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Victoria University Antarctic Research Expedition Science and Logistics Reports 1986-87: VUWAE 31

McMURDO SOUND SEISMIC STUDIES

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McMURDO SOUND SEISMIC STUDIES

Abstract

There were two parts to the K045 scientific programme. The first involved the downhole velocity logging of the CIROS-1 drillhole at the completion of drilling. This work encountered severe noise problems which may make the signal irrecoverable. The second part of the event involved the determination, using seismic reflection techniques, of the dip, faults and strong reflectors in the geologic structure in the immediate vicinity of the CIROS-1 drillhole as well as the testing of various seismic reflection techniques for any future reflection projects on the sea-ice. Despite transport and manning problems 2 seismic lines were shot running at right angles from the CIROS-1 drillhole and records appear to have primary reflected energy down at least to 800 metres sub-seafloor. Tests proved explosives a more useful sound source than marine airguns.

Background

While velocity measurements are made on the recovered core the effects of pressure, temperature, pore fluid etc., mean that the velocity of the core samples can differ markedly from the in situ velocities which the downhole technique will measure. The reliability of the tie between the recovered core and seismic reflection and refraction lines in the CIROS-1 hole vicinity is dependent on how well the in situ sediment velocities are known. Most of the available literature on vertical seismic profiling refers to the use of three component seismometers deployed down the drillhole. There is little published regarding the use of a hydrophone in a sea-ice situation. From land operations one of the main requirements is to separate the desired first arrival signal from tube waves, strong resonant wave modes set up in the water column. Other than a well-cemented hole the next most preferred configuration for seismic energy transmission is an open hole. Consequently the shot hole was placed 50 metres from the drill rig and the downhole logging was performed in an uncased hole (until the collapsed hole was encountered).

It is important to link the geological structure determined by drilling with the horizons detected in other seismic surveys in the western Ross Sea, particularly the multichannel seismic survey conducted by the S.P. LEE, in order to date these horizons and their deformation. The reflection survey will also hopefully reveal some of the deformation history in the vicinity of the drillhole as well as the possible presence of reflecting strata at depths greater than reached by the drilling project.

Two sound source types, explosive and marine airgun, are available. Explosives are the traditional sound source used in 'land' seismic work in the Antarctic. Marine airguns, lowered through a hole in the sea-ice, have been used in the northern Arctic regions and are potentially a less expensive sound source than explosives. K045 was to test the feasibility of this sound source versus explosive.

Personnel

Event leader Bryan Davy, Geophysics Division, DSIR
Graham Alder, Victoria University of Wellington
Hernan Moreano Andrade, Instituto Oceanografico
de la Armada, Equador.
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Results

(a) Downhole velocity survey

A Mark Products P27 deep hydrophone was lowered down the CIROS-1 drillhole in steps varying from 10 to 20 metres with the seismic travel time from a surface explosive event being recorded at each level on an ES-1200 Nimbus seismic recorder. The difference in travel time to each level should provide the velocity of the intervening layers. The hydrophone was connected beneath the MWD Water and Soils Division logging cable. Unfortunately the outer earth-return nature of this cable meant that it acted like an aerial picking up background electromagnetic noise of a similar level to the voltages produced by the hydrophone. Much of this background noise appeared of a regular sinusoidal nature and it is hoped that selective filtering of the digitally recorded signal can yet yield the first arrival information required. Examination of the raw seismic record indicates seismic arrivals from the surface explosion but the nature of these seismic modes was indeterminate as was the presence of first arrival information. Halfway down the hole, at a depth of 317.8 m the hydrophone encountered a hole cave-in. Due to the uncertain value of the data being recorded as well as the difficulties anticipated in clearing the blockage it was decided to abandon the downhole survey at this stage.

(b) Seismic reflection profiling

Two 24 channel 6-fold seismic reflection lines designed both to determine the geological structure in the CIROS-1 immediate vicinity and to evaluate marine airgun sound sources as an alternative to explosive operations were surveyed running at right angles away from the drill hole. In order to speed up surveying operations two 12 channel ES-1200 Nimbus seismic recorders, belonging to Geophysics Division DSIR and Victoria University respectively, were linked for the first time to form a 24-channel system. The sound source was lowered through a 12 inch diameter hole drilled through the sea ice using an ice auger system developed by Alex Pyne, Victoria University. These holes were originally spaced every 70 metres with the reflected sound being detected by a line of geophones set in the ice with an individual spacing of 35 metres, overall spread length of 815 metres and a near-geophone to shot offset varying between 70 and 420 metres- After 6 drill holes 12 of the geophones were 'rolled along'. This configuration yields 6-fold common midpoint coverage. The first line shot was orientated at 314 degrees E, towards the MSSTS-1 hole, and although it did not reach this hole as hoped for at first, with a 2.5 km length it did cross the S.P. Lee multichannel seismic line. A second 2 km line with orientation 224 degrees E was surveyed with twice the shot spacing i.e. 140 metres in order to cover a greater distance, although halved resolution, within the time and explosive resources available.

Generally airgun operations proved slow and complicated with shuttle sealing and airgun firing and recovery difficulties. Where airgun records were obtained they appear on the unprocessed records to be significantly inferior in resolution and penetration to those obtained by explosives despite the stacking of airgun records, in order to achieve some significant ground coverage and also due to undermanning on the project the survey concentrated on the use of an explosive sound source. Reflecting horizons appear on the raw records to be detectable to depths of 700-800 metres at least, although such identification is made difficult by the presence of strong horizon multiples over the same time window.

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Recommendations

Preliminary examination of the seismic record suggests primary reflected energy is present to at least 800 metres sub-seafloor. Multi-channel seismics enables the removal of much of the sea-floor multiple reflector energy as well as the provision of velocity information. Thus where it is unlikely a multichannel marine seismic ship con venture the above system provides a viable alternative. Explosive sound sources despite their greater expense are the much preferred option due to their simplicity of operation and superior sound source characteristics. The joining of the two twelve channel Nimbus recorders successfully provided a 24-channel system halving the number of shots required for a single 12-channel recorder. A tracked heated cab/room environment is required for the Nimbus recorders. With an explosive sound source the rear transport could be toboggan based. The seismic line, if possible, should be bulldozed and surveyed.

Any future downhole velocity logging using a hydrophone transducer needs to be linked to the surface by a shielded logging cable.

Acknowledgements

Thanks to Alex Pyne for his assistance in planning and organisation as well as the use of the ice auger, derrick and associated sledges.

Thanks to Geophysics Division technical staff for the work configuring the Nimbuses for dual operation, constructing a portable airgun firing unit and various triggering adapters as well as general seismic equipment support.

Thanks to Eric Broughton of the Victoria University technical workshop for the work on development of the compressed air supply system.