Broadband seismometry beneath the seafloor

The recently completed Ocean Seismic Network Pilot Experiment (OSNPE) at a site southwest of Oahu, Hawaii showed that high-quality broadband (0.001-10 Hz) seismic data can be acquired in the deep oceans using portable seismographs. The OSNPE showed that surficial burial of the seismometer sensor in the seabed resulted in dramatically superior long-period (<0.1 Hz) performance relative to an identical sensor sitting on the seafloor. The custom-built burial system used on the OSNPE relied on the controlled application of a large mass to push the seismometer into the seafloor. The physical dimensions of the pressure case and the shear strength of the seabed sediments controlled the magnitude of the burial force. Instrument recovery was accomplished with the use of an ROV. An improved burial system and buriable seismometer are currently being designed. It will utilize a broadband borehole seismometer of much smaller diameter resulting in a substantial reduction in required burial force. The new burial system will eliminate the need for ROV recovery.