Ocean Seismic Network Pilot Experiment

Abstract

[1] The primary goal of the Ocean Seismic Network Pilot Experiment (OSNPE) was to learn how to make high quality broadband seismic measurements on the ocean bottom in preparation for a permanent ocean seismic network. The experiment also had implications for the development of a capability for temporary (e.g., 1 year duration) seismic experiments on the ocean floor. Equipment for installing, operating and monitoring borehole observatories in the deep sea was also tested including a lead-in package, a logging probe, a wire line packer and a control vehicle. The control vehicle was used in three modes during the experiment: for observation of seafloor features and equipment, for equipment launch and recovery, and for power supply and telemetry between ocean bottom units and the ship. The OSNPE which was completed in June 1998 acquired almost four months of continuous data and it demonstrated clearly that a combination of shallow buried and borehole broadband sensors could provide comparable quality data to broadband seismic installations on islands and continents. Burial in soft mud appears to be adequate at frequencies below the microseism peak. Although the borehole sensor was subject to installation noise at low frequencies (0.6 to 50 mHz), analysis of the OSNPE data provides new insights into our understanding of ocean bottom ambient noise. The OSNPE results clearly demonstrate the importance of sediment borne shear modes in ocean bottom ambient noise behavior. Ambient noise drops significantly at high frequencies for a sensor placed just at the sediment basalt interface. At frequencies above the microseism peak, there are two reasons that ocean bottom stations have been generally regarded as noisier than island or land stations: ocean bottom stations are closer to the noise source (the surface gravity waves) and most ocean bottom stations to date have been installed on low rigidity sediments where they are subject to the effects of shear wave resonances. When sensors are placed in boreholes in basement the performance of ocean bottom seismic stations approaches that of continental and island stations. A broadband borehole seismic station should be included in any real-time ocean bottom observatory.

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“…Such an observation system has been developed for logging (Amitani et al, 2002) but is not used routinely in boreholes. Installation of cabled sensors into a borehole by controlled wireline has also been achieved (Stephen et al, 2003). In this case, the downhole cable was payed out under the seafloor station before reentry.…”

Eos, Transactions, American Geophysical Union Volume 87, Number 31, 1 August 2006

“…Sonobuoys [Reid et al, 1973] have had success in the past in recording local earthquake signals, but they have been too noisy to provide an acceptable signal-to-noise ratio for all but the strongest earthquakes [Kebe, 1981;Cotaras et al, 1988]. Ocean bottom seismometers (OBS) [Zhao et al, 1997;Laske et al, 1999;Stephen et al, 2003] and moored hydrophones [Smith et al, 2004] are capable of addressing the coverage gap, but they are expensive to manufacture (~US$50,000 for a three-component OBS) and deploy (~$20,000 per day of ship time). Unable to communicate their recordings remotely without prohibitively expensive cabling, stationary underwater devices have to be retrieved at regular intervals for the data to be analyzed in the computer lab.…”

A future for drifting seismic networks

“…Sonobuoys [Reid et al, 1973] have had suc cess in the past in recording local earth quake signals, but they have been too noisy to provide an acceptable signal-to-noise ratio for all but the strongest earthquakes [Kebe, 1981;Cotaras et al, 1988].Ocean bottom seismometers (OBS) [Zhao et al, 1997;Laske et al, 1999;Stephen et al, 2003] and moored hydrophones [Smith etal.,2004] are capable of addressing the coverage gap, but they are expensive to manufacture (~US$50,000 for a three-component OBS) and deploy (-$20,000 per day of ship time). Unable to communicate their recordings remotely without prohibitively expensive cabling, stationary underwater devices have to be retrieved at regular intervals for the data to be analyzed in the computer lab.…”