A Self-Orienting Ocean Bottom Seismometer Package for Cabled-to-Shore Systems

A small boat deployable triaxial OBS package designed for deployment on oceanic slopes of up to 11 degrees has been developed and tested. The package is designed for studies on continental shelves to a depth of 200 meters. With a base to height ratio of 0.5 the 25 KG package (in water) self-orients a Mark Products L4C 1 HZ three component sensor mount with a 67 second orientation time constant using a silicon fluid damped couette coupler. The gimballed mount is oriented to within +/- 25 minutes of vertical within two hours after the package is stabilized on the ocean bottom thus accommodating any long term shifts in the oceanic sediments. A 12 kilometer single conductor ocean water return circuit sea cable supplies 600 milliwatts of power to the OBS electronics. A four channel seismic subcarrier frequency multiplex is combined with a single digital subcarrier each with a 25 Hz bandwidth is generated by the underwater system electronics. The frequency multiplex is introduced to the sea cable line driver where the signals are equalized to compensate for the cable´s high frequency roll-off. Two of these units were deployed in the Gulf of Pozzuoli, Italy this past June to fill a gap in a sensor array which is being used to monitor seismic activity occurring at Phelgraean Fields, an active volcanic area 1 Km north of Naples, Italy.