The first 36-metre core on board the Pourquoi Pas?

The REPREZAI expedition took place in January 2011. The objective was to study the cycles of the internal architecture of the Zaire turbidite system through the analysis of the migration of deposits (channel-lift systems) and the search for control factors. The CALYPSO1 corer collected 26 sediment samples at depths ranging from 4 011 to 5 178 metres.

The excellent quality of cores made at deep sites with the new Dyneema® cable on the Pourquoi Pas ? during the CABTEX test mission, was confirmed. All programmed coring operations were completed. Analysis of measurements using CINEMA2 software shows that the cores were barely distorted and that the position of sedimentary layers is practically undisturbed. Recovery rates were also very good, on average 90% for this expedition.

Because the geotechnical characteristics of the soil cored in the deep delta of Zaire show very low shear resistance, coring with a tube of 36 metres was attempted. The manoeuvre to launch and recover this type of corer is a delicate task but thanks to the crew of the Pourquoi Pas ? the operation was completed safely and successfully. This is the first 36-metre core made on board the Pourquoi Pas ? and it bodes well for further long cores.

The recovery rate of this core, made in 4 963 metres of water, is 92% for an effective core length of 32.80 metres.

The kinematics of the coring operation plotted by the CINEMA software from the measurements made on the ballast and the trip plate show that the operation is nominal and close to the optimum until the end of the sampling phase. The corer’s piston is stationary for 80% of the time. Only a slight subsampling phase is observed at mid penetration of the corer. The calculation of the position of the sedimentary layers in the core shows that they are at most 0.40 m out of step with lithology.

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1 The CALYPSO is a giant Kullenberg-type plunger corer, developed by Y. Balut (IFRTP) on board the N/O Marion Dufresne. It can reach a length of 60 m.

2 CINEMA : Software for the modeling of the elastic recalculation of  core cables to understand and anticipate the adjustments of corers to ensure the stationarity of the piston, sine qua non condition of an optimal quality of sample.