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doi:10.2204/iodp.proc.302.102.2006

Geophysical data collected during Expedition 302

Single-channel seismic reflection and 15 kHz echo soundings profiling were conducted during Expedition 302 to build upon the existing geophysical site survey database in the vicinity of the four drill sites (M0001–M0004) located along profile AWI-91090 (Figs. F1, F7). The acquisition of these additional seismic reflection data had three main objectives:

1. To image the upper 450–500 m of the Lomonosov Ridge sediment sequence at improved resolution,

2. To cross profile AWI-91090 in the vicinity of Expedition 302 drill sites, and

3. To further extend the seismic database of the Lomonosov Ridge in the vicinity of Expedition 302 drill sites in order to facilitate integration of the Expedition 302 core data with the acoustic stratigraphy on the Lomonosov Ridge as whole.

Methods

A schematic illustration of the Expedition 302 seismic reflection acquisition and underway postprocessing setup is shown in Figure F8. The individual components are described below.

Navigation

The seismic acquisition system was installed in a winch compartment located on the Oden’s aftdeck. From this location, there was no direct cable access to the ship's navigation system, and, therefore, a separate Ashtech 12-channel Global Positioning System (GPS) receiver was installed to receive positions for the seismic reflection profiling. The GPS antenna was mounted on the aftdeck’s starboard side, and all positions were corrected in the Meridata data acquisition software (MDCS) (Fig. F8) to account for the distance between the GPS antenna and the seismic source and streamer.

Seismic reflection

A PAR 1600 air gun with a 0.65 L (40 inch³) air chamber and wave shape kit was used as the seismic source. The seismic signal receiver consisted of a single-channel streamer with an active length of 16 m. The streamer contained 100 AQ-1 hydrophone elements and a preamplifier. The shot interval was set to 2.7 s, and, following a time delay of 1.0 s, the received signal was sampled at 4 kHz during a 1.3 s sweep. Data acquisition was performed using a top unit from Meridata (Fig. F8) connected to a personal computer. All acquired data were stored on a hard disk in the custom Meridata format. Apart from the data acquisition software (MDCS-Meridata data acquisition) (Fig. F8), the system included software for signal post-processing and interpretation (MDPS-Meridata processing and interpretation) (Fig. F8).

The seismic source was towed using a specially designed steel depressor as a tow vehicle (Fig. F9) for two main reasons: (1) to keep the seismic source as close to the Oden’s fantail as possible to avoid problems with ice and (2) to keep the air guns deeper than the Oden’s noisy and turbulent propeller stream. Towing seismic equipment in heavy ice conditions necessitated that proven methods developed from previous Arctic Ocean seismic reflection surveys (Jokat et al., 1992; Kristoffersen, 1997; Kristoffersen et al., 2001) be employed. The streamer was attached to the steel tow vehicle in order to prevent it from being towed into the propeller stream or caught by ice. Between the ship and the steel depressor, the cables were protected from ice by a 25 m long reinforced plastic hose (Fig. F9). The towed seismic devices were designed to be small so that they could be launched and retrieved quickly.

The Russian nuclear icebreaker Sovetskiy Soyuz assisted during the seismic reflection survey by breaking ice and forming leads ahead of the Oden along the profiling track. This made it possible to keep close to the planned track, although some maneuvers around major floes were unavoidable to keep the two-ship convoy from stopping (Fig. F7).

Bathymetry

A Marimatech E-Sea Sound MP35 dual-frequency survey echo sounder set to 15 kHz was installed shortly before the Expedition 302 departure. Depths from this echo sounder were continuously logged in the Oden’s database during most of Expedition 302. A sound velocity of 1463 m/s was used to calculate water depths from the echo soundings.

Results

Seismic reflection

Seismic reflection profiling was accomplished on one occasion during Expedition 302. The survey was planned along a track crossing profile AWI-91090 orthogonally at several locations; two of these crossings were located near Sites M0002 and M0003 (Figs. F7, F10). Time did not permit the completion of the entire planned 41 km track because of ice conditions. However, a total of 22 km of seismic reflection profiles was acquired at an average ship speed of 2.4 kt (Fig. F7). The initial part of the survey (profiles 48250112–4825026) was conducted in nearly 10/10 sea ice cover and heavy ice pressure, which resulted in a high noise level due to icebreaking and a ship speed too low (<1 kt) for high-quality seismic reflection acquisition. The ice pressure gradually decreased and, as a result, the ship speed could be increased, which improved data acquisition during the remaining survey (Fig. F10).

Following completion of drilling operations at the last site, seismic profiling was planned for 6 h in the vicinity of Site M0004. However, increasing wind and high ice pressure resulted in an unsuccessful survey. The ship speed could not be maintained above 1 kt because of ice, and only ship noise was recorded.

Bathymetry

The new Marimatech echo sounder installed on the icebreaker Oden performed poorly during the entire expedition. In the deep areas (depth = >2000 m), practically no useful bathymetry data were collected (drilling operations on the Lomonosov Ridge took place in water depths ranging from 1200 to 1300 m). Because the Oden was clearing ice continuously around the drillship Vidar Viking, an abundance of echo soundings could have been collected around each of the drill sites (Fig. F11). Hundreds of crossing track-lines were logged, which made it possible to compare the depth data logged at the crossovers (Fig. F11). The results from this comparison, together with a three-dimensional (3-D) analysis, show the poor performance of the echo sounder (Fig. F12). Because the area had previously been surveyed during the ARK-VIII/3 cruise with Polarstern’s Hydrosweep system (Fütterer, 1992), no further efforts were made to make use of the echo sounding data collected during Expedition 302.

Acoustic stratigraphy at Expedition 302 coring sites

The acoustic stratigraphy in the surveyed area is remarkably consistent. Profile 48250420, which runs along AWI-91090 and passes close to Site M0003, provides a representative view of the seismic stratigraphy (Fig. F10A). The stratigraphic level for the pronounced unconformity identified by Jokat et al. (1992) is marked by a prominent reflector (Fig. F10A, F10B). Profile 48250531 passes <100 m from Site M0002 (Fig. F10B), again showing the unconformity as a prominent reflector (Fig. F10B).

Prominent reflectors in the seismic reflection profiles collected during Expedition 302 are readily correlated with reflectors in profile AWI-91090 (Fig. F13). Jokat et al. (1995) have divided the upper ~450–500 m thick and horizontally stratified sediment section of the Lomonosov Ridge stratigraphy into four units (Units LR6–LR3). These units have been interpreted to represent important stages in the Cenozoic evolution of the Lomonosov Ridge and, thus, the Arctic Ocean. Because of its higher resolution, the Expedition 302 seismic reflection data provide additional information regarding the acoustic stratigraphy within units LR6–LR3. For example, within unit LR5, a set of reflectors are resolved at higher resolution than earlier data. These reflectors are clearly visible in Figure F13 where profile 48250420 crosses profile AWI 91090. Figure F14 shows the approximate maximum drill depth at the four Sites M0001–M0004.

The Expedition 302 high-resolution seismic profiles add new information to the geophysical database. Based on previous multichannel seismic profiles, the uppermost 450–500 m of the Lomonosov Ridge sediments can be subdivided into four seismic stratigraphic units LR3–LR6 (Jokat et al., 1995) (Fig. F13). The base of unit LR3, the unconformity at the base of the Tertiary sediments, is a prominent reflector in the Expedition 302 profiles as well as in AWI-91090 (Fig. F13). In profile AWI-91090, there is an almost equally strong reflector some 100 m below the base of LR3. In the higher resolution Expedition 302 profiles, no reflector exists at this level. Similarly, in profile AWI-91090, there is a reflector ~100 m below the seafloor that has no clear equivalent reflector in the Expedition 302 seismic profiles. In AWI-91090, this prominent reflector is interpreted as the boundary between units LR5 and LR6 (Fig. F13). In the Expedition 302 profiles, this interval is almost seismically transparent. The LR4/LR5 boundary is distinct in the AWI-91090 profile as well as in the Expedition 302 profiles. In the AWI-91090 profile, there is again a reflection some 100 m below the LR4/LR5 boundary, interpreted as the boundary between units LR3 and LR4. In the Expedition 302 profiles, this reflector is not present. Thus, only the unit boundary LR4/LR5 and the base of unit LR3 are identified in both the Expedition 302 profiles and the AWI-91090 seismic line (Fig. F13).

The Cenozoic sequence in the Expedition 302 profiles is subdivided into two seismic stratigraphic units, corresponding to LR5/LR6 and LR3/LR4. The seismic reflectors are essentially flat-lying throughout the entire section. The internal reflectors in the upper LR5–LR6 unit are weak and discontinuous, indicating small impedance contrasts. Undulating reflectors at several levels suggest that the seafloor has, at times, been rather uneven. The basal part of Unit LR5–LR6 contains a unit of seismically stratified reflectors (Fig. F13). The true nature of these reflectors remains to be further explored through core-seismic integration.

In the Expedition 302 profiles, the LR3–LR4 unit features a rather diffuse horizontal reflector in its middle part (Fig. F10). Slightly undulating reflectors occur in the lower and upper parts of this unit. Similar to the LR5–LR6 unit, the seafloor has evidently been rather uneven at times.

The LR3 reflector, marking the base of Cenozoic sediments on the Lomonosov Ridge (Jokat et al, 1995), appears to be complicated and composed of reflections from two or more sediment surfaces. The erosional unconformity, sculptured in bedrock, is rough. Subsequent infilling of this irregular surface has created the flat strong reflector seen in the Expedition 302 data (Fig. F10).

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