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Victoria University Antarctic Research Expedition Science and Logistics Reports 1979-80: VUWAE 24

ONSITE OPERATIONS

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ONSITE OPERATIONS

Onsite handling and description of core (ARP)

The procedure for extracting core from the core tube varied depending on the core size being drilled. HQ core (dia. ≈ 63 mm) was recovered in a split tube ("splits") within the core tube. The 10 foot long splits containing core were hydraulically extracted from the tube and transferred intact and unfrozen onto the specially constructed bench in the Science Hut. The smaller sized NQ & BQ core tubes did not have splits. Core therefore was directly removed from the tube onto an HQ 'split' by gently tapping the tube and extruding the core with a steel rod. Often the tube had to be heated in a "Herman Nelson" hot air hose to unfreeze water surrounding the core before extraction could proceed. NQ & BQ cores therefore suffered some sub-horizontal fracturing resulting from core extraction. These fractures, however, were fresh and easily distinguished from natural fracturing within the core. Boxing, initial examination and description was done on the unfrozen core in the Science Hut.

The primary purpose of the onsite description was to record as accurately as possible the depth from which core was recovered and any features which might be changed by freezing and transportation to the core lab.

Information from the driller often enabled a more accurate determination of from where core was recovered during the coring run. This was particularly important when recovery was less than 100%. The position of core recovery was shown at the graphic log, a double line being used to indicate the bottom depth of the run when the run was not the full 10 feet. When doubt existed about the exact depth of "the core, it was assumed to have come from the top of the run.

Fracturing was noted on the graphic log and where possible determined as natural or induced from drilling and core extraction. Some mudstone lithologies initially recored and intact cylindrical blocks suffered extensive conchoidal fractures on subsequent freezing.

The colour of the core was compared with the "Revised Standard Soil Colour Charts" (Japan). Bioturbation and bedding was often most easily seen when the core was fresh and wet.

Seismic (Sonic) Velocities (PCF)

Routine on-site measurements of sonic velocity, density and chemistry of the core were planned. Due to low and fluctuating temperatures in the Science Hut and the cramped space, only sonic velocities were finally performed. Sonic velocity measurements were undertaken with a PUNDIT, determining the travel time between two transducers of an 82 kHz pulse, as described in Barrett & Froggatt (1978). Only whole, coherent segments of core free from fractures or clasts and at least 50 mm long were chosen. Sampling between transducers and core was improved by washing measuring points on the core with water and use of water as a complant. Grease or oil was avoided to lessen core contamination. Consistent readings about the core axis were obtained in this manner. Measuring was performed as soon as possible after cote recovery, usually as soon as the core was boxed and before initial logging. Except for Core 2, the cores remained unfrozen. Time after recovery was usually less than one hour although delays of up to eight hours occurred.

Velocities measured ranged between 1.78 and 5.10 km.s−1 for sediments and 3.44 to 5.92 km.s−1 for basement clasts of dolerite, granite etc. Clast velocities agree well with the data of Barrett & Froggatt 11978). However, sediment velocities were generally higher than determined from seismic refraction profiles in McMurdo Sound (Barrett 1979, Nor they et al., 1975) and more varied than for DVDP 15 where many of the cores were frozen (Barrett s Treves, 1976). Velocities determined on each core or unit within a core have, initially been averaged (Table 2) and plotted against depth (Figure 3). A steep uniform velocity gradient of V = 19.OH + 1.6 (V = velocity in kms and H = depth in km) is shown to 110 m sub-bottom where there is a marked velocity reversal. A further reversal occurs at 190 m. Reflectors labelled A, K, B & C (Northey et al., 1975) have been tentatively identified at 141 m, 168 m, 180 m and 210 m sub-bottom respectively. The lithology of each reflector is a well page 8 cemented medium sandstone up to 0.5m thick with velocities of between 4 to 5 km.s−1, compared to 2.0 - 3.5 km.s−1 for less cemented material adjacent to the reflectors. There is no apparent correlation between these reflectors and lithologic units. There is an overall good correlation, however, between lithology and reversals in the overall velocity gradient.

Sonic velocities determined with the PUNDIT were found to provide quick, easy and reliable measurements on the core. These allowed the actual Velocity profile to be built up as drilling proceeded with consequent readjustment of projected target depths using the available seismic reflection records. Only reflectors A, C, & D were recognised on the reflection profiles beneath MSSTS 1, so once A had been penetrated, adjusted velocities gave projected depths of C at 204 m and D at 270 m sub-bottom. Based on two-way travel times from Track XXII (Northey et al., 1975) depth estimates were revised after each reflector was recognised. Calculated and actual depths are compared in Table 3. General close agreements between depths indicates reliability of average core velocities.

TABLE 2 ft. Sonic Velocity (Km.s−1) averaged for each MSSTS core

TABLE 2 ft. Sonic Velocity (Km.s−1) averaged for each MSSTS core

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Figure 3 Plot of sonic velocity against depth for core from MSSTS 1.

Figure 3 Plot of sonic velocity against depth for core from MSSTS 1.

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TABLE 3 Calculated and actual depths to recognised reflectors at MSSTS 1

TABLE 3 Calculated and actual depths to recognised reflectors at MSSTS 1

Gas Composition (KK)

The gas composition dissolved in circulating water from drill hole has been analyzed by using a gaschromatograph (Shimadzu GC-30AT) coupled with a pen recorder. The amounts of O2, N2 and CH4 are measured by a Molecular Shieve 5A column, and those of air Components (O2 + N2). CO2, CH4 and C2H6 by a Porapac-Q column.

The equipment was set in a small hut placed just near the drill-rig. The gas analysis was made at constant column temperature (15°C). Helium was used as carrier gas.

The time table of working at drill site is:

Nov. 1-2 Set-up of equipments at drill site

Nov. 8-17 and 20-23 Analyses of gas composition of water and ice samples.

Because the drilling condition was not good at the time of Nov. 8-23, only five measurements were made for circulating water from the bottom of drill hole. Gas composition of room air, atmospheric air, and dissolved gas in sea water were also measured for comparison.

Preliminary results are summarized in Table 5. As seen from the table, CH4 and C2H6 were not found in circulating water samples. Low oxygen contest (6–15%) and rather high CO2 content (1–6%) are commonly observed for water samples except for sample no.5, the gas composition of which is the same as that of sea water. However, main components (O2 and N2) are not much different from the atmospheric air. This fact suggests that the dissolved gas in the circulating water is mainly from atmospheric air, probably sucked in by the pump used for water circulation. The low oxygen content may have caused by contact of sucked air with the down hole sediment, which is in a highly reducing state.

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TABLE 4 Composition of gases from MSSTS1 (in vol.%).
Sample No. Date of Sampling Downhole Depth (a) O2 N2 CO2 CH4 C2H6
1 Nov. 12 179 11.6 83.0 0.4 Not detected Not detected
2 Nov. 15 183 9. 85.4 5.6 Not detected Not detected
3 Nov. 15 186 6.1 92.8 1.1 Not detected Not detected
4* Nov. 15 186 14.6 85.4 1.0 Not detected Not detected
5 Nov. 15 202 31.1 68.9 Not measured Not detected Not measured
6 Nov. 21 sea water 30.1 68.8 1.1 Not detected Not detected
7 Nov. 21 atmospheric air 21.2 78.8 0.04 Not detected Not detected
8 Nov. 21 room air 18.0 81.7 0.31 Not detected Not detected
9 Nov. 8 sea ice (0.26 ml air/sr.-ice)

Surveillance of the Sea Ice (BAS)

Four bench marks number BM1 to BM4 were established on a line bearing 300° and situated at 25, 50, 100 and 200 metres respectively from the rig (Fig. 3). These, together with a point on the Science Hut 10 m from the rig and a point on the rig itself were levelled throughout the drilling operation. Until October 19 a Kern DkM1 theodolite was used, thereafter a Wild level; observational error is estimated at less than ±5 mm. Differential vertical movements of all stations relative to BM3 are plotted in Figure 4 which shows that BM1 and BM2 descended rapidly with respect to BM3 in the period October 10 to 19 when weight was being concentrated at the rig. There followed a 10 day period during which a small but significant rebound occurred. This was greatest and most pronounced at BM2, the farthest station from the rig measured other than the datum bench mark BM3. After October 28 BM1 & 2 and the Rig show a consistant subsidence relative to BM3 while BM4 had risen, Which is taken to indicate that BM3 in fact descended relative to sea level and that the zone of subsidence due to loading at the rig exceeded 200 m in diameter. The total subsidence of the rig relative to BM4 in the period October 28 to November 21 was 65 mm.

Late in the drilling operation when snow had ablated from the sea ice surface a number of old healed cracks were discovered in a region 40 m from the rig. In contrast to the experience at DVDP 15 no relative movement was observed on any existing cracks and no new cracks in the vicinity of the rig or camp were observed during the entire operation.

As at DVDP 15 differential slackness on the guy ropes of the drill mast occurred. The slack guys were on the side of the mast loaded by the main lifting pulley. The amount of slackness increased with load and was attributed to deformation of the mast.

Ice thickness was monitored and is graphed in Figure 4. On September 19 when the site was first visited the ice was 1.98 m thick. The ice thickness increased to 2.33 m by November 14 after which there was a slight decline to 2.28 m at the termination of drilling. All thicknesses were measured in a small region about 10 m from the rig. During the bathymetric survey of the drillsite, ice thickness was measured in 16 holes and found to be the same to within 20 mm over the entire area.

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Figure 4 Behaviour of the sea ice at MSSTS 1 during the drilling

Figure 4 Behaviour of the sea ice at MSSTS 1 during the drilling

  • A - vertical movement of the sea ice
  • B - vertical in ice thickness
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Downhole Temperature

On October 8 during a 30 hour break in drilling, downhole temperatures were measured using a thermistor and bridge circuit on loan from the Physics Department, VUW.

Seven temperature readings at known vertical spacing were obtained in the hole plus two further readings in the water column above.

The thermistor was calibrated prior to the field season with cable and bridge held at room temperature (about 15°C] and only the thermistor temperature lowered. Using this calibration a temperature of −3.3°C was obtained for the seawater above the hole. As it is known that the water temperature in McMurdo Sound is remarkably constant at −1.8°C it appears that either the instrument had drifted since calibration or the bridge circuit was affected by low ambient temperatures. A re-calibration was made on site using Mercury thermometers and a commercial thermistor temperature probe. The new calibration curve was approximately parallel to the old but gave a temperature for seawater closer to the expected value of −1.8°C.

Downhole temperatures derived from both calibrations are plotted against depth in Fig. 5. Absolute values of temperature are known to be in error but the estimate of temperature gradient is not thought to be badly affected. The gradient, 35°C/km, is slightly higher than the value of 31°C/km obtained at DVDP 15.

Figure 5 Downhole temperatures from MSSTS 1.M.

Figure 5 Downhole temperatures from MSSTS 1.M.

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Tidal Measurements (BAS)

A tide gauge consisting of a line anchored to the sea floor and running over a pulley suspended above the sea ice (Fig. 6) was set up in the Science Hut adjacent to the rig. Tidal movements were recorded by taking off the movement through a pulley reduction system onto a chart recorder. Usable records were obtained for the periods 1 to 25 September inclusive and 1 to 24 November inclusive. Until 9 September a reduction of 13.6:1 was used, thereafter 10.1:1. The maximum tidal amplitude was 0.89 m. In addition to the main tidal variations with periods of about 24.20 and 12.20 hrs small cyclical movements with amplitudes of a few mm and periods of 5–50 sec occurred. These are attributed to attenuated ocean swell and were too small to be measured by the recorder.

Bathymetric Survey of the Site (BAS)

Water depths were measured by weighted metre line on a, 200 metre grid surrounding the site. Results are summarized in Figure 7. The sea floor around the site has a very subdued topography with an approximately north-south strike and gentle dip to the east.

Figure 7 Bathymetry within a kilometre of MSSTS 1.

Figure 7 Bathymetry within a kilometre of MSSTS 1.

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Figure 6 Tide gauge used at MSSTS 1, and the record from October 12 to 19, 1979.

Figure 6 Tide gauge used at MSSTS 1, and the record from October 12 to 19, 1979.

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VUWAE "Drillers" (A. McP)

Three student members of VUWAE 24 worked as offsiders on the Longyear 44 drill rig alongside MOW drillers during the MSSTS project. Shift work began on October 16 in 8 hour shifts with one VUWAE member in each of three of the four drill crews. Some of the jobs done by the student offsiders are described below. The rig set up is shown in Figure 8.

Two ten foot lengths of drill pipe were joined to make 20 foot "stands". This was accomplished by means of chain vices and stilsens.

Floor work in the drill shack included joining and breaking of "stands" to and from the drill string, monitoring pump pressure, and general maintenance.

Derrick work consisted of detaching the winch howser from the "stands" while "tripping out" (removal of the entire drill string from the hole), and attaching it while "tripping in". This was often very uncomfortable as the derrick was exposed to all weathers.

Core pulling, or "grabbing" included breaking the drill string and lowering a light wire line with an "overshot" attachment down the hole. The "overshot" latches onto the core barrel, which is then pulled up the hole by winch. The core was then removed from the barrel by a high pressure water jet or other means.

Mixing of mud was always enjoyable, though often rather more grubby. This consisted of mixing the various drill muds and gels in the mud tanks with the help of motorized stirrers. The mud was then pumped down inside the drill string, returning up the casing and bringing with it rock clips from the drilling.

We also acted as general "dogs body", which included any tiling from getting the coffee (and not forgetting the biscuits) for smoko, to refuelling the rig and pump at the end of the shift.

The VUWAE members also assisted in the packing up of the camp and dismantling of the rig. All rig gear and camp buildings were dismantled and towed on sleds by D4 caterpillar back to Scott Base. Because this was such a long (9 hours), slow, monotonous and potentially dangerous trip, the D4 was escorted by a student in a Snotrac. "Riding shotgun" was at times a rather tedious task but enjoyed by most as it gave an opportunity to be relatively alone and reflect on the Antarctic surroundings.

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Figure 8 Layout of the drilling rig at MSSTS 1.

Figure 8 Layout of the drilling rig at MSSTS 1.