RISP drill site survey

RISP drill site survey

J. W.

J.

CLOUGH, K. C. JEZEK, and D. ROBERTSON Department of Geology and Geophysics Geophysical and Polar Research Center The University of Wisconsin, Madison Madison, Wisconsin 53706

A camp was operated at the proposed Ross Ice Shelf Project (RISP) drill site (28°22'30"S. 168°37' 33"W.) from November 22 to December 4, 1974, in part to conduct geophysical measurements in advance of the drilling program scheduled for 1975-1976. Measurements included radio-echo and seismic reflection soundings, seismic refraction profiles, and a radar survey of bottom crevasses. The ice thickness determined by radio-echo

shown in figure 1, presumably are thin cracks extending up into the shelf and filled with frozen brine. Reflections from the top of the crevasses arc recorded as hyperbolic arcs, as shown schematically in the lower portion of figure 1. The general agreement between the shape of the recorded signal arriving after the bottom echo, and the arcs computed for reflection from the lips of an infinitely thin crevasse, suggest that these crevasses are narrow. There are no obvious surface topographic features corresponding to the crevasses. A map of the survey area (figure 2) shows two distinct crevasses (heavy solid lines) running through the survey area.

sounding is 420 ±10 meters. Seismic reflection

measurements indicate a water layer thickness of 238 meters, corresponding to an ocean bottom depth of 600 meters below sea level. A single seismic long refraction shot yields an apparent velocity for the crystalline basement of 6.8 kilometers per second between two geophone spreads spaced 2 kilometers apart; this is much higher than the average cross-spread velocity (5.4 kilometers per second). The difference in apparent velocities suggests irregularities in the basement topography and permits considerable variation in the "true" velocity (and consequently in the thickness of the sediment layer) (Bentley and Clough, 1972). The average cross-spread velocity of 5.4 kilometers per second is in fair agreement with a velocity of 5.7 kilometers per second (Bentley et al., 1974) found nearby on the ice rise at 7°S. M. (air navigation grid coordinates) and elsewhere in the Ross Sea Embayment (Houtz and Davey, 1973; Bentley and Clough, 1972). Assuming a sediment velocity of 2.0 kilometers per second, a minimum sediment thickness of 1.0 kilometers is indicated. A survey of bottom crevasses (Clough, 1974) was conducted to help select the exact drill site. Profiles were made along a series of parallel, colorcoded flag lines to determine the position of bottom crevasses in the area. The crevasses, which are

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This is contribution number 325 of the Geophysical and Polar Research Center, Department of Geology, University of Wisconsin, Madison.

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Figure 1. Three parallel radio-echo profiles In the drill site area. Crevasses extending vertically up from the bottom of the shelf are shown schematically at the bottom of the figure, along with the resulting radio-echo signal pattern.

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Figure 2. The drill site area and the position of profile lines and principal bottom crevasses (heavy lines) shown in figure 1. Less distinct features are represented by stippling. The location of the "J-9" strain network is shown by dashed lines. A suggested location for drilling operations is along the "A" line on either side of the central bottom crevasse.

This geophysical survey was supported by National Science Foundation grant Gv-36963X.

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References

Bentley, C. R., and J. W. dough. 1972. Seismic refraction measurements of antarctic subglacial structure. In: Antarctic Geology and Geophysics (R. J Adie, editor). Oslo, Universitetsforlaget. 683-691. Bentley, C. R., J W. dough, and J D. Robertson. 1974. RISP geophysical work. Antarctic Journal of the U.S., JX(4): 157-159. Clough, J W. 1974. RISP radio-echo soundings. Antarctic Journal of the U.S., IX(4): 159. Houtz, R., and F. J. Davey. 1973. Seismic profiler and sonobuoy measurements in Ross Sea, Antarctica. Journal of Geophysical Research, 78(17): 3448-3468.

Glaciological measurements on the Ross Ice Shelf

this program, which aims to measure ice strain rates, snow accumulation rates, 10-meter temperatures, and ice velocities at a network of stations 50 kilometers apart over the entire ice shelf. These, together with results from other RISP programs, will be used to formulate a dynamic model of the ice shelf to aid interpretation of borehole results and to provide insight into the history of the ice shelf. The 1974-1975 glaciological party consisted of P. Gurling and P. Burton, both of the British Antarctic Survey (BAs), and L. Gibson, V. Petrosky, and ourselves, all of The University of Nebraska, Lincoln. The first part of the 1974-1975 season was devoted to remeasurement of strain networks already planted in the grid northwest part of the ice shelf (figure). With the help of a BAS Twin Otter, 41 of the 47 Kehle-type strain rosettes (Zumberge et al., 1960) were located and successfully remeasured using Wild T-16 theodolites and Tellurometer CA-1000 distance measurers. During periods of cold weather (less than —18°C.), the Tellurometers were unreliable; for future measurements we intend to enclose the instruments in electrically heated jackets. For part of the season, malfunction of the Otter's inertial guidance system delayed the work, and although rosette remeasurement took

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Positions of strain networks on part of the Ross Ice Shelf.

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