(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.