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Core-log-seismic integration

During Expedition 343, we used LWD data and core measurement data (see “Physical properties” and “Logging while drilling” for background on data collection and processing) to establish ties to the seismic data obtained in 2011 after the Tohoku-oki earthquake.

Seismic data

The high-resolution seismic data set was obtained by JAMSTEC during the KY11-E05 cruise in October and November 2011 of the R/V Kaiyo. The sounding source was a 320 in3 cluster gun array, and the shot interval was 37.5 m. The receiver was a 1.3 km, 192 channel digital streamer cable with 6.25 m channel spacing. The maximum offset was too short for velocity analysis at the target depth of this expedition. The velocity model used for stacking and poststack time migration was obtained from analysis of another multichannel seismic survey, KR11-E03, which was obtained with a 6 km streamer cable. Specifically, a velocity model obtained from stacking velocity analysis of Line D05, located 4 km south of our drill site, was used. The above velocity model resulted in a migrated image that was obscured by migration noise, so we adopted a constant velocity model of 1525 m/s, which appropriately migrated the seafloor reflection and resulted in a much better image of the deeper part.

Depth conversion of the seismic data

In order to accurately correlate the core, log, and seismic data, we needed to convert the seismic reflection profiles to depth sections. Sonic logging and a check shot survey were not conducted during Expedition 343, so for initial depth conversion we converted and slightly modified the root mean square velocity model obtained from the prestack time migration velocity analysis of Line D05 to interval velocity. The resulting depth section was later calibrated using the logging and core measurements collected during the expedition. The final calibrated depth section was used for core-log-seismic integration purposes.

Synthetic seismograms

We created synthetic seismograms to help correlate the LWD logs with the high-resolution seismic data. Density and sonic wave velocity were not directly measured during LWD; therefore, in order to create a reflectivity series and synthetic seismograms, these data were estimated using resistivity logs. The method used to calculate density is detailed in “Logging while drilling.” Velocity was calculated from estimated density with the following empirical relationship:

V = 304800/(241.011 – 43.346ρ), (50)

where V is velocity and ρ is the estimated bulk density. This relationship was determined by cross-plotting density and sonic velocity recorded from ODP Leg 186 Sites 1150 and 1151 and later using measurements from discrete core samples. The synthetic seismograms were compared with the seismic section to help match the depth of the main reflectors and provide a quality check of the logging and core measurement data used to calculate the synthetic seismograms. Both Petrel (by Schlumberger) and Geolog/SeisEarth (by Paradigm) software packages were used to construct synthetic seismograms.