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Ice Thickness Determination at Wilkes Ice thickness determination at Wilkes BY G. A. ALLEN and R. WHITWORTH Bureau of Mineral Resources, Geology and Geophysics, Melbourne Introduction Aerial photography by the U.S. Navy Antarctic Expedition of 1947-48 was the beginning of detailed mapping in the Budd Coast area of Antarc- tica. Summer surveys of ice-free areas by Australian, Russian, and U.S. parties continued until Wilkes station was set up in Vincennes Bay in 1956 by the U.S.A. During the IGY, U.S. scientists established a sub- sidiary station called S-2 about 50 miles south-east of Wilkes for glacio- logical purposes. Heights along the trail to S-2 were determined by barometric altimeters. Some gravity observations were made around Wilkes by Sparkman during a gravity tie from McMurdo Sound. A spot ice thickness by radio echo sounder was reported by Waite in 1958 but its locality is uncertain. In 1959, control of the station was transferred to the Australian National Antarctic Research Expedition (ANARE). In 1961 the first seismic ice thickness measurements were made by geophysicists of the Bureau of Mineral Resources (BMR) in co-ordination with ANARE. This survey was along a north-south traverse, and reached 300 miles south of S-2 (Jewell, 1962). It revealed a valley in the ice 40 to 80 miles south of S-2, beneath which the rock plunged to 7000 ft. below sea-level. Jewell suggested that the trough diverted ice flow into the Totten and Quincy Adams-Vanderford Glaciers. A traverse to the south-east of S-2 in 1962 showed that the ice surface to the east of Wilkes was dome-shaped (Walker, 1966). Gravity work on this traverse and on the summer traverse to Vostok suggested that there was a rock ridge along the southern edge of this dome. Bouguer anomaly gradients caused by rock structure were large enough to indicate a major tectonic feature coincident with the trough. A reconnaissance traverse east of S-2 by Kirton in 1963 further outlined the shape of the dome and rock surface in the eastern part of the dome (Kirton, 1965). To summarize, these surveys outlined an ice dome about 100 miles in diameter on the edge of the Antarctic continent. The dome was separated from the continent proper by a trough in the rock that could be a major tectonic feature sub-parallel to the coast, as had been reported elsewhere in Antarctica (Zhivago, 1962). The ice dome formed a small-scale ice-cap conveniently situated for intensive glaciological and geophysical studies. As a result, ANARE glaciologists commenced a detailed investigation 405 406 ISAGE of ice movement and snow accumulation over the northern sector of the dome in 1964 (Morgan, 1966; McLaren, 1967; Budd, 1968), whilst the BMR began a systematic geophysical survey over the dome and trough. The systematic survey As the previous surveys had shown the Bouguer anomalies to be greatly influenced by the underlying geology, a grid of seismic stations with spacing of 10 miles was adopted. Gravity readings were made at one-mile intervals between shot points; readings at this spacing were expected to define the rock surface adequately under moderate ice thickness when tied into the seismic control. Barometric readings at one-fifth mile intervals with simultaneous observations every mile were used for height control. The survey formed a rectangular grid of traverses between shot-points. In this way uncertainties in the Worden gravity meter drift and barometric heighting were considerably reduced after adjustment of the network of observations by least squares. The grid method proved very effective for the gravity observations. For example, on the spring traverse of 1964, the standard deviation of the adjusted intervals between shot points was 0-08 milligals, despite a very erratic meter drift. The accuracy of the barometric levelling was an order of magnitude lower by comparison. During the same traverse, the barometer pair difference was fairly consistent with a standard deviation of variation equivalent to 6 ft. Yet the standard deviation of the adjusted 10-mile traverses was 27 ft, an error several times larger than expected. The major cause of error appears to be local atmospheric fluctuations such as temperature inversions. Partly as a result of these errors, it has proved difficult to combine the previous surveys and glaciological traverse observations with the grid survey. Differences of up to 160 ft have occurred at traverse inter- sections. Ad hoc adjustments to the heights on other surveys were made to make the results compatible with the grid survey. After adjustment of the heights, differences in Bouguer anomaly values at the intersections were found. These are thought to be caused by navigational errors in an area of rapid horizontal changes in gravity. Results Heights (Fig. 1). The ice dome forms a fairly symmetrical feature separated from the main Antarctic plateau by a valley that lies between the Totten and Vanderford Glaciers. Several minor valleys run into the main valley. The symmetry of the dome may be due in part to the sparsity of data in the east and north-west. Small waves of a few tens of feet occur in the ice surface along the southern flank of the dome. In the south- west, the waves are sufficiently intense to cause tensional crevassing. The waves probably reflect underlying rock ridges rather than pressure waves in the ice. MASS BUDGETS : REGIONAL STUDIES 407 CAK POINSETT FIG. 1. Ice surface topography. Free-air anomalies (Fig. 2). If isostatic compensation occurs, the free- air anomalies would be expected to average zero. Compensation of a feature appears to occur regionally over an area of 250 km across. It is therefore marginal whether a body of the size of the ice dome would be compensated, or whether the strength of the crust would bear the load. The rugged rock surface and intense geological gravity anomalies make the free-air anomalies complex. This complexity, plus the present lack of data in some areas makes it impossible to determine the degree of compensation reliably. Ice Bouguer anomaly results (Fig. 3). Over most of the dome, the Bouguer anomaly features are broad and shallow, with an easterly trending "high" in the north and a "low" with indistinct trend in the S-2 area. Localized features occur over rock ridges under the fairly thin ice cover. An arcuate gravity ridge marks the southern edge of the dome area. The northern boundary is not very marked, but the southern edge exhibits extremely high gradients, particularly in the west where gradients exceed 20 milligals/mile. The areas of maximum gradient form a linear feature aligned with the northern edge of the Vanderford Glacier valley. 408 ISAGE LEGEND u~Ll • 'Hlfh' anomaly 'Low' anomaly Contour Intervall 10 mllllpls Free air anomalies In mllllgali STATUTE MILES FIG. 2. Free air anomalies. South of the dome there is an intense, complex Bouguer anomaly "low"; in the west its trend is north-westerly, but it swings round to an east-west trend towards the Totten Glacier. Seismic shooting. It is useful to digress at this point to mention certain aspects of the seismic work. Shallow 3- to 6-ft holes were used almost exclusively. The corrections for the uppermost low-velocity layers were obtained using a power-law function, of the type V(z) = azb for the velocity depth relationship (Faust, 1951). The applicability of this func- tion to the seismic time-distance plots is of some interest as it implies that considerable anisotropy in velocity may therefore occur (Gassman, 1951 ; White and Sengbush, 1953). A maximum velocity of 12,500 ft/sec was determined by several refraction profiles in the survey area. MASS BUDGETS : REGIONAL STUDIES 409 _ _| Hljh anomaly r —i "Low' anomaly lur Inccrvil 10 mllllgals Bou|uer anomaly In mllllgalt FIG. 3. Ice Bouguer anomalies. Difficulty was experienced in identifying the ice/bedrock reflection on some seismic records. Instead of a single discrete reflection event, often a zone or zones of events were recorded. Possibly these are reflections from several areas of a rough bedrock surface, or multiple reflections from a thick basal moraine layer. As we had no definite evidence to the contrary, the beginning of the first event was taken as the bedrock reflec- tion. Ice thickness measurements uncorrected for dip were used to control the gravity reductions. Rock Bouguer anomalies (Fig. 4). The Bouguer anomalies caused by geological structure were derived from the ice Bouguer anomaly at each seismic shot—point by correcting for the thickness of ice. The formula 410 ISAGE FIG. 4. Rock Bouguer anomalies. used was: RBA = IBA — hr (pr — pi), where RBA = rock Bouguer anomaly, IBA = ice Bouguer anomaly, hr = thickness of rock above or below sea level, pr = density of rock and pi = density of ice. The total range in rock Bouguer anomalies exceeded 120 milligals, and gradients as high as 7 milligals/mile were detected. The major features are the broad "low" over most of the dome area, which may extend as far as the coast in the east, and the complicated pattern of "highs" and "lows" in the south. The Bouguer anomaly "highs" generally correspond with considerable depressions in the rock surface suggesting that these areas may be underlain by corresponding rises in the base of the crust. MASS BUDGETS : REGIONAL STUDIES 411 Rock elevation In feet above teilevel Value» bated on Selimlc lutlom only FIG. 5. Rock topography. Rock topography (Fig. 5). The application of corrections for geology greatly alters the picture of the rock topography given by the ice Bouguer anomaly results. For example, within a distance of ten miles the ice thickness estimated using gravity in 1962 differs by up to 2000 ft from the thickness measured by seismic shooting in 1965.
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