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Victoria University Antarctic Research Expedition Science and Logistics Reports 1994-95: VUWAE 39

IMMEDIATE SCIENTIFIC REPORT K044 1994-95: Seismic Experiment Ross Ice Shelf

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Event K044
Seismic Experiment Ross Ice Shelf- Again "Seris-A"

Antarctica New Zealand November 24th, 1994 to January 6th, 1995

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1. Summary

The Transantarctic Mountains form one the highest and longest mountain chains in the world. Mechanisms for forming the mountains are debated, though it is believed that the mountains are a result of rifting between East and West Antarctica. The boundary between East and West Antarctica is marked by where the mountains meet the Ross Ice Shelf/Ross Sea. The occurrence of rifting suggests that at the mountain front a depression, or graben, should exist in the West Antarctic crust.

During December 1994, the K044 geophysical traverse collected data on the Ross Ice Shelf intended to enhance and extend the image of the crustal structure of the boundary between East and West Antarctica that resulted from the 1990/91 Seismic Experiment on the Ross Ice Shelf ("Seris", see ten Brink et al, 1993). The traverse at ~82°S covered a total distance of almost 200 km, starting at the edge of the Transantarctic Mountains and moving north-east onto the Ross Ice Shelf.

Data collected included GPS data for navigation and position finding, gravity data, magnetic data and radio echo sounding measurements of ice thickness.

The gravity data collected confirms earlier data and has initially shown some interesting features on the new portion of the traverse, though at this stage it is unclear what these variations represent. It is possible that the variations seen could be associated with the graben that was not imaged during the 1990/91 work closer to the mountain front.

Measurements of the Earth's magnetic field were made over the length of the traverse, such data were not collected on the earlier traverse. Once effects from the daily variation in the magnetic field and the general shape of the background field have been removed, we expect to see magnetic anomalies relating to the transition between West and East Antarctic crust.

Measurements of ice thickness made with an ice penetrating radar suggest that the Ross Ice Shelf becomes thinner away from the mountains, a result that would be expected given that further from the mountains, ice is further from its source. The main purpose of the radar measurements was to make ice thickness measurements up the Robb Glacier. Such measurements would have been used in reprocessing of seismic data from the 1990/91 traverse. However poor weather and time constraints meant that this part of the event was not carried out.

Future work may involve making the ice thickness measurements not achieved this year on the Robb Glacier, along with magnetic measurements. It is also feasible that a larger seismic expedition could be carried out should results from the Seris-A traverse suggest this is warranted. Such an expedition could easily be extended still further across the Ross Ice Shelf.

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2. Proposed Program

The principal objective of this event was to enhance and extend the geophysical image of the sub-surface portion of the Transantarctic Mountain front that was gained from the 1990/91 Seris Expedition on the Ross Ice Shelf.

The 1990/91 seismic work did not locate a graben structure adjacent to the Transantarctic Mountain front, a graben that is believed to have formed as a result of rifting between East and West Antarctica. This expedition extended the Seris traverse a further 100 km north-east onto the Ross Ice Shelf with measurements of gravity, magnetics and ice thickness. With these measurements we will investigate the possibility that the expected graben exists further away from the mountain front.

Additionally we aimed to obtain detailed measurements of ice thickness on the Robb Glacier portion of the 1990/91 expedition for reprocessing of seismic data. This information would then provide a clearer picture of the deep structure of the East/West Antarctica boundary.

3. Scientific Endeavours and Achievements

Navigation along the 200 km "Seris-Again" (Seris-A) traverse route utilised the satellite based Global Positioning System (GPS). Approximate co-ordinates of desired measurement sites were pre-determined in New Zealand and then stored in a GPS receiver as "way points". Navigation to a way point when in the field is achieved by following the bearing indicated by the GPS unit. Navigation in this manner was accurate to about ±100 m.

A typical day of making measurements involved travel of up to 50 km on skidoo. We opted to operate up to 25 km either side of a base camp on successive days, making measurements and then returning to camp, thereby avoiding the need to set and break camp every day. Such a method was fuel efficient in that only light loads of science and survival equipment were carried on a single sledge behind each of two skidoos. Roughly every third day, it became necessary to shift camp. Whole days were dedicated to this task. Camp was moved at most 60 km in a day.

Measurements of the Earth's gravity and magnetic fields were made at 2 km intervals along the 200 km traverse. In addition to use in navigation, co-ordinates of measurement sites were also determined using GPS. We hope to obtain positions and elevations with accuracy's of the order ±2m. To achieve this accuracy, we logged data from satellites for approximately 15 minutes at each site. This data will be corrected to a continuously logging base station GPS left at camp, and this base station will in turn be corrected to data logged at McMurdo Station by the USGS.

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Images of the crustal structure of the Transantarctic Mountain front obtained from the Seris traverse of 1990/91, did not locate any graben structure adjacent to the mountains. The absence of this graben can suggest that the age of the mountain uplift is older than thought (ten Brink et al, 1993). However, there is a possibility that the graben formed further from the mountain front. In search of this graben, the K044 team extended the 1990/91 traverse a further 100 km north-east onto the Ross Ice Shelf.

Preliminary indications from gravity data suggest that interesting features lie below the ice and sea water under the new portion of the traverse. Closer to the mountains the gravity data are flat, reflecting the presence of flat lying sediments in this region, as imaged in the seismic data from the Seris traverse (ten Brink et al, 1993). The new data indicates that the gravity profile does not remain flat further out from the mountains, perhaps indicating structures associated with deformation occurring in the West Antarctic crust at the same time as mountain uplift. At this stage it is unclear exactly what the data indicate and further processing is necessary.

Soundings of ice thickness were made over the full length of the Seris-A traverse at 4 km intervals. Such measurements involved laying out antenna arrays onto the snow surface and recording reflections of electromagnetic waves from an ice-water or ice-rock interface. Given that the whole traverse was carried out on the Ross Ice Shelf, we only expect to have reflections from an ice-water interface. It is unclear at this stage how the signals from such an interface behave. Despite this, field examination of the data seemed to suggest that the ice was becoming thinner away from the Transantarctic Mountains, perhaps reducing from about 400 m to 200 m. This observation appears to be consistent with other work on the Ross Ice Shelf (USGS, 1972).

In order to reprocess seismic reflection data and improve an image of the deep crustal structure of the mountain front, it was planned to continue the traverse up the Robb Glacier with the priority being to obtain detailed ice thickness measurements. Such measurements would be a valuable "static correction" for use in reprocessing of the Seris seismic data. Poor weather and time constraints meant that this part of the expedition was not possible.

It had also been hoped to obtain magnetic data up the Robb Glacier. A significant magnetic anomaly is expected to be present at the mountain front as the cross is made between the very different crusts of West and East Antarctica. Despite the Robb Glacier portion of the traverse not eventuating, magnetic data collected on the portion of the traverse covered may have been close enough to the mountain front to begin to see this anomaly.

For the future, given the desire to obtain the detailed ice thickness measurements from radio echo sounding on the Robb Glacier, an expedition may eventually be proposed to cover the portion of the Seris-A traverse not covered this season. At the same time magnetic data would be recorded in page 4 search of any magnetic anomaly associated with the boundary between East and West Antarctica. Such work would probably involve a similar time period and logistical effort to this years traverse.

With seismic reflection and refraction sounding, a more detailed examination of the geology underneath the Ross Ice Shelf can be achieved. Should the results of the Seris-A work suggest the existence of the extensional graben expected to be present, then the traverse can form the basis of a logistically larger seismic expedition to further probe the area examined this year. Such a traverse could easily be extended still further north-east across the Ross Ice Shelf.

Making geophysical measurements requires sensitive and fragile instruments, and making the measurements in Antarctica is made more difficult by the harshness of the Antarctic environment In an effort to improve the chances of survival of this equipment in Antarctica, methods have been devised at Victoria University to protect the instruments. Many of these modifications are based on experience from earlier designs used on the polar plateau during the East Antarctic Seismic Traverse of 1993 (Bannister, 1994).

Perhaps the most sensitive instrument used was the gravity meter. This meter requires operation at a fixed temperature of 48°C, and it is extremely susceptible to damage from even the smallest of bumps. With the traverse being undertaken using skidoos towing sledges, it was necessary to consider the roughness to be experienced by the gravity meter whilst riding over sastrugi'd terrain on the sledges.

In order to keep the gravity meter warm and safe from bumping, a large box was made in which to house the meter. The meter itself (with rough dimensions 15x20x30 cm) was enclosed, for warmth, by a perspex housing insulated with closed cell foam. Based on past experience, closed cell foam is necessary as the moisture held in open cell foams tends to freeze, thereby expanding the foam and misshaping it The housing was designed to allow normal operation of the meter without the need to continually remove the housing. A battery used to keep the meter's internal heater operating was also contained within the box, thereby making it a self contained unit. The battery only needed to be removed for charging. Future versions are planned to include a built in solar panel to maintain the battery at operational capacity.

To protect the meter from jarring while riding on the sledges, a spring loaded suspension platform was built into the box upon which the meter rested during transport. The platform absorbed the shock that occurs from travelling over bumpy ground.

Other insulated boxes were constructed to house the magnetometer and barometers, and a separate box to hold computers. Space was allowed to include bottles of hot water with the computers, and by doing so, we were able to maintain temperatures in the computer box above zero for the duration of a days work.

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These adaptations were given a rigorous test whilst in the field, the result being that the equipment endured the field season successfully.

Much of the equipment used on the traverse, as in any geophysical project, requires the use of battery power. In the cold environment of Antarctica, batteries tend to discharge quicker and recharge less efficiently. We made substantial use of solar energy for battery recharging. With a solar panel we found that use of a generator was only required after prolonged periods of poorer sunlight conditions. By avoiding the use of a generator, we also avoid the tedious task of refuelling in the cold.

4. Publications

Results from the traverse will be processed and analysed during 1995 and presented as a Bachelor of Science (Honours) thesis by Julie Quinn at Victoria University.

A publication for an international journal is likely to be prepared towards the end of 1995. Such a publication will present the results from the traverse to a wider audience interested in the implications derived from our results.

A presentation will be made at the upcoming 7th International Symposium on Antarctic Earth Sciences in Siena Italy, in September 1995. The presentation will aim to expose to a wider audience some of the methods developed for protecting sensitive geophysical instruments in Antarctica. At Scott Base prior to our traverse, other scientists expressed interest in these methods. Hence we see it as worthwhile to make the methods known in the hope that it can be of use to others.

5. Acknowledgments

Event K044 would like to thank the staff at Scott Base for their part in making the "Seris-A" traverse a success. Alex Pyne of the Antarctic Research Centre at Victoria University loaned the event GPS equipment and generously gave up his own time to introduce us to the system. Discussions on GPS techniques with Perry Gilbert (Dosli) were invaluable and we thank Barbara Littel and Larry Hotham of the USGS for providing GPS data from McMurdo. The Seris-A expedition was initiated and organised by Tim Stern at Victoria University, and it was he who arranged the financial support of the Internal Grants Committee, Victoria University. Ron Hackney's study in New Zealand is made possible by support through the New Zealand government Commonwealth Scholarship and Fellowship Plan. Eric Broughton and Workshop staff at Victoria University provided much assistance in preparing the scientific equipment for the field. Thanks to Ed Waddington from the University of Washington for the loan of a radar transmitter.

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Bannister, S., 1994. Seismic Investigation, Wilkes Basin. Report to New Zealand Antarctic Program 1993/94: Event K101. Institute of Geological and Nuclear Sciences, Science Report 94/21, 31pp.

ten Brink, U., Bannister, S., Beaudoin, B. and Stern, T., 1993. Geophysical Investigation of the tectonic boundary between East and West Antarctica, Science 261, 45-50.

United States Geological Survey, 1972. Ross Ice Shelf. 1:1 000 000 Map of Ice Thickness and Bathymetry.