Observatoires sismiques sur le fond de la mer

Obtaining a detailed image of the Earth's interior is considered by the earth scientist as an important step towards comprehension of the functioning of the globe as a system. Although the first images were obtained by Wiechert, Gutenberg and others at the end of the XDCth century from earthquake modelling, it was not thought to extend seismic experiments and observations into marine areas until the late 1930's. Detecting and localizing nuclear blasts was an essential stimulus of the actions undertaken in the 1960's to em place seismological observatories on the seafloor. The plate-tectonic theory, in which the oceanic areas play a major role, greatly accelerated the evolution of modern oceanology. Since 1970, benefiting from scientific programs and the parallel development of microelectronics and computer science, the ocean bottom seismology has become a recognized discipline of geophysics, The contribution of Ifremer's department of Marine Geosciences, in helping to advance this area of research, has been significant since 1972. Our first system was designed in 1972 for work in shallow water (up to about 200 m depth) on the continental shelf, and was used to explore the deeper geological structure of the English Channel and the Western Approaches. The need for refraction studies to explore the deep structure of the margins and ocean-continent transition zones led us to design a deep-sea Ocean Bottom Seismograph (OBS) MK I in 1975, rated to 6 000 metres depth. This system introduced, for the first time, the concept of "isolated seismic sensors". This principle was adopted later on by other instrument designers. The OBS MK II was constructed in 1978, namely to study interferences between the instrument and the "isolated seismic sensors" as well as to test, under real operating conditions, an instrumental design which makes use of the "long inverted pendulum" concept. A series of four instruments were built, which also incorporated some innovative features in OBS technology, such as high density digital recording (Schlumberger 2000) permitting 130 hours of continuous recording for the instruments. A specialized instrument, the Ocean Bottom Vertical Seismic Array (OBVSA) was designed and built in 1984 primarily for lithospheric and tomographic investigations where low noise levels and separation of the wavefronts, obtained through the directional characteristics of the array, are important. The array, having multiple seismic channels, is emplaced vertically on the seafloor, up to maximum water depths of 6000 metres. A new system, the Ocean Bottom Seismic System (OBSS), was designed in August 1990 aiming at the construction of a variety of instruments from basic modules. This concept has been developed in response not only to the need for more and more elaborate seismic observations, but also to meet stringent operational and cost requirements.