Marine Geophysics and Hydroacoustics

Western Australian Continental Shelf

SO257 - WACHEIO: Western Australian Climate History from Eastern Indian Ocean Sediment Archives

 

Summary

During Cruise SO257, a Geometrics GeoEel streamer consisting of four solid state sections (32 channels) and one standard 1.7L Generator Injector (GI)-gun were used to acquire 2D high-resolution multichannel seismic data. The aim was to acquire high-resolution shallow seismic data across a number of transects during the cruise. More ...

The main topic and objectives of cruise SO257 was to decipher the paleooceanographic environment along the western australian Shelf integrated in a larger project "Western Pacific Warm Pool" in the framework of the Integrated Ocean Discovery Project IODP Expedition 363. The original plan was to acquire high resolution seismic data as a pre-site survey for the deep drilling campaign in order to select suiatable sites and avoid drilling in mass wasting deposits. Unfortunately the drilling campaign took place a year ahead of the SO257. Nevertheless we surveyed the area around drill site U1482 in great detail and hence we can correlate our seismic data with sedimentological and geophysical properties obtain by the drilling campaign. Three people from Kiel university were part of the seismic crew but we got support from international scientist from Australia.

General Cruise Information

Based on the analysis of high-resolution climate archives from a suite of sediment cores retrieved along thseismic crewe continental margin of Western Australia (between 15° and 32°S) and on the development of a high-resolution chronology, using radiometric dating and benthic isotope stratigraphy, we will: (1) test the hypothesis of southward (northward) shifts of the westerlies and the southern margin of the tropical rainbelt during Southern Hemisphere warming (cooling) events; (2) test the hypothesis of a reduction in tropical convection and weakening of the Walker circulation during tropical warming, thus evaluating consequences for the prediction of tropical rainfall during future global warming; (3 explore the effects of tropical and Southern Hemisphere warming on the “Indian Ocean Dipole” and “Southern Annual Mode” along the  coast of Western Australia. To achieve these goals, we will reconstruct the displacement of climatic belts along the western coast of Australia during main Southern Hemisphere warming/cooling phases within the last 135 000 years.

Seismic Survey

The GI-Gun was deployed with the starboard side crane and towed ~20 m behind the ship's stern. The gun had an elongated buoy connected to the gun-hanger by two rope loops which stabilized the gun in a horizontal position at ~2 m depth. The Injector of the GI gun was triggered with a delay of 42 ms with respect to the Generator signal, which basically eliminates the bubble signal. The gun was shot between 150 to 163 bar. Shooting intervals varied between 6 and 8.5 s (depending on water depth) resulting in a shot point distance between ~15 and ~20 m at 4.5 knots. The gun worked reliably for most of the cruise, but failed for one seismic profile, and had to be serviced. Data from this profile are of lower resolution than similar lines as one chamber of the airgun did not fire. A trigger was generated by a custom-made unit allowing to adjust shooting rates and delay times based on the water depth. A digital streamer (Geometrics GeoEel) was used for receiving the seismic signals. The system consists of a tow cable (~50 m, with variable length in the water, one vibration isolation section (10 m long) at the end of the active part of the streamer, and up to 4 active sections (each 12.5 m long). An active section contains eight channels (channel spacing of 1.5625 m, total length of 12.5 m) resulting in 32 channels for the entire streamer. Initial lines used only 3 active sections, but repair of a fourth section mid-way during the cruise allowed 4 active sections for the remainder of the cruise. One A/D converter module belongs to each active section.

equipment
General concept and pictures of the seismic equipment used on cruise SO257. A 1.7L GI-Gun was used as source (left) and a Digital Geometrics Streamer with 4 active sections received the reflected signals.

These A/D converter modules are small Linux computers. Communication between the A/D converter modules and the recording system in the lab is via TCP/IP. A repeater was located between the deck cable and the tow cable (Lead-In). Data were recorded using acquisition software provided by Geometrics. The analogue signal was digitized with 2 kHz. The data were recorded as multiplexed SEG-D files. One file was generated per shot. The acquisition PC allowed online quality control by displaying shot gathers, a noise window, and the frequency spectrum for each shot. The cycle time of the shots is displayed as well.

seismic

Comparison of 2D high resolution seismic data acquired during SO257 (left) and industry seismic data (right).

Thirty seismic profiles were acquired during the cruise. The shallow parts of these data reveal much higher resolution imaging of the strata compared to industry multichannel seismic data. Seismic reflection profiles imaged a number of submarine mass transport features, including head scarps and slump complexes around the margin. Whilst final processing is not complete, initial processing of the images shows disrupted seafloor with irregular to chaotic reflections on parts of the seafloor. Potential faults are imaged on several profiles.

 

Person to contact: Katja Lindhorst

Participants and Partner Institutes (Seismic Crew):

 

Scientific Cruise:

  • May 2017: FS SONNE - SO257 WACHEIO: Western Australian Climate History from Eastern Indian Ocean Sediment Archives