For all ODP logging operations, tool depth is measured by the length of cable spooled out from the logging winch. The recorded depth is then shifted to depth below seafloor, as identified by the gamma ray log (Kanazawa, Sager, Escutia, et al., 2001). Because the ship oscillates at the sea surface due to wave motion, among other sources of irregular tool motion, the actual tool depth departs from the measured cable length and the logging speed is not constant. Both the MGT and HNGS data were collected at cable speeds of 250–300 m/hr. The tools experienced variations in logging speed due to ship heave and hole conditions, and during the repeat MGT pass, an electronic fault in the wireline heave compensator resulted in further increases in vertical tool motion. Such differences in operational conditions likely contribute, in part, to differences observed between logs.
For the MGT, accurate common-depth stacking depends on the precise time shift of data from detectors in the sensor array. Rapid variations in the logging speed must be carefully taken into account. To accomplish this, time shift corrections are applied to restack the data based on depth and cable speed. The effect of residual ship heave on the count statistics of the stacked MGT log is therefore minimized.