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.