S11D-01 INVITED
Seabed property estimation from ambient noise recordings
Processing and imaging of multi-component seismic data requires detailed knowledge of the shear velocity model of the subsurface. The near surface shear velocity is particular difficult to obtain using conventional seismic methods based on the analysis of shot generated data acquired in deep water. An analysis is presented of ambient noise recorded by an ocean bottom seismic cable developed for seismic exploration and monitoring. The data-set consists of 33 minutes of ambient noise recordings acquired in 270 m water depth. The ocean bottom seismic cable system has a hydrophone and three orthogonal geophones spaced at a 12.5 m interval over a total length of 5600 m. Analysis of the data revealed that below the frequency of 0.075 Hz the hydrophone data were found to be dominated by pressure variations due to sea surface waves. However, the corresponding seafloor compliance (the transfer function between pressure and velocity) was not observed on the vertical geophone component due to the limited 33 minutes recording length. Above 4 Hz the data were dominated by waterborne noise. The ambient noise wavefield at frequencies between 0.5 and 3 Hz was dominated by Scholte waves most likely generated by wave interaction. In addition Love waves were also observed but their excitation mechanism remains unexplained. Scholte and Love wave phase-velocities were picked in the frequency-wavenumber domain. In this domain the slowest velocity is apparent for inline propagation. Scholte wave phase-velocities were inverted for a near surface shear velocity model using a linearized inversion method. In an alternative approach, the spectral ratio of the vertical component and total ambient noise wavefield for the Scholte wave band was calculated and inverted for a near shear wave velocity model. This near surface model is in agreement with other published shallow shear velocity models and synthetics for this model fit well with the observed Scholte and Love wave phase velocity and the Scholte wave spectral ratio data.
S11D-02 INVITED
Ocean acoustic interferometry using noise and active sources
The Green's function between two receivers in the ocean is estimated by cross-correlating the signal recorded at each receiver. The signal source is either the ambient noise field, or a specific configuration of active sources. When using ambient noise fields it can be demonstrated that the Green's function emerges for a spatially distributed noise field by time-averaging of random sources. When using active sources, they should ideally surround the two receivers completely in order to get unbiased estimates; however, a source lowered vertically down the water column was used as this is experimentally less complex. Both the ambient and active source methods have been applied to experimental data obtained on horizontal bottom arrays and vertical line arrays. The recorded correlations have been used to determine travel paths between the receivers, for array localisation, and to determine ocean bottom properties. http://www.mpl.ucsd.edu/people/gerstoft/
S11D-03
The origin of deep ocean microseisms in the North Atlantic Ocean
Oceanic microseisms are small oscillations of the ground, in the frequency-range 0.05-0.3 Hz, associated with the occurrence of energetic ocean waves of half the corresponding frequency. In 1950 Longuet-Higgins suggested in a classical theoretical paper that microseisms originate from the interaction between oppositely- traveling components in the ocean wave spectrum. The theory provided an estimate of the magnitude of the corresponding microseisms in a compressible ocean, which has never been tested quantitatively. In this presentation we report explicit calculations of microseisms amplitudes making use of hindcast ocean wave spectra from the North Atlantic Ocean, and comparison of the calculations to seismic data collected at stations in North America, Greenland, and Iceland. We find that a particularly energetic source area stretches from the Labrador Sea to a region south of Iceland, where climatological conditions are conducive to generating oppositely traveling waves of same period, and where the ocean depth corresponds to an "organ-pipe" resonance of the compression waves generated by the opposing wave-wave interaction, as predicted by the theory. It is demonstrated that deep ocean nonlinear wave-wave interactions are sufficiently energetic to account for the observed seismic amplitudes in North America, Greenland and Iceland.
S11D-04
Correlation of Seismic Noise From an OBS Aarray off Costa Rica
The CRSEIZE experiment of 1999-2000 (DeShon and others, 2006), an array of 14 "ONR" OBSs were deployed in an array across the forearc of the subduction zone west of Costa Rica. The station spacing was about 15 km and the array was about 35km by 60km. Half the sensors were PMD broadband electrochemical seismometers and half were 1-Hz inertial sensors. Since the length of this deployment was about six months, the dataset offers an opportunity to exploit recently-developed techniques exploiting microseismic noise as a tool for structural analysis. See Shapiro and others, 2005, as a starting point. For this technique to work, there must be coherent observations of propagating seismic waves between pairs of stations. Practically, this means that the there must be some frequency ranges where propagating seismic signals are not contaminated by local non-propagating noise. Twelve-hour samples of field data, which were sampled at 64 Hz, were desampled to 1 Hz for this reconnaissance analysis. Data from the vertical sensor were whitened and cross-correlated, basically following Gerstoft and others, 2006. The resulting correlograms were noisier than the best data which is being shown from land sensors but there are recognizable arrivals, mostly at periods of around 7 and 3.3 seconds. No well-dispersed waveforms have been seen yet. At the present time, there are too few arrivals to identify unequivocally.
S11D-05
Seismological Evidence for Increasing Oceanic Storm Intensity
Several major tropical cylones during the 2007 hurricane season have generated wave-induced seismic signals detectable by seismic instrumentation in the Global Seismographic Network (GSN) and Advanced National Seismic System (ANSS) (ex. Flossy, Dean, Felix, Henrietta). From these storms, seismic background energy "noise" between 6 and 25 s period is dominated by a persistent "microseism" arising from energy transferred from ocean gravity waves to elastic Rayleigh waves. Microseism power spectral density (PSD) is dominated by a primary peak (10-20s) that is generated by waves breaking on coastlines and by a (much stronger) secondary peak centered near (5-10s) that is generated by the half-period periodic variation of sea bottom pressure due to standing wave components generated from wave-wave interaction of the ocean gravity wave field. The microseism peaks can vary in amplitude by several orders of magnitude due to station proximity to coastlines and wave amplitudes, which have a strong seasonal dependence. Such observations demonstrate the utility of microseisms as an integrative proxy for assessing long-term and regional scale sea swell changes induced by changes in global storm activity. We examine changes in the microseism amplitude and use it as a proxy for decadal-scale changes in storm-wave amplitude, a topic of considerable interest in the debate about the impact of global climate change on oceanic storm frequency and intensity. High-quality continuous digital records from the GSN and its precursor networks now extend back over 30 years at the longest-operational sites. In this abstract, we demonstrate the development of an oceanic storm trigger algorithm by observing the current storm season using data from the ANSS and GSN and then apply the resulting methods to an investigation of oceanic wave climate changes over three decades. Limited spatial distribution and length of long-term seismic observational records causes some ambiguity for climate change detection and analysis. Inferior instrumentation, inconsistent techniques, and incomplete data can lead to artificial trends and, consequently, to incorrect interpretations. An advantage of our technique is that it is independent of seismic data age, quality and completeness. We measure the change in microseism intensities over time by computing a measure called the microseism index (Grevemeyer et al., 2000) at 14 long operating GSN and precursor stations. The oldest station in our study, HG.ALQ, began operation in 1972 and is a precursor to IU.ANMO in Albuquerque, NM. For the period 1972-2007 we detect clear microseismic signal and demonstrate an increasing trend through time. The positive trend in microseism levels suggests that oceanic wave maximum heights have increased, and correlates with other metrics showing increasing surface sea and air temperatures and oceanic storminess.
S11D-06 INVITED
Five Years of Data at the Monterey Ocean Bottom Broadband Seismic Station (MOBB)
We present an overview of the results obtained at MOBB in the past 5.5 years of its continuous operation. In particular we focus on the observations of the long-period ocean surface gravity waves (infragravity waves; 0.002 to 0.05 Hz) and different methods to remove the long-period background and signal-generated noise from the seismic observations. MOBB was installed 40 km offshore in the Monterey Bay at a water depth of 1000 m in April 2002 in collaboration between Berkeley Seismological Laboratory and Monterey Bay Aquarium Research Institute (MBARI). It is located west of the San Gregorio Fault and represents the first step towards extending the onshore broadband seismic network in northern California westward of the Pacific-North America plate boundary. MOBB comprises a three- component broadband seismometer Guralp CMG-1T, sensitive over a wide frequency range, from 50 Hz to 2.8 mHz (360 s), a water current meter measuring current speed and direction, and a differential pressure gauge. At present, the station is autonomous and the data are on average retrieved every 4 months using MBARI's remotely operated vehicle Ventana. Work is under way to connect it to the MARS (Monterey Accelerated Research System) cable so that it will contribute continuous real time data to the northern California earthquake monitoring system. Lessons learned from the MOBB deployment as well as noise removal techniques that are specific to the ocean bottom installation will provide us reference for future installations of broadband seismic stations in the oceans. When compared to the quiet land stations, ocean bottom seismic station MOBB shows increased background noise in the band pass of interest for the study of regional and teleseismic signals. This is mainly due to deformation of the seafloor under the pressure forcing by infragravity waves. Also observed is additional signal- generated noise which is due to the reverberations in the shallow sedimentary layers as well as in the water layer. We present results of removing the long-period background noise from the seismic observations by subtracting the coherent signals derived from the pressure measurements. We also present results of the modeling of the signal-generated noise in the near surface layers and examples of removing the signal- generated noise through deconvolution. Comparison of the energy of the long-period noise (> 20 s) at MOBB to the energy of the short-period ocean waves recorded at the local buoys showed that the infragravity waves are generated locally, when the short-period ocean waves reach the shore. We have observed two types of modulation of the infragravity signal. First, the entire infragravity band signal is modulated in-phase with tides. Second, a longer-period modulation of the infragravity signal is observed and is best correlated with the energy of the 14 s period ocean waves. Understanding of the relation between the observed infragravity signal and local and distant sea conditions may contribute to unraveling the coupling mechanism responsible for the earth's low frequency hum.
S11D-07
Forcing by Infragravity Waves Interacting Over the Deep Ocean Basins is Important to Maintaining the Earth's Seismic Normal Mode Background (the 'Earth's Hum')
The recent discovery that the seismic normal modes of the Earth are excited to a nearly constant level during seismically quiet days (‘the hum of the Earth') has lead to much speculation as to what drives the observed background in the absence of large Earthquakes. Other authors have shown that the hum cannot be explained by the many small earthquakes occurring each day and have also precluded excitation by "slow" earthquakes as the source. The nonlinear interaction of low frequency ocean waves (infragravity waves) generates high phase velocity components that excite the seismic normal modes of the Earth. I show the expected forcing of Earth normal modes by infragravity waves interacting over the deep ocean basins is sufficient to explain the background seismic acceleration spectrum observed during seismically quiet days. Previously, I had shown that waves interacting over the continental shelves could also explain the background spectrum. It is likely that both pelagic and shelf ocean wave sources contribute significantly to the forcing of the Earth's hum and both coastal and pelagic source regions have been identified using seismic techniques. Infragravity wave spectral amplitudes in deep water over the Pacific basin are relatively constant, thus explaining the small observed variation in the hum. However, time varying source regions are also observed in the hum, and may be best explained by intense near shore sources. Observations suggesting enhanced spectral levels for two seismic modes that couple strongly with atmospheric modes (OS27 and OS39) have been cited as evidence that the source of the hum lies in the atmosphere. However, the wave interaction mechanism also couples energy into infrasound and thus may explain these observations. A calculation of the coupling between ocean waves and normal modes expected from interacting ocean waves reproduces the vertical acceleration spectrum observed at quiet seismic sites from 2 to 40 mHz including the small "hump" between 5 and 15 mHz and the observed rise at higher frequencies. The model diverges above 40 mHz because the ‘single frequency' microseism peak is generated by a different mechanism. The shape of the hum spectrum is controlled primarily by the elastic properties of the Earth with the attenuation structure controlling mode amplitudes.
S11D-08
Vector Acoustics, Vector Sensors, and 3D Underwater Imaging
Vector acoustic data has two more dimensions of information than pressure data and may allow for 3D underwater imaging with much less data than with hydrophone data. The vector acoustic sensors measures the particle motions due to passing sound waves and, in conjunction with a collocated hydrophone, the direction of travel of the sound waves. When using a controlled source with known source and sensor locations, the reflection points of the sound field can be determined with a simple trigonometric calculation. I demonstrate this concept with an experiment that used an accelerometer based vector acoustic sensor in a water tank with a short-pulse source and passive scattering targets. The sensor consists of a three-axis accelerometer and a matched hydrophone. The sound source was a standard transducer driven by a short 7 kHz pulse. The sensor was suspended in a fixed location and the hydrophone was moved about the tank by a robotic arm to insonify the tank from many locations. Several floats were placed in the tank as acoustic targets at diagonal ranges of approximately one meter. The accelerometer data show the direct source wave as well as the target scattered waves and reflections from the nearby water surface, tank bottom and sides. Without resorting to the usual methods of seismic imaging, which in this case is only two dimensional and relied entirely on the use of a synthetic source aperture, the two targets, the tank walls, the tank bottom, and the water surface were imaged. A directional ambiguity inherent to vector sensors is removed by using collocated hydrophone data. Although this experiment was in a very simple environment, it suggests that 3-D seismic surveys may be achieved with vector sensors using the same logistics as a 2-D survey that uses conventional hydrophones. This work was supported by the Office of Naval Research, program element 61153N.