Images: Ocean Seismic Network Seafloor Observatories
This map shows 20 regions for locating seafloor seismic observatories in order to have 128 stations evenly spaced around the globe (red boxes). The six starred boxes have been selected as preliminary test sites. Yellow lines mark plate boundaries.
The borehole seismometer deployed at OSN-1 sits on its "teeter-totter" on
the fantail of R/V
Thompson (University of Washington) in Honolulu Harbor.
The autonomous seafloor data recording package for the borehole seismometer is deployed from R/V
Thompson at OSN-1. The system is designed to acquire three channels of seismic data at 20 samples per second for four months.
These seismic traces show the vertical vibration of the earth at Oahu (blue), OSN-1 (red), and southern California (green) caused by a magnitude 6.1 earthquake in Afghanistan on February 4, 1998. The earthquake event is the large amplitude arrival occurring in the middle of the traces. The left half of each trace shows the ambient noise prior to the event. In this figure the response at the OSN-1 observatory is better than at Oahu and comparable to Southern California.
This "ambient noise spectrum" shows the magnitude of the earthA?s vibration in the frequency band from 0.001 hertz to 10 hertz for the vertical (Z) and two horizontal (H1 and H2) components of the borehole seismometer at OSN-1. The black dashed lines show the range of typical levels for land stations. Around 0.1 hertz and below 0.01 hertz for the horizontals, the seafloor station is as noisy as the noisiest land stations. Between 0.01 and 0.1 hertz on the vertical component and all components near 10 hertz the seafloor station is as quiet as the quietest land stations
Procedure for installing the borehole seismic system in an existing borehole from a conventional, nondrilling, research vessel. The ship is maintained within a 10-meter watch circle by dynamic positioning and the Global Positioning System navigation. Acoustic transponders on the seafloor assist in locating the lead-in package relative to the re-entry cone. A camera and lights on the lead-in package at the bottom of the borehole seismometer also assist in locating the cone. Once re-entry is completed the system is lowered until the seafloor data recording package lands in the cone. The control vehicle has propulsion and additional navigational aids to manipulate the string near the seafloor. Once the system has been tested in place, the tether at the top of the recording package is released and the control vehicle returns to the surface. Recovery of the recording package is carried out with a grappling hook attached to the bottom of the control vehicle. At OSN-1 the water depth is 4,407 meters, the re-entry cone is 5 meters across, and the borehole seismometer was emplaced 248 meters below the seafloor.