(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.