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

AAMT Surveys

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AAMT Surveys

Method

The AAMT method involves the generation of a magnetic field directed into the earth, using a large transmitter loop. The signal is initiated at low frequency (10 Hz) and is increased in steps to a maximum of 8000 Hz; the low frequency signal is able to penetrate to greater depths than the higher frequency signals. A block diagram of the transmitter is shown in Figure 21.

Figure 21. Block diagram of the A.A.M.T. transmitter.

Figure 21. Block diagram of the A.A.M.T. transmitter.

The two receiving stations consisted of three induction magnetometers set along orthogonal axes. The phase and amplitude of the signal in the R, X, Z directions, relative to a reference signal was measured using an EG & G, BROOKDEAL ELECTRONIC, PRINCETON APPLIED RESEARCH 5206 two phase lock-in analyser. These were placed at distances from the transmitter loop determined by the skin depth.

It is possible to obtain a quick impression of the subsurface by comparing the amplitude of the H(Z) and H(R) components. The H(X) component should be very small if the magnetometer is directed correctly at the centre of the transmitting loop.

This group ran into a series of problems. Firstly, some of the equipment failed due to low temperatures. The lock-in analysers required several hours of warming up to get them into their operating range of 10-40 °C. Eventually, they were discarded and the spectrum analyser was used. This reduced the number of receiving stations to one. The spectrum analyser turned out to be ideal because it enabled the recorder to see the full range of spectral lines over an interval of about 100 Hz. This meant that you could discern the signal from any noise or spurious signals. This was very convenient in the region of 50 Hz where we got a lot of interference from the power generators we were using. The disadvantage was that we lost all the information about the phase of the signal.

Secondly, the ice thickness was greater than had been anticipated. He found it difficult to locate the ice/rock boundary even at a distance of 2 km from the side of the glacier. We decided to abandon the A-A′ profile and to experiment in an area where the RES indicated that the ice was less than 500 m deep. The transmitter was located at two different positions and an array of receiving stations was set up (see Figure 22).

A quick analysis of the data produced a graph of apparent resistivity versus depth. This graph corresponds to the subsurface at the midpoints between the transmitter and receiver.

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Figure 22. Location of A.A.M.T. stations.

Figure 22. Location of A.A.M.T. stations.

We found that two main types of curve could be distinguished (see Figures 23 and 24). He ran into problems when we set up a receiver less than 400 m from the transmitting loop; this was corrected by using a small loop (25m × 25m) for close readings.

Curves similar to Figure 23 were found at Rx1, Rx2r when using Tx1 and at Rx2r, Rx4r, Rx1p, and Rx2p when using Tx2. These can be seen to tie around Tx2 and towards the edge of the glacier.

Curves like Figure 24 were found at Rx3r when using Tx1 and at Rx1r, Rx11, Rx21 when using Tx2. There were grouped around Tx1.

All other stations produced data which was too scattered to interpret sensibly.

A model to produce the curve in Figure 23 would require a thin layer of low resistivity (about 100 m) at a depth of between 1000 m and 1500 m. This layer has the effect of displacing the curve to the left. This could be a water layer at the ice/rock contact. The ice appears to have a resistivity of about 10,000 - 20,000 m.

Figure 24 does not curve sharply to the left. Thus the low resistivity layer is missing and we have a two layer case. The resistivity of the ice is about 25,000 m and the rock is about 5,000 m. The resistivities correspond quite well to literature values. However, the depths indicated do not agree with those found using the RES method.

In addition to magneto-telluric sounding the AAMT group observed natural fluctuations in the magnetic field. Polaroid photographs were taken of the Spectrum analyser scope at regular intervals.

A Geo-electric sounding was also performed. This used the Schlumberger electrode configuration with the potential electrodes separated by 10m and the distance L (see Figure 25) varying from 20m to 600m. Figure 26 shows the resulting graph of apparent resistivity verses spacing L. The curve is a two layer case with a surface layer resistivity of 35,000 m. It is difficult to get any information about the underlying layer due to the high resistivity of the surface.

Acknowledgement

Other workers involved in these surveys were H. Giesel (Science Co-ordinator); H. Engelhardt and R. Lamers (RES); E. Blohm and F. Kuhnke (AAMT); G. Merkel (Surveyor).

Publications

The BGR will publish a collection of papers of the GANOVEX IV results in a volume. In addition, an aeromagnetic map will be published by the BGR and USGS, and it is intended that a summary of the salient findings of the expedition be published in the Journal of Geophysical Research.

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Figure 23. Results of A.A.M.T. sounding.

Figure 23. Results of A.A.M.T. sounding.

Figure 24. Results of A.A.M.T. sounding.

Figure 24. Results of A.A.M.T. sounding.

Figure 25. Schlumberger electrode configuration.

Figure 25. Schlumberger electrode configuration.

Figure 26. Graph of resistivity versus depth for geo-electric sounding.

Figure 26. Graph of resistivity versus depth for geo-electric sounding.

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Future Research

GANOVEX V, to occur in the 1986/87 season, is proposed to be a ship-based geological expedition of the Pacific Coast area of northern Victoria Land and is to include some further geophysical and geological work based at Gondwana Station, including studies of Mt Melbourne.

Management

The members of this event gained a great deal personally by having participated in the BGR programme and would encourage NZARP to participate in future GANOVEX expeditions as invited. However, from a scientific point of view, it would be more advantageous for NZARP to develop proposals with the BGR for joint or independent investigations associated with the aims of the expedition.

Acknowledgements

We are grateful to the expedition leader, H. Durbaum, and each member of the GANOVEX IV party for inviting us to participate in the programme and for providing logistical support, and the DSIR Antarctic Division for support through the different stages of this event. We thank P. Barrett, R. Dibble, and T. Stern who helped plan the event, and T. Hatherton for the use of a DSIR Geophysics Division Worden gravity meter and barometer. The assistance of G. Ball (Mountain Guide) and J. McConchie (Field Guide) is greatly appreciated. He acknowledge financial support from the Internal Research Committee of Victoria University of Wellington.