A Geophysical Investigation of the Dart Glacier, South Island, New Zealand.
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This study had the dual aims of investigating the physical dimensions of the Dart glacier and evaluating the success of the geophysical techniques used in achieving the first aim. The data collection methods were; measurement of the gravity field, Schlumberger resistivity soundings and horizontal resistivity traverses. The analysis was made via; 2-D and 3-D modelling of the bouguer gravity anomaly;1-D ( 10 points/decade filter) modelling of the resistivity soundings and 2-D modelling of the horizontal traverses and the soundings with a finite difference program.
The gravity method proved the most effective for determining the shape of the glacier along profiles over white ice and for determining the presence of ice buried under gravels. In all cases the method was limited by equivalence variations.
Some models of the glacier, resulting from two-dimensional gravity modelling, were increased in thickness by up to 80% when modelled with three-dimensional gravity while other 2-D models were not changed.
Resistivity measurements (Schlumberger soundings and traverses) were successful in investigating the thickness and lateral distribution of the gravels adjacent to the lower glacier. However, the technique was unsuccessful in determining the thickness of the ice. This was due to limited depth penetration and 3-D variations in the glacier shape. Two-dimensional resistivity modelling was unsuccessful in interpreting these 2-D and 3-D effects on the resistivity measurements.
Hammer seismic refraction was unsuccessful on the glacier as the velocity of the ice (~4000 ms-1) is similar to the velocity of the basement schist (~4500 ms-1). The similarity of the velocities required excessively large cable spreads to measure the thickness of the glacier.
An important feature of the study is the interaction of the gravity and the resistivity techniques. The resistivity results were interpreted first and the subsequent results were used in the interpretation of the gravity data. The resistivity data was then reinterpreted taking the results of the gravity interpretation into consideration. The data from the gravity and resistivity methods was interpreted in this cycle until any anomalies were explained. Together the methods were able to determine that there was electrical layering in the upper glacier, that the gravels beside the lower glacier were not extensive and the gravel substratum was slumped schist. Independently the gravity and resistivity techniques were unable to conclusively show the above conclusions.
Due to equivalence variations of the models the shape of the glacier cannot be defined exactly but trends from the upper to lower glacier are evident. Modelling the gravity data from the upper-most profile (C line, 4.25 km down from the head of the glacier) shows that the glacier, on this profile, is 110 m thick. Down-valley from the M line (6.4 km down the glacier) the glacier thickens to a maximum of at least 400 m thick. Down-valley from the D line (7.2 km down the glacier) the ice thins, but the full down-valley extent of the ice has not been determined. Gravity measurements show that beyond the presently exposed toe the ice continues down the valley but is covered by a layer of river-lain gravels. Measurements were only made to 450 m down valley from the toe (8.8 km down the glacier). Here, the low gravity values obtained indicate that ice up to 200 m thick was below the river-lain gravels.
The glacier gets narrower towards the toe. On the C line it is wide and shallow (width/depth ~5.5) while at the toe it is in a narrow deep valley (F line width/depth ~1.1).
The gravels to the south-east of the toe are not extensive. Resistivity soundings show that these gravels are increasing in depth down the valley. A sounding on the F line shows gravel thickness of up to 61 m. One hundred metres to the south-west (down valley and parallel to the glacier) the gravels are up to 96 m thick. Gravity modelling shows that the gravels continue to increase down the valley and then thin to <= 15 m at the down-valley limit of the gravity measurements.
The resistivity soundings show the gravels to be layered, with higher resistivity layers of 1000 to 3000 Ohm m in the upper 26 m and a layer of lower resistivity (400 to 800 Ohm m) below. The slumped schist was interpreted to have a resistivity of 1900 to 6000 Ohm m, while solid schist has resistivities of 5000 to 8000 Ohm m. The resistivity soundings showed the electrical layering of the upper glacier to consist of; an upper shallow layer (fixed to 1.05 or 1.8 m) of ice with resistivities ranging from 2.2 to 8.5 M Ohm m, a middle layer (which can be varied from 12 to 90 m thick) with resistivities from 40 to 2000 M Ohm m and a lower layer between 10 and 100 M Ohm m. Resistivity soundings on the upper ice also detected anisotropy in the glacial ice.
Slumped schist was shown to be up to 200 m thick and resting directly against the southeastern edge of the lower glacier. The main part of the slump that rests against the glacier is a 1.8 km section in the middle of the slump. The recession of the glacier had removed support for the schist on the south-eastern side of the valley. The schist has subsequently slumped to rest against the glacier.
The glacier surface is being planed down by surface water erosion and buried under gravels. The gravel cover is slowing the ablation and preserving large thicknesses of ice from melting
The gravity method proved the most effective for determining the shape of the glacier along profiles over white ice and for determining the presence of ice buried under gravels. In all cases the method was limited by equivalence variations.
Some models of the glacier, resulting from two-dimensional gravity modelling, were increased in thickness by up to 80% when modelled with three-dimensional gravity while other 2-D models were not changed.
Resistivity measurements (Schlumberger soundings and traverses) were successful in investigating the thickness and lateral distribution of the gravels adjacent to the lower glacier. However, the technique was unsuccessful in determining the thickness of the ice. This was due to limited depth penetration and 3-D variations in the glacier shape. Two-dimensional resistivity modelling was unsuccessful in interpreting these 2-D and 3-D effects on the resistivity measurements.
Hammer seismic refraction was unsuccessful on the glacier as the velocity of the ice (~4000 ms-1) is similar to the velocity of the basement schist (~4500 ms-1). The similarity of the velocities required excessively large cable spreads to measure the thickness of the glacier.
An important feature of the study is the interaction of the gravity and the resistivity techniques. The resistivity results were interpreted first and the subsequent results were used in the interpretation of the gravity data. The resistivity data was then reinterpreted taking the results of the gravity interpretation into consideration. The data from the gravity and resistivity methods was interpreted in this cycle until any anomalies were explained. Together the methods were able to determine that there was electrical layering in the upper glacier, that the gravels beside the lower glacier were not extensive and the gravel substratum was slumped schist. Independently the gravity and resistivity techniques were unable to conclusively show the above conclusions.
Due to equivalence variations of the models the shape of the glacier cannot be defined exactly but trends from the upper to lower glacier are evident. Modelling the gravity data from the upper-most profile (C line, 4.25 km down from the head of the glacier) shows that the glacier, on this profile, is 110 m thick. Down-valley from the M line (6.4 km down the glacier) the glacier thickens to a maximum of at least 400 m thick. Down-valley from the D line (7.2 km down the glacier) the ice thins, but the full down-valley extent of the ice has not been determined. Gravity measurements show that beyond the presently exposed toe the ice continues down the valley but is covered by a layer of river-lain gravels. Measurements were only made to 450 m down valley from the toe (8.8 km down the glacier). Here, the low gravity values obtained indicate that ice up to 200 m thick was below the river-lain gravels.
The glacier gets narrower towards the toe. On the C line it is wide and shallow (width/depth ~5.5) while at the toe it is in a narrow deep valley (F line width/depth ~1.1).
The gravels to the south-east of the toe are not extensive. Resistivity soundings show that these gravels are increasing in depth down the valley. A sounding on the F line shows gravel thickness of up to 61 m. One hundred metres to the south-west (down valley and parallel to the glacier) the gravels are up to 96 m thick. Gravity modelling shows that the gravels continue to increase down the valley and then thin to <= 15 m at the down-valley limit of the gravity measurements.
The resistivity soundings show the gravels to be layered, with higher resistivity layers of 1000 to 3000 Ohm m in the upper 26 m and a layer of lower resistivity (400 to 800 Ohm m) below. The slumped schist was interpreted to have a resistivity of 1900 to 6000 Ohm m, while solid schist has resistivities of 5000 to 8000 Ohm m. The resistivity soundings showed the electrical layering of the upper glacier to consist of; an upper shallow layer (fixed to 1.05 or 1.8 m) of ice with resistivities ranging from 2.2 to 8.5 M Ohm m, a middle layer (which can be varied from 12 to 90 m thick) with resistivities from 40 to 2000 M Ohm m and a lower layer between 10 and 100 M Ohm m. Resistivity soundings on the upper ice also detected anisotropy in the glacial ice.
Slumped schist was shown to be up to 200 m thick and resting directly against the southeastern edge of the lower glacier. The main part of the slump that rests against the glacier is a 1.8 km section in the middle of the slump. The recession of the glacier had removed support for the schist on the south-eastern side of the valley. The schist has subsequently slumped to rest against the glacier.
The glacier surface is being planed down by surface water erosion and buried under gravels. The gravel cover is slowing the ablation and preserving large thicknesses of ice from melting
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xiv. 179 p. ill. 30 cm.
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1988Nicol
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Citation
Nicol, S, “A Geophysical Investigation of the Dart Glacier, South Island, New Zealand.,” Otago Geology Theses, accessed February 7, 2025, https://theses.otagogeology.org.nz/items/show/222.