Site selection for Site 1149 was initially based on multichannel seismic (MCS) lines and sonobuoy data obtained during cruise C2005 of the Robert Conrad in 1976. A short SCS and 3.5-kHz survey was conducted on approach to Site 1149, which confirmed the general seismic character and unit thicknesses observed in the MCS records. Holes 1149A, 1149B, and 1149C are located at the intersection of JOIDES Resolution SCS lines 1 and 3, which is ~3.5 km northwest of the intersection of SCS line 1 and C2005 line 39 (Fig. F1B). Hole 1149D is located ~5 km to the southwest on C2005 line 39. Approximately 50 km of SCS data were shot with two synchronized 80-in3 water guns and received using a single-channel 100-m-long streamer. The guns and streamer were both towed at 12-18 m depth. Water guns were fired every 13 s, equivalent to ~36 m at 5.4 kt. The seismic data from each shot were sampled every 1 ms from 0 to 11 s and were digitally recorded after applying an antialiasing filter with a corner frequency at 250 Hz.
Depth in meters below seafloor (mbsf), lithology, lithologic unit divisions, and biostratigraphic ages from Holes 1149A and 1149B (Shipboard Scientific Party, 2000c) are summarized in Figure F2. Results from Holes 1149C and 1149D are not considered in this study because cores were washed and/or taken discontinuously through most of the sediment section and downhole measurements were not obtained.
The wet bulk density of discrete samples was obtained during routine measurements of index properties at Site 1149 during Leg 185 (Shipboard Scientific Party, 2000c, 2000a). Laboratory P-wave measurements used in this study were made using the Hamilton Frame velocimeter PWS3 contact probe system (Boyce, 1976; Blum, 1997). The PWS3 measurement is conducted across the split core axis (x-direction) using vertically oriented transducer pairs, with the upper transducer pressed against the split surface and the lower pressed against the core liner. Velocities were also measured in three mutually perpendicular directions (x, y, and z) using the PWS3 system on discrete samples of igneous and sedimentary rocks (lithologic Units III and IV and oceanic crust), which were sawed as oriented cubes. Velocity and density values were obtained every 1.5 m at the same depth in Unit I and Subunit IIA where core recovery was ~95%; however, wet bulk density values for an ~30-m interval in Subunit IIB could not be obtained because the pyncnometer did not attain a stable helium pressure (Shipboard Scientific Party, 2000c).
Downhole measurements of velocity were made every 0.15 m in Hole 1149B using the long-spacing sonic sonde (LSS) as part of the combination of instruments (tools) known as the Formation MicroScanner-sonic tool string. The LSS consists of two transmitters spaced 0.61 m apart, which are located 2.4 m below two receivers that are also spaced 0.61 m apart. Velocity is calculated from measured traveltime between transmitter/receiver pairs over a known distance (i.e., 2.4, 3, 3, and 3.6 m), resulting in a maximum of four traveltime values at each measurement depth. During postcruise processing, all traveltimes resulting in velocities outside the range of 1524-3805 m/s were discarded (Lamont-Doherty Borehole Research Group). This simple reprocessing successfully corrects most noisy data because the inaccurate traveltimes are sufficiently extreme and there is an eightfold redundancy of measurements at each depth. The velocity values used in this study (known as VP2) were derived from the median values of the processed traveltimes.
Downhole density measurements were made in Hole 1149B with the Hostile Environment Litho-Density Sonde (HLDS) located in the middle of the 30-m-long triple combination tool string. The HLDS utilizes an eccentralizing arm to maintain sensor contact with the borehole wall to a maximum hole diameter of 46 cm. Data accuracy is degraded by washouts where borehole diameter exceeds 46 cm. Density values above 180 mbsf within the pelagic clay are considered unreliable and are not used in this study because of extended washout zones (Fig. F3).
Velocity and density data used to calculate synthetic seismograms were obtained from combining laboratory and downhole logging measurements. Downhole tools record continuous measurements of in situ borehole properties, regardless of core recovery, at a relatively fine sampling interval of ~15 cm. Downhole measurements are used where possible and are especially valuable in intervals of low core recovery (e.g., below 180 mbsf). Laboratory measurements were used in discrete depth intervals where downhole measurements were either not available or were considered unreliable. Core and logging depth scales match exactly at the lithologic Unit II/III boundary (~180 mbsf), and depths reported for all major lithologic unit boundaries are from core depths.
In Hole 1149A, there was nearly 95% core recovery using the advanced hydraulic piston corer (APC) and extended core barrel (XCB) to the base of the pelagic clay interval at 180 mbsf (lithologic Units I and II) (Fig. F2). Downhole measurements of density and velocity were very limited in this interval (poor quality and/or minimal coverage) (Fig. F3); therefore, laboratory velocity and density values from Hole 1149A are used from 0 to 180 mbsf. A constant density of 1.36 g/cm3, which is the density value measured at the top of Subunit IIB, was also used for the lower portion of this unit from 149 to 180 mbsf. Laboratory measurements can accurately reflect actual velocity and density trends in such intervals of high recovery, minimal core disturbance (i.e., APC), and relatively frequent sampling (every 1.5 m). Core recovery, however, dropped dramatically to ~10%-15% when using a rotary core barrel (RCB) in the interval of interbedded chert/porcellanite/clay and chert/chalk/marl, characterizing lithologic Units III and IV below 180 mbsf (Fig. F2). Although backfill prevented logging of the volcanic basement, hole conditions remained stable long enough to log most of the sedimentary section within this interval of poor core recovery. The LSS, in the middle of the 33-m-long tool string, obtained velocity measurements from 160 (Subunit IIB) to 351 mbsf (Unit IV) ~59 m above the top of oceanic crust and 94 m above total hole depth (Fig. F3). A constant velocity (the average log velocity of lithologic Unit IV [2485 m/s]) was used to extend the log velocity from 351 mbsf down to the top of oceanic crust at 410 mbsf. The average laboratory-derived velocity of 4821 m/s was used to continue the velocity data from 410 to the total depth of 445.2 mbsf (Fig. F2).
Density values were acquired as shallow as 63 mbsf (Unit I) and extend down to 398 mbsf, ~12 m above the top of oceanic crust and 47 m above total hole depth (Fig. F3). A constant density, the average log density of the lowermost 50 m of lithologic Unit IV (2.2 g/cm3), was used to extend the log density from 398 mbsf down to the top of oceanic crust at 410 mbsf. The average laboratory-derived density of 2.66 g/cm3 was used to extend the density data to the total depth of 445.2 mbsf (Fig. F2). The combination of laboratory and downhole logging data used to make continuous velocity and density profiles are listed in Table T1.