GEOPHYSICAL INVESTIGATION IN NVL (K045)- Jean Olson and Richard Kellett
The 1984/85 GANOVEX IV geophysical expedition in northern Victoria Land investigated the crustal structure in a large area between the Ross Sea and the Polar Plateau. The main geophysical programme included airborne magnetic and radio echo-sounding surveys using 2 Dornier 228 aircraft, and numerous geophysical surveys in areas of geologic significance and where magnetic anomalies were detected. The programme involved about 40 scientists and technicians based at Gondwana Station, Terra Nova Bay, and at three field camps.
The members of this event were invited to assist in the main geophysical programme, under the direction of the expedition leader. Their work included making gravity surveys, processing aeromagnetic data, and assisting in radio echo-sounding and magneto-telluric surveys.
|(a)||investigate the boundary fault between the Ross Sea and the Transantarctic Mountains;|
|(b)||investigate the boundary fault between the West Antarctic fold belt and the East Antarctic Shield;|
|(c)||investigate the Bowers Zone and its tectonic significance including its extention into the Ross Sea area;|
|(d)||investigate the northern part of the Victoria Land Basin and its connection with the Bowers Zone.|
|(a)||An aerial survey of magnetics and radio-echo sounding using 2 Dornier 228 aircraft covering approximately a 650 km by 200 km area over the Transantarctic Mountains and the Ross Sea in flight lines about 4 km apart. Part of this research was a joint BGR-USGS co-operation.|
|(b)||Geophysics ground-check groups to make geophysical measurement of areas with a geologic significance and where major anomalies are detected by the airborne magnetic profiling.|
|(c)||A helicopter-supported gravity survey from the Ross Sea to the East Antarctic Shield and in areas where special anomalies are detected. Local radio echo-sounding measurements accompanying the gravity measurements would be made to establish the ice thickness.|
The main programme would be carried out by about 35 scientists and technicians based at Gondwana Station, Terra Nova Bay, and in small field camps; a second phase of the airborne programme would be carried out from McMurdo. Transport of the main party and cargo between Christchurch and McMurdo and other logistical support would be provided by the U.S.A. and N.Z.; all other transport would be done with the 2 Dornier aircraft, 3 Squirrel 350 Helicopters, and 3 Skidoos stationed in the field.
The two members of this event were invited to participate in the main geophysical programme, under the direction of the expedition leader.
This work was to be a continuation of the BGR geologic expeditions GANOVEX I and III in northern Victoria Land, using geophysical methods. The results of those expeditions are published in the volumes: "GANOVEX 1979/80" and "GANOVEX III 1982/83" page 27 (Volume 1), N. Roland (editor), each published by the Federal Institute of Geosciences and Natural Resources and the Geological Surveys of the various Federal German States; Volume 2 of the latter, to include geologic maps, is in press.
|16 Nov. - 19 Dec.||Gondwana Station||Tested and installed geophysical instruments, made helo-supported gravity surveys, processed aeromagnetic data.|
|19 Dec. - 6 Jan.||Priestley Glacier Field Camp||White-out conditions prevented planned helo-supported gravity surveys, made short gravity profile on foot.|
|6-10 Jan.||Gondwana Station||Processed aeromagnetic data, analysed gravity data.|
|15-20 Dec.||Gondwana Station||Tested geophysical instruments, prepared for the field.|
|20 Dec. - 21 Jan.||Ht Queensland Field Camp||Assisted in radio echo-sounding and magneto-telluric profiles made from skidoos.|
|22-27 Jan.||Gondwana Station||Helped pack up and winterise camp.|
Geophysical Surveys on Browning Pass
A geophysical survey was made along part of Browning Pass (Figure 15) to determine the basement structure beneath the ice. The survey, not originally planned in the geophysics programme, was made following a decision to do work near Gondwana Station during periods of bad weather or when helicopters were not available to support planned surveys. Browning Pass was chosen for study because of the suggestion of a fault that extends along the pass, on the basis of differential folding of Priestley schist on each side of the pass (Skinner, 1983). The objective of the survey was to determine whether a fault could be detected using geophysical methods. The survey consisted of gravity magnetic, and radio echo-sounding measurements, but only the gravity survey is described below.
Measurements were made along a 1 km by 3 km grid oriented orthogonal to the trend of the pass, at points separated by approximately 250 m in each direction of the grid. Positions and relative elevations were surveyed with an EDM instrument. Gravity measurements were made with a Worden gravity meter. Gravity measurements were corrected for drift by linear interpolation of base-station measurement made approximately every 2 hours. Average drift was about 0.25 mgals per 2 hours. Gravity was also corrected for relative elevation, but terrain corrections have not yet been applied.page 28
Figure 16 is a map showing the relative positions of the stations and the partially reduced relative gravity observations. Interpretations will be made after the data are fully reduced and modelled in conjunction with the radio echo-sounding and magnetic observations.
Other workers involved in this survey were R. Saltus (magnetics and geodesy), G. Druivenga (radio echo-sounding and geodesy), and G. Merkel (geodesy).
Skinner, D.N.B., 1983. The Geology of Terra Nova Bay. In: Antarctic Earth Science (Oliver, R.L.; James, P.R.; and Jago, J.B.; eds.) Australian Academy of Science, Canberra, A.C.T.
Figure 16. Relative Gravity observed on Browing Pass in the area indicated on Fig. 15.
Geophysical Surveys near the Priestley Glacier Field Camp
Gravity, magnetic and radio echo-sounding profiles were made near the Priestley Glacier camp, orthogonal to the Priestley Glacier; the profiles could not be extended beyond the moraine of the glacier due to crevasses. These surveys were done when planned surveys were not possible due to bad weather or because of unavailable helicopter support. A contact between schist and gabbro was observed in a spur near the camp, and the aim of the profiles was to detect the continuation of the contact beneath the ice. However, since no significant variations in any of the measurements were observed, it was concluded that the contact probably occurred beneath the glacier.
Measured ice thicknesses were about 700-800 m.
Other workers involved in this profile were G. Delisle (magnetics), R. Thierbach (radio echo-sounding), and H. Geipel (geodesy).
Geophysical Surveys of the Campbell Glacier near the Mt Queensland Field Camp
Geophysical surveys of the Campbell Glacier were carried out with radio echo-sounding (RES) and active audio magneto-telluric (AAMT) methods in order to detect ice thickness and structure, and to obtain information of the resistivity of the ice and underlying rock.
This work was planned subsequent to problems which made the original plans unfeasible. The RES equipment was originally to be used in the airborne surveys, but after the Dornier aircraft "Polar 2" was damaged on 16 December, the equipment was salvaged and adapted for ground-based surveys. The AAMT work was originally planned as part of helicopter-supported ground-check group to be based near the polar plateau, but it was found that the temperatures near the plateau were too low for the operation of the instruments and that the equipment was too bulky and heavy to be moved easily by helicopter.
Figure 15 shows the area covered by the survey.
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.
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.page 32
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.
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.page 34
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.
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).
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.page 35
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.
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.
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.