caused by antennae or system architecture. Therefore, after a series of iterative data interpretation and analysis efforts, including collegiate conferral upon both system design and post-acquisition analysis, it appears that no readily-extractable bedrock data may be available from the acquired data.

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Conclusion

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Inaugural radar sounding of the Bonar Glacier in New Zealand was conducted using a purpose built ∼ 8 MHz impulse radar system. Discrete radar returns were recorded, however, questions remain over the content and interpretation of the recorded radar signal. Advice is sought from members of the glaciological community on suggested methods by which ice thickness / depth to bedrock data may be extracted from the existing dataset.

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Acknowledgements

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This research was funded by a Faculty Research Grant from the Faculty of Science, Health, Education and Engineering at the University of the Sunshine Coast. Considerable assistance was provided by Rickard Pettersen, University of Upsalla and Laurent Mignon, Blue Systems Integration. Their assistance in facilitating the progression of this work to this point is most appreciated.

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References

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Arizona Software, 2010. GraphClick, version 3.0.2.

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Chinn, T. J., 2001. Distribution of the glacial water resources of New Zealand, Journal of Hydrology (NZ), 40(2), 139–187. Hubbard, B. and N. Glasser, 2005. Field Techniques in Glaciology and Glacial Geomorphology, John Wiley & Sons, Ltd. Icefield Instruments Inc., 2000. Ice Penetrating Radar - Instructions for use, Whitehorse, Yukon, Canada., 1.3b ed., minor changes May 25, 2005.

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Narod, B. B. and G. K. C. Clarke, 1994. Miniature high-power impulse transmitter for radio-echo sounding, Journal of Glaciology, 40(134), 190–194.

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Pettersson, R., 2014. Gecko GPR data acquisition software.

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