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Victoria University Antarctic Research Expedition Science and Logistics Reports 1984-85: VUWAE 29

RES Surveys

RES Surveys

Method

This method used high frequency signals which are reflected off the ice/rock interface. The data is stored on a video cassette and can be replayed through a printer to obtain an immediate visual record.

The layout of the equipment is similar to that used in seismic surveying. A block diagram of the instrumentation is shown in Figure 17. The transmitter and receiver were mounted on sledges and the sampler-scope and the recording section were situated on a skidoo which was used to tow the sledges. Figure 18 is a plan view of the whole unit. The skidoo was usually driven at a constant speed of 10 kmph and the signals were received at a rate of 4 per second. This gave a good single fold coverage of any reflector. Experiments were made using damped and undamped antennae to determine the best transmitter-receiver separation. All antennae were dipoles which varied in wavelength from 8-16 m.

Common midpoint profiles were also carried out. This method, described below, allows the records to be stacked and thus provides an enhanced signal-to-noise ratio.

For these surveys, the transmitter and receiver were placed 10 m on either side of a point of interest, and then moved away from the midpoint in 5 m intervals. A two minute sweep was recorded at each interval, as well as a single pulse. The maximum separation from the midpoint was 95 m.

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Results

The high resolution antennae picked up many reflecting layers within the ice but were unable to find the ice/rock interface (Figure 19 shows a photograph of a high resolution record). Crevasses produced diffraction patterns in the upper part of the record which masked some of the layers. This was only a problem in the first 4 kilometres of the A-A′ profile. Beyond this, we could see down to about 7 microseconds (two-way travel-time) or approximately 600 m. The base was not visible.

On the Mt Melbourne side of the A-A′ profile, the base appeared as a strong reflection, at a depth of about 1100 m and rose rapidly to the order of 10 m on the A-A′ profile. Using undamped antennae we were able to penetrate down to 1200 m. He found that the base was only visible continuously near the edges of the glacier. An example of an undamped record is seen in Figure 20.

A change in the nature of the reflector at 800 m is indicated by the quality of the reflections which was excellent above that depth and poor below.

Figure 17. Block diagram of R.E.S. equipment.

Figure 17. Block diagram of R.E.S. equipment.

Figure 18. Plan view of the R.E.S. equipment mounted on skidoo and sledges.

Figure 18. Plan view of the R.E.S. equipment mounted on skidoo and sledges.

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