A leveling system for an ocean-bottom seismometer

As part of a new broad-band, ocean-bottom seismometer (OBS) system that has been developed at the Woods Hole Oceanographic Institution, a gimbaled leveling system was designed and built. The goal of the broad-band system is to measure ocean-bottom vibrations from a period of 120 seconds up to 20 Hertz. During system deployment, a sphere containing the seismic sensor is dropped into sediment on the ocean bottom. Seismic sensors need to be leveled before use, and it is not practical to accurately control the attitude of the sphere as it settles on the ocean bottom. The sphere holds the seismic sensor in gimbals whose axes have brakes. The gimbal axes have brakes to prevent tilting in response to slow horizontal accelerations that would complicate long-period seismometer response. To level the seismic sensor, the brakes are released, the righting moment of the seismic sensor in the gimbals levels the seismometer attitude, and the brakes are reasserted. The brake systems were designed to have zero play and had to be modified to raise the lowest system natural frequency above 20 Hertz. This paper describes the mechanical aspects of the system and the modifications needed to push up the mechanical resonances. Twenty-five of the OBS systems have been built and deployed off Hawaii as part of the Plume-Lithosphere Undersea Melt Experiment (PLUME)