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The mechanical behaviour of sediments is usually studied from laboratory core analyses which do not take into account the real state of in-situ stresses. Therefore, the aimed purpose is presently to determine the in-situ geotechnical properties which mainly depend on the relationship between fine-grained particles and intergranular fluids. At IFREMER, these achievements have been pursued in the framework of 2 scientific program: "Nodules and Sedimentary Processes and Instability on Slopes, for which surficial measurements were conducted on selected seafloor targets usually pre-defined from high resolution geophysical data." 2 kinds of instrumentation were developed in correlation to 2 different strategies of handling at sea. There is the instrumentation of the submersible "Le Nautile" equipped with a vane shear device, which enabled us to measure for the 1st time the in-situ undrained shear strength of pelagic sediments in the nodule environment. The possibility of accurately choosing on the seafloor the testing sites by means of a submersible, enable us to achieve this spectacular operation of ground-truthing. The other set of instrumentation is an autonomous equipped frame tethered by a cable. It is a supported rig which can be handled by any customary oceanographic vessel and is designed to penetrate the seafloor up to 2 m with a piezo-cone.

In July 1988, the first re-access to a deep sea borehole was performed using the logging shuttle NADIA 1 and the manned submersible NAUTILE during the cruise FARE. This operation was conducted on the site DSDP 396 B, drilled by the Glomar Challenger within the framework of the Deep Sea Drilling Project in 1976, close to the axis of the mid Atlantic Ridge at 23 degree N. A temperature logging hole and a fluid sampler was introduced in that hole and lowered to -300 m below the sea floor. It was the first time such an operation was done without a drill string and a drilling ship equipped with a dynamic positioning system. From this experience, a new logging shuttle, NADIA 2, was developed. NADIA 1 was designed with the objective of demonstrating the feasibility of the reoccupation of a deep sea borehole. NADIA 2 is a real logging unit, able to operate most of the logging tools used in the Ocean Drilling Programme. NADIA 2 can also be used as an ocean bottom observatory with an autonomy of weeks to a few months on the sea floor.