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doi:10.2204/iodp.pr.317.2010

Traveltime/depth conversion

Relating seismic sequence boundaries to features observed in cores is critical to any test of continental margin sequence stratigraphy. Sequence boundaries were originally assigned predicted depths based on synthetic seismograms created using sonic and density logs from ODP Site 1119 and commercial exploration well Clipper-1. Ages were also assigned to the sequence boundaries using ties to Site 1119 and the exploration wells, principally Clipper-1 (Lu and Fulthorpe, 2004).

During the safety reviews by the Environmental Protection and Safety Panel (EPSP), predicted sequence boundary depths were modified using a new analysis of check shot and sonic log information from Clipper-1 and Site 1119. Seismic stacking velocities were not used for traveltime/depth conversion because they were substantially higher than those derived from Site 1119 and Clipper-1, which are in general agreement. The final traveltime/depth conversion function used to estimate sequence boundary depths prior to Expedition 317 was

Y = 317X² + 758.3X,

where Y = depth below seafloor in meters, and X = two-way traveltime in seconds.

Pending postcruise analysis, depths quoted for seismic sequence boundaries in this report continue to be derived from this predicted function. A sonic log was obtained from Hole U1351B to ~490 m WSF, but it requires postcruise processing before it can be used to provide an alternative traveltime/depth conversion. A sonic log was also obtained from Hole U1352B to ~420 m WSF, and a synthetic seismogram was generated to provide an alternative traveltime/depth conversion. However the resultant velocities proved to be anomalously lower than the predicted function, which is itself evidently too slow, at least for traveltimes corresponding to the deepest penetrations. For example, if the Marshall Paraconformity correlates, as seems likely, with seismic horizon "Green," the precruise predicted function underestimates its depth by ~30–35 m.

Results from Hole U1353C may be more representative. A sonic log to ~230 m WSF yielded an excellent synthetic seismogram match to the seismic data and estimated depths to seismic sequence boundaries within the logged interval (U13–U10) that were 5–10 m deeper than predicted precruise. This may represent an improved traveltime/depth conversion for this interval, although the difference may be within the margin of error for both sets of traveltime/depth estimates. A sonic log was also obtained from Hole U1354C to ~380 m WSF at the end of the cruise, and results have not yet been assessed.

In summary, the precruise predicted function continues to be used for core-log-seismic correlations in this report. However, postcruise analysis may lead to modifications of this function based on further evaluation of Expedition 317 wireline logging and physical properties data.

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