49. ANALYSIS OF NOISE ON DIGITAL SONIC LOG DATA AT LEG 120 SITES. DERIVED SYNTHETIC SEISMOGRAM AND CORRELATION WITH MCS DATA AT SITE 7471

During Leg 120, weather conditions and time constraints restricted logging operations to Sites 747 and 750; only the seismic-stratigraphic combination was run at these sites. For some intervals, the sonic digital tool produced extremely noisy velocity logs that made interpretation of the data very difficult, even impossible. Three types of noise were observed: phase skipping, oscillations caused by noncompensated heave, and discrete aberrant velocity values. The heave-related oscillations recorded on the velocity logs were eliminated by deriving a simple model simulating the tool movement. The aberrant velocity values, essentially caused by failures in recording the signal from one of the two transmitters, were eliminated by inspection of the individual waveforms using an interactive software package. This software program was also used to pick the compressional arrival time. A composite velocity log accounting for all these corrections and complemented for the unlogged part of the hole with the core-measured velocities was used to derive a synthetic seismogram at Site 747. The synthetic seismogram was correlated with a multichannel seismic reflection profile and the lithologic units, which allowed us to define four seismic sequences that could be identified with the K2-K3, P1-P2, PN1, and NQ1 sequences of the Raggatt Basin in the Southern Kerguelen Plateau