INTRODUCTION

Ocean bottom seismometers (OBS) have been widely used during the last thirty years to collect seismic data for the determination of the structure of oceanic lithosphere and earthquake activity. Understanding the process of the creation of crust and lithosphere at mid-ocean ridges is a major goal of marine geophysics. Seismic refraction and reflection studies provide information on crustal structure. The distribution of earthquake activity in time and space contains information on the forces active in the spreading process. The achieve these important scientific goals, a number of research groups have developed ocean bottom seismometers (Ambuter and Solomon, 1974; Prothero, 1974, 1976,1979, 1981; Johnson et al., 1977; Mattabom and Solomon, 1977; Sutton et al., 1977; Avedik et al., 1978; Latham et al., 1978; Heffler and Varret, 1979; Kasahara et al., 1979; Koelsch and Purdy, 1979; Moore et al., 1981; Kirk et al., 1982; Whitmarsh and Lilwall, 1983). Recent developments in electronic devices and improvements in battery design have allowed the ocean bottom seismometer to become an operational tool. Considerable flexibility is provided by using low-power microprocessors. So the instrument complexity has shifted from hardware that is difficult to modify to software algorithms. This paper describes a new instrument which uses current electronics technology to achieve significant improvements in performance. This instrument is the first one of a planned network which will be used by the French scientific community.

Sensors : Seismometer Levelling

A seismometer measures the motion of the ground on which it is emplaced. A well-designed instrument will detect this motion with high fidelity and low noise over a broad range of amplitudes and frequencies. Data quality from previous experiments has often been disappointing because of poor signal quality and quite complicated. Many authors have dealt with it and proposed some solutions to eliminate both the noise and the cross-coupling problem which occurs between the vertical motion of the seafloor and the horizontal motion of the sensors (Duschenes et al., 1981; Sutton et al., 1981; Trehu and Solomon, 1981; Prothero and Eickemeyer, 1982; Trehu, 1985). The best way to reduce the mechanical noise is to separate the geophones from the main pressure vessel and to place them at a distance on the seafloor. This small pressure package must be designed to reduce the noise from all potential sources to acceptable level. The main instrument must also be designed not to generate too much noise. We have chosen a compact instrument with a large baseplate on the seafloor to avoid oscillations of the OBS induced by ocean currents near the seafloor (Kasahara et al., 1980; Duennebier et al., 1981; Kasahara et al., 1981).

Fig. 1 Schematic drawing of the geophones mounting/levelling arrangement (available in full paper)

Horizontal seismometers must be leveled to less than 2°, once the OBS is on the sea bottom, Figure 1 is a cutaway drawing of the deployed geophone package. The ground motion sensors are three orthogonal 2 Hz geophones. Sensors and preamplifiers are housed inside a titanium case. Gimbals and housed inside a titanium case.

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