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

3. Scientific Endeavours and Achievements

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.