Sonic Logging in the Deep Drill Hole 0 at Byrd Station€¦ · speedy logistics, both logger and...
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1600 3800 3900 4000 4100 P-WAVE VELOCITY (rn/sec.) Sonic Logging in the Deep Drill Hole at Byrd Station CHARLES R. BENTLEY and JOHN W. CLOUGH Department of Geology and Geophysics University of Wisconsin During the 1969-1970 field season at Byrd Sta- tion, ultrasonic velocity measurements were carried out for the first time in a deep borehole in ice. Man- ufacture of the logging equipment by the Simplec Corporation was completed in Dallas, Texas, on December 13, 1969. In a remarkable example of speedy logistics, both logger and operator were de- livered to Byrd Station just five days later. The log- ger, modified from standard equipment used as a stratigraphic tool in exploratory drill holes on land, comprises an electronic package, a crystal transducer, and four crystal receivers hung in a vertical string. The receivers are spaced at intervals of 67, 129.5, and 67 cm, and travel times can be measured between any preselected pair. After an initial period of minor repair and modi- fication to the equipment, velocity measurements were made in late December and early January. The principal measurements comprised P-wave velocity determinations, made with the first and fourth re- ceivers to provide maximum velocity resolution. Travel-time differences were thus read between re- ceivers 263.5 cm apart, so that, with a time error of approximately 1 sec, the nominal precision of rela- tive velocity determinations was about ± 6 m/sec. These measurements were supplemented by one set of measurements with a smaller spacing as a check against systematic error. Attempts to determine S-wave velocities were probably unsuccessful (defi- nite results must await more detailed examination of the data). A preliminary plot of the velocity of vertically propagating compressional waves as a function of depth is shown in the figure. The maximum depth was limited to 1,500 m by the presence of the drill- ing cable in the hole at greater depths. The major features are a gradual and continuous downward velocity increase between about 200 and 1,150 m depth, a rapid increase between 1,150 and 1,300 m, and a leveling-off to a nearly constant velocity be- low 1,400 m. The bottom segment is too short to as- certain whether any velocity increase remains. Smaller-scale velocity variations between 400 and 1,200 m depth appear to be real features of the velocity log, although their reality as features in the ice remains to be examined by further analysis of the data. The effect of increasing density on the wave ye- 0 200 400 600 cI) q) 800 000 1200 1400 Preliminary plot of the velocity of vertically traveling compres- sional waves as a function of depth in the deep drill hole at Byrd Station. locity is largely limited to depths less than about 250 m. At greater depths, the continuing velocity in- crease is probably due primarily to the effect of crystal anisotropy. The velocity along the c-axis of an ice crystal is about 200 m/sec faster than the poly- crystalline average. The velocity structure in the deep hole is thus consistent with a gradually increas- ing concentration of vertically oriented crystals down to a depth of 1,200 m, with an abrupt modification to a highly preferred orientation below. This picture agrees generally with that suggested by Gow et al. (1968) from preliminary thin-section analysis, except for a sharper and somewhat deeper upper boundary to the zone of strong anisotropy. Reference Gow, A. J . , H. T. Ueda, and D. E. Garfield. 1968. Antarctic ice sheet: preliminary results of first core hole to bedrock. Science, 161: 1011-1013. 110 ANTARCTIC JOURNAL
Transcript of Sonic Logging in the Deep Drill Hole 0 at Byrd Station€¦ · speedy logistics, both logger and...
16003800 39004000 4100P-WAVE VELOCITY (rn/sec.)
Sonic Logging in the Deep Drill Holeat Byrd Station
CHARLES R. BENTLEY and JOHN W. CLOUGH
Department of Geology and GeophysicsUniversity of Wisconsin
During the 1969-1970 field season at Byrd Sta-tion, ultrasonic velocity measurements were carriedout for the first time in a deep borehole in ice. Man-ufacture of the logging equipment by the SimplecCorporation was completed in Dallas, Texas, onDecember 13, 1969. In a remarkable example ofspeedy logistics, both logger and operator were de-livered to Byrd Station just five days later. The log-ger, modified from standard equipment used as astratigraphic tool in exploratory drill holes on land,comprises an electronic package, a crystal transducer,and four crystal receivers hung in a vertical string.The receivers are spaced at intervals of 67, 129.5, and67 cm, and travel times can be measured betweenany preselected pair.
After an initial period of minor repair and modi-fication to the equipment, velocity measurementswere made in late December and early January. Theprincipal measurements comprised P-wave velocitydeterminations, made with the first and fourth re-ceivers to provide maximum velocity resolution.Travel-time differences were thus read between re-ceivers 263.5 cm apart, so that, with a time error ofapproximately 1 sec, the nominal precision of rela-tive velocity determinations was about ± 6 m/sec.These measurements were supplemented by one setof measurements with a smaller spacing as a checkagainst systematic error. Attempts to determineS-wave velocities were probably unsuccessful (defi-nite results must await more detailed examinationof the data).
A preliminary plot of the velocity of verticallypropagating compressional waves as a function ofdepth is shown in the figure. The maximum depthwas limited to 1,500 m by the presence of the drill-ing cable in the hole at greater depths. The majorfeatures are a gradual and continuous downwardvelocity increase between about 200 and 1,150 mdepth, a rapid increase between 1,150 and 1,300 m,and a leveling-off to a nearly constant velocity be-low 1,400 m. The bottom segment is too short to as-certain whether any velocity increase remains.Smaller-scale velocity variations between 400 and1,200 m depth appear to be real features of thevelocity log, although their reality as features in theice remains to be examined by further analysis of thedata.
The effect of increasing density on the wave ye-
0
200
400
600cI)
q)
800
000
1200
1400
Preliminary plot of the velocity of vertically traveling compres-sional waves as a function of depth in the deep drill hole at
Byrd Station.
locity is largely limited to depths less than about250 m. At greater depths, the continuing velocity in-crease is probably due primarily to the effect ofcrystal anisotropy. The velocity along the c-axis ofan ice crystal is about 200 m/sec faster than the poly-crystalline average. The velocity structure in thedeep hole is thus consistent with a gradually increas-ing concentration of vertically oriented crystals downto a depth of 1,200 m, with an abrupt modificationto a highly preferred orientation below. This pictureagrees generally with that suggested by Gow et al.(1968) from preliminary thin-section analysis, exceptfor a sharper and somewhat deeper upper boundaryto the zone of strong anisotropy.
ReferenceGow, A. J . , H. T. Ueda, and D. E. Garfield. 1968. Antarctic
ice sheet: preliminary results of first core hole to bedrock.Science, 161: 1011-1013.
110 ANTARCTIC JOURNAL
