Leg 184 used the standard JOIDES Resolution configuration of single-channel seismic reflection equipment, as modified from the description of Comas, Zahn, Klaus, et al. (1996). The survey was conducted using an 80-in3 water gun, which was towed ~20 m behind the stern at a depth of ~13 m to prevent it from broaching during the swell conditions. The gun was fired at 10-s intervals with ~2000 psi. The ship's speed was ~5 to ~6 kt depending on conditions. The hydrophone streamer was a 100-m Teledyne model 178, which contains 60 active hydrophones. The streamer was towed at a depth of ~15-20 m. The midpoint of the active portion of the streamer lay ~250 m astern. Analog seismic reflection data were displayed on Raytheon model 1807M and EPC model 9802 recorders. The seismic signal was digitized in real time and displayed on a Sun workstation and for processing. The data were written to DAT (4 mm) and Exabyte (8 mm) tapes in SEGY format. Shipboard processing used the SIOSEIS software and included automatic gain control and filtering to sharpen the signal.
The 3.5-kHz seismic system aboard the JOIDES Resolution consists of an EDO 248C transceiver mounted in a sonar dome and a Raytheon model 1807M line scanner recorder. The system has an effective acoustic cone of ~20° to ~30°. Only analog 3.5-kHz records are available for the Leg 184 site locations.
SONNE95, Legs 1 and 2 (April-May 1994), collected 2972 km of seismic reflection profiles on the northern continental margin of the South China Sea, mostly near the Pearl River Mouth Basin (Sarnthein et al., 1994). The seismic reflection system consisted of four air guns (three Geco Prakla and one SSI) with a total volume of 8.7 L. The guns were triggered at 25-m intervals, which at 4.5 kt gives a seismic record every 11 s. The signal was received by a Geco Prakla eight-channel hydrophone streamer, which had receiver-to-receiver distance of 12.5 m and a common midpoint distance of 6.25 m. Processing of the digital data was accomplished using the SEISTRIX 3 software and included digital filtering, muting, trace editing normal move-out correction, stacking, deconvolution, and migration (Wong et al., 1994).
SONNE95 used the PARASOUND profiling system, which has a narrow beam (~4°) that uses a differential frequency of ~4 kHz. This system increases vertical resolution and suppresses hyperbolic echoes where the surface slopes are low. Only analog PARASOUND records are available for the Leg 184 site locations.
Comparisons between the reflector sequences observed in the seismic lines and the characteristics of the sediment cores and logs were made by calculating the acoustical impedance as a function of two-way traveltime (TWT). Acoustical impedance was estimated as the product of the sonic velocity (meters per second) from the long-spaced sonic log and the hostile environment lithodensity sonde bulk density (g/cm3) from the logged section of each site. The acoustical impedance was then smoothed with a 30-point moving window (~15 m in length) to eliminate the spikes caused by poor hole conditions but preserve the major impedance features. To calculate the TWT for each impedance estimate, we first smoothed the velocity data with a 30-m (67 points) moving window to approximate the seismic wavelength and give a smooth velocity structure for calculating depth relationships. We next calculated the interval TWT between logging measurements (TWT = [2·D]/V, where D = depth interval between log measurements in meters and V = smoothed velocity). These interval TWTs were cumulatively summed up from an initial depth, which varies from site to site depending on the hole conditions and on where the logs became reliable. We used the physical properties core-log data for bulk density and P-wave velocity to span the gap from core top to the logged interval. These shipboard values are usually lower than the in situ log values and have been adjusted to best fit the log data and give a continuous density and velocity profile for each site (see "Site 1143 [SCS-9]," "Site 1144 [SCS-1]," "Site 1145 [SCS-2]," "Site 1146 [SCS-4]," and "Sites 1147 and 1148 [SCS-5C]"). Because the reflection of seismic waves is sensitive to the relative change in impedance as well as the absolute amplitude, we examined the first derivative of the acoustical impedance log for some sites. These data will be presented in the Leg 184 Scientific Results volume of the Proceedings of the Ocean Drilling Program.
As noted by Ludmann and Wong (1999), "The nomenclature of sequences and unconformities in the Pearl River Mouth Basin is not unambiguous in the literature." This comment applies to the remainder of the SCS as well because several systems of seismic reflector (and associated or assumed unconformities and sequence boundaries) use the same symbols but assign different ages to the features. We follow the nomenclature of Ludmann and Wong (1999), who modified the system of Guong et al. (1989) for both sequences and unconformities. However, we include Pliocene/Pleistocene Reflector TN from Jiang et al. (1994; see Fig. F6 in the "Leg 184 Summary" chapter) and the Oligocene/Miocene Reflector T5 from Chen et al. (1987; as illustrated in table 1 of Ludmann and Wong, 1999). The nomenclature and age assignments for these boundaries and their associated reflectors are summarized in Table T2.
Briefly, we adopt T0 as a late Pleistocene reflector at ~0.45 Ma and TN as the Pliocene/Pleistocene reflector at ~1.85 Ma. T1 is taken as the Miocene/Pliocene reflector (~5.2 Ma), T2 is the middle/upper Miocene reflector at ~10.2 Ma, and T4 marks the lower/middle Miocene boundary at ~14-15 Ma. Although Ludmann and Wong (1999) do not recognize T5 (Chen et al., 1987; ~26 Ma) in the Pearl River Mouth Basin, we retain it: some of the Leg 184 reflectors seem consistent with this age feature. Ludmann and Wong (1999) consider T7 (~32 Ma) to be the mid-Oligocene breakup unconformity associated with the Nanhui Movement. This boundary marks the end of the rifting and the beginning of the drifting phase for the SCS basin. Although these reflectors are not always uniquely identified in the Leg 184 seismic lines and cores, they provide a useful frame of reference. The well-dated Leg 184 sediments may serve to better constrain the age of these features.