Present knowledge of the western Great Australian Bight seismic stratigraphy is based on extensive, high-quality multichannel seismic reflection data, together with a single oil exploration drill hole (Jerboa-1), which provides minimal information about the Cenozoic succession (Fig. F11). Characteristics of the various seismic data sets are summarized in Table T11. The original Leg 182 drilling proposals (James and Feary, 1993; Feary et al., 1994, 1995) were based on detailed seismic stratigraphic interpretation (Feary and James, 1998, reprinted as Chap. 2) of a grid of 2350 km of high-quality, regional two-dimensional seismic reflection lines collected and processed by the Japan National Oil Corporation (JNOC) in 1990 and 1991, over an area of 155,000 km2 on the continental shelf and upper slope of the western Great Australian Bight. An additional 1380 km of moderate-quality regional two-dimensional seismic lines collected by Esso Australia in 1979 and reprocessed by JNOC were also used to fill gaps in the JNOC data set.
In February 1996 the Australian Geological Survey Organisation (AGSO) performed a multichannel seismic site-survey cruise (Feary, 1995) on the Rig Seismic in support of the Leg 182 drilling proposal (Fig. F12). This cruise collected 1800 line-kilometers of high-resolution, two-dimensional seismic data as 0.5-nmi-spaced grids centered on each site, together with tie lines between sites (individual site-survey seismic grid maps are presented in the "Seismic Stratigraphy" sections of the site chapters). The Australian Geological Survey Organisation Survey 169 (AGSO169) data were collected using a 1000-m 80-channel analog streamer, using Seismic Systems Inc. GI air guns deployed in full bubble-suppression mode (configured with 45-in3 generator and 105-in3 injector chambers). A single array of three GI guns was used in shallow water (<750 m) and twin arrays of three GI guns were used in deep water (750-4500 m), running at 2000 psi (effectively 1800 ± 200 psi) at 3 m depth. Data are 3.5-s records in shallow water and 8.5-s records in deep water (12.5-m shot interval in shallow water, 25-m shot interval in deep water), with a 1-ms sample rate. Data acquisition specifications were in accordance with current exploration industry standards. Primary and independent backup differential Global Positioning System navigation systems resulted in extremely high navigational accuracy, and excellent weather conditions resulted in low streamer noise levels (normally 5-10 µB).
The regional Cenozoic seismic stratigraphy (Feary and James, 1998, reprinted as Chap. 2), derived from interpretation of the JNOC regional seismic data set, is based on division of the Cenozoic succession into seven unconformity-bounded seismic sequences. The high-resolution site-survey data set permitted the identification of additional unconformity surfaces within many of the regional seismic sequences; however, as there is insufficient resolution to extend these boundaries regionally, they are traced on an individual site basis only.
Interpretation of the stacked and migrated two-dimensional seismic data was performed on a GeoQuest IESX-2D seismic interpretation workstation during the precruise proposal and safety evaluation processes. A GeoQuest workstation with the complete seismic and interpretation data sets was available on board throughout the leg, so it was possible to evaluate coring results during drilling to aid in operational decisions and to contribute to data interpretations. In particular, it was possible to immediately derive the detailed characteristics of seismic sequences intersected at each site (presented in the "Seismic Stratigraphy" sections of the site chapters) and to make limited predictions based on the results of earlier sites. The availability of the workstation with the complete interpretation package was invaluable when drilling difficulties prevented successful coring in Holes 1129A and 1129B at the original Site 1129 location, and provided the basis for siting the successful Holes 1129C and 1129D 0.5 nmi south of the original location.
Correlation of two-dimensional seismic reflection profiles to one-dimensional data derived from cores or downhole logs is a key step toward understanding drilling results within a regional context (Mayer, 1994). The principal requirement for such correlation is to establish the relationship between the two-way-traveltime scale of seismic reflection data displays and the depth scale of coring. Check-shot surveys provide the optimum time-depth control to calibrate integrated sonic curves derived from interval transit-time data and to provide individual time-depth tie points. Before Leg 182, stacking velocity functions derived during seismic reflection data processing were used to estimate depths to drilling targets. The procedure undertaken during Leg 182 was for processed check-shot results (WST; see "Downhole Measurements") to be plotted on depth to two-way-traveltime graphs to determine the relationship between depths of lithofacies and/or biostratigraphic boundaries encountered during coring, and sequence boundaries and horizons located on seismic data. Because sonic logs only provide data up to the base of drill pipe, the integrated sonic trace derived from interval transit-time data was unconstrained within the time domain. Check-shot tie points provided the necessary time constraint so that the integrated sonic trace could define the actual time-depth relationship at each site. Check-shot results were also used to compare the final time-depth relationship with predictions based on stacking velocities. These plots show that the actual time-depth relationship defined by check-shot surveys occurred close to or at the lower limit of the envelopes defined by stacking velocity curves, indicating that stacking velocities consistently underestimate actual sediment velocities. Where check-shot surveys were not run, this consistent relationship between stacking and sediment velocities was used to produce an optimum time-depth estimate, with the integrated sonic trace being placed close to the lower limit of the envelopes defined by stacking velocity curves.