OSS IV: Noise Levels, Signal-to-Noise Ratios, and Noise Sources

Duennebier, F. K.; McCreery, Charles S.; Harris, D. Cessaro; Fisher, R K; Anderson, Colin

doi:10.2973/dsdp.proc.88.107.1987

Cited by 7 publications

(5 citation statements)

References 9 publications

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“…At the microseism peak (0.25 Hz) the borehole sensor can actually be up to 6 dB quieter. The OSN-1 observations are similar to the OSS-IV observations in the band 2-40 Hz acquired in a similar geological environment in the northwest-Pacific [Duennebier et al, 1987a].…”

Section: ] Statessupporting

confidence: 80%

“…Observations and models of ambient noise near the seafloor indicated a preponderance of interface waves, perhaps generated by scattering from seafloor heterogeneities [ Bradley , 1994; Bradner et al , 1965; Dougherty and Stephen , 1988; Duennebier et al , 1987a; Latham and Nowroozi , 1968; Latham and Sutton , 1966; Orcutt et al , 1993a; Orcutt et al , 1993b; Schreiner and Dorman , 1990; Webb , 1992]. Also large eddies in the ocean near the seafloor induce pressure fluctuations which tilt the seafloor and generate a seismic signal at frequencies below a few Hertz [ Webb , 1988].…”

Section: Introductionmentioning

confidence: 99%

“…In shallow water studies, experiments had shown that burying the seismometer reduced ambient noise levels [ Duennebier et al , 1991; Sutton and Duennebier , 1988; Trevorrow et al , 1989a; Trevorrow et al , 1989b]. In deep water, a number of studies had also observed that ambient noise at frequencies above the microseism peak was consistently quieter for both vertical and horizontal sensors in boreholes than at the seafloor [ Adair et al , 1984; Bradley , 1994; Bradley et al , 1997; Duennebier et al , 1987a; Hedlin and Orcutt , 1989; Stephen et al , 1994]. In one study where the VLF noise was not quieter in the borehole than on the seafloor, the sensor had been clamped at only 190m depth into soft sediment [ Carter et al , 1984].…”

Section: Introductionmentioning

confidence: 99%

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Ocean Seismic Network Pilot Experiment

Stephen

Spiess

Collins

et al. 2003

Geochem Geophys Geosyst

104

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[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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Section: ] Statessupporting

confidence: 80%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Ocean Seismic Network Pilot Experiment

Stephen

Spiess

Collins

et al. 2003

Geochem Geophys Geosyst

104

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“…The 48‐hr smoothed median ground velocities on each instrument were fit with a model as follows:

m_{t} = a + ((), b_{1} + c_{1} w_{t}) + e {v_{t}}^{2} + f {s_{t}}^{2} for w_{t} < normalB

m_{t} = a + ((), b_{2} + c_{2} w_{t}) + e {v_{t}}^{2} + f {s_{t}}^{2} for w_{t} \geq normalB

where m is the modeled noise ground velocity, t an index of discretized time, w is wind speed, v is maximum horizontal wave velocity 1 m above the seafloor, s is predicted bottom current, B is the windspeed at the breakpoint for a piecewise linear function with two segments, and the other terms on the right‐hand side ( a , b 1 , b 2 , c 1 , c 2 , e , and f ) are constants solved for by regression. In this model, wind speeds are fit with a piecewise function to account for the noise floor observed at lower windspeeds (Duennebier et al, 1987; McCreery et al, 1993), and noise is dependent on the square of wave velocities and bottom current velocities to match the relationship of Trehu (1985). We solved the regression in two steps.…”

Section: Methodsmentioning

confidence: 99%

“…At higher frequencies of 4–30 Hz, the same noise floor is also observed at low wind speeds. As the wind speed increases above a threshold of ~8 m/s, noise levels rise with no observed ceiling which is thought to be due to acoustic noise generated by the spray blowing off whitecaps with a second source near land from surf breaking against the coast (Duennebier et al, 1987; McCreery et al, 1993). Bottom currents are also an important noise source on vertical and horizontal channels.…”

Section: Introductionmentioning

confidence: 99%

Physical Sources of High‐Frequency Seismic Noise on Cascadia Initiative Ocean Bottom Seismometers

Hilmo

Wilcock

2020

Geochem Geophys Geosyst

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Physical sources of high-frequency seismic noise in the ocean are investigated using data from the Cascadia Initiative (CI) ocean bottom seismometer (OBS) network, hindcasts of wind speed, waves, and the bottom currents predicted by a regional ocean circulation model and observed at sites on cabled observatories. Seismic data in the 5-12 Hz band are considered because it is best for detecting regional earthquakes and lies between the frequencies of local microseisms and the seasonal whale calls. Median noise levels in this range vary by~20 dB between sites at a given depth but on average decrease with increasing depth. On the continental shelf, the orbital motions of ocean waves are a major source of noise while at the quietest sites in the deep ocean, noise increases when wind speeds exceed~10 m/s. On the continental slope and abyssal plain within about 100 km of the slope, seismic noise is not predicted at specific sites by the bottom currents in the ocean circulation model. In these regions, ocean currents are inferred to be the primary source of noise, because noise varies on tidal periods, is low on buried seismometers, and has spatial variations broadly consistent with those of median absolute currents. Comparisons between OBSs suggest that high-frequency noise is reduced by low-profile hydrodynamic designs but not by shielding. Many OBSs also record numerous short duration events on and near the continental shelf that have been attributed elsewhere to animals bumping into the sensor or gas bubbles moving through sediments. Plain Language Summary Environmental noise generated by waves, wind, and ocean currents limits the ability of seafloor seismometers to detect the ground motions from earthquakes. This research uses data from a large deployment of seafloor seismometers off the coast of the Pacific Northwest of America to understand the seismic noise at frequencies of 5-12 Hz which coincides with the frequency band used to detect regional earthquakes. Instruments deployed in shallow water on the continental shelf record high noise levels caused by motions from ocean waves that reach the seafloor. The quietest sites are found in the deep ocean far from the coast, and here the noise is created by wind blowing spray from whitecaps. At intermediate sites on the continental slope and deep ocean just seaward of the continental slope, the noise results from ocean currents that are related to ocean circulation and tides. Some shallow instruments also record large numbers of short signals that are not earthquakes and may be either a result of animals bumping into the seismic sensor or gas bubbles moving through sediments. The best way to reduce seismic noise is to bury the seismic sensors, but when this is infeasible, the instruments should have a short hydrodynamic design.

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On the spectra shape of seismic noise and earthquakes recorded in the ocean

Ostrovsky¹

1990

Physics of the Earth and Planetary Interiors

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OSS IV: Noise Levels, Signal-to-Noise Ratios, and Noise Sources

Cited by 7 publications

References 9 publications

Ocean Seismic Network Pilot Experiment

Ocean Seismic Network Pilot Experiment

Physical Sources of High‐Frequency Seismic Noise on Cascadia Initiative Ocean Bottom Seismometers

On the spectra shape of seismic noise and earthquakes recorded in the ocean

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