Removing Infragravity-Wave-Induced Noise from Ocean-Bottom Seismographs (OBS) Data Deployed Offshore of Taiwan

Vertical ocean-bottom seismograph (OBS) data at frequencies below 0.05 Hz are contaminated by noise induced by infragravity waves. We constructed the transfer function between pressure and velocity data from OBSs deployed in Taiwan waters to remove the wave pressure-induced noise from seismic recordings. Data were analyzed from five portable broadband OBSs deployed each for 10 months at water depths from 1740 to 4600 m and from a cabled, shallow-buried seismograph (EOS1) installed on the seafloor at 300 m depth. Removing long-period noise from these OBS data improves the identification of teleseismic phases such as P, S, SS, Pdiff , and PKIKP that are otherwise ambiguous or unidentifiable. For EOS1, infragravity-wave signals completely mask the P and S waveforms in the 10–50 s period band suitable for centroid moment tensor (CMT) solutions for most of the local events. Application of the transfer functions to predict and remove wave deformation yielded clean prominent P and S waveforms at these periods and aided in the CMT determination for small events jointly with land stations. The relative amplitudes of the wave-number-normalized transfer function for some of the OBSs are mostly determined by the thickness of the sediment at the OBS site

High-Resolution Seafloor Absolute Pressure Gauge Measurements Using a Better Counting Method

Vibrating quartz force transducers are the critical component of most deep-sea pressure and depth gauges in use in oceanography, producing a frequency output that varies with pressure. Accurate and low drift pressure measurements can be obtained by precisely measuring this frequency. In most imple- mentations, the frequency is determined by counting the number of cycles of a high-frequency standard oscillator occurring during a large number of cycles of the lower-frequency quartz force oscillator. Res- olution is limited by the sampling interval (length of counting) and the frequency of the frequency stan- dard. Alternative counting methods can provide significant (20–40 dB) improvements in resolution at sampling rates above 1 Hz. Each counting method can be described as a different filter applied to the output of a counter of the frequency standard gated at each transition of the transducer quartz oscillator. Improvements in resolution can be understood as the result of minimizing the aliasing of higher-frequency counting noise into the spectrum below the Nyquist frequency. A simple multipole infinite impulse re- sponse (IIR) filter designed to limit spectral leakage of high-frequency noise minimizes the noise spectrum and thereby optimizes the resolution of the pressure output. The resultant noise spectrum rises as fre- quency squared above 1 Hz, independent of the sampling rate. At frequencies below 1 Hz, it is limited by noise in the electronics driving the force transducer quartz oscillator. Resolution increases with frequency of the frequency standard up to about 200 MHz, plateauing for higher frequencies due to other noise sources (likely electronic)

Determining the orientations of ocean bottom seismometers using ambient noise correlation

The cross-correlation of multicomponent ambient seismic noise can reveal both the velocity and polarization of surface waves propagating between pairs of stations. We explore this property to develop a novel method for determining the horizontal orientation of ocean bottom seismometers (OBS) by analyzing the polarization of Rayleigh waves retrieved from ambient noise cross-correlation. We demonstrate that the sensor orientations can be estimated through maximizing the correlation between the radial-vertical component and the phase-shifted vertical-vertical component of the empirical Green's tensor. We apply this new method to the ELSC (Eastern Lau Spreading Center) OBS experiment data set and illustrate its robustness by comparing the obtained orientations with results from a conventional method utilizing teleseismic P and Rayleigh wave polarizations. When applied to a large OBS array, the ambient noise method provides a larger number of orientation estimates and better azimuthal coverage than typically is possible with traditional methods

The effect of local wind on seismic noise near 1 Hz at the MELT site and in Iceland

The mantle electromagnetic and tomography (MELT) experiment on the east Pacific rise near 17°S was the first large teleseismic experiment on a midocean ridge. During the six-month deployment, no compressional arrivals were well recorded above 0.5 Hz. In comparison, the ICEMELT experiment in Iceland recorded compressional arrivals at 1-2 Hz from about 2 earthquakes per month. We compare noise spectra from the two experiments and show that this difference in detection is at least in part a result of noise. Near 1 Hz, seismic noise in the oceans is produced locally by wind-generated waves. At both experiment sites, 1-Hz noise levels are well correlated with local sea-surface-wind speeds derived from satellite observations. For a given wind speed, 1-Hz noise levels are about 10-20 dB lower in Iceland. At the MELT site, cross-correlations of wind speed with the logarithm of noise in a narrow-frequency band yield correlation coefficients exceeding 0.7 at frequencies between 0.4 Hz and 2 Hz. Noise levels at 1 Hz increase with wind by 1.3-1.4 dB per m/sec for wind speeds less than 10 m/sec. For the ICEMELT experiment, high correlation coefficients extend to markedly higher frequencies for coastal stations, and there is a 10-dB drop in 1-Hz noise levels 100-km inland. Noise levels increase by about 0.8 dB per m/sec. The strong correlation between wind speed and l-Hz seismic noise provides justification for using satellite wind speed data to search for locations on the global spreading system where there is a better probability of recording high-frequency arrivals. The calmest sites are found on the northern east Pacific rise, near the equator in all oceans, and near 34 ° N and 22 ° S on the mid- Atlantic ridge.This study was supported by the National Science Foundation under grant OCE-9414299.Peer Reviewe

Post Eruption inflation of the East Pacific Rise at 9°50′ N

In June 2008, we installed a geodetic network at 9°50′ N on the East Pacific Rise to track the long‐term movement of magma following the 2005/6 eruption. This network consists of 10 concrete benchmarks stretching from the ridge to 9 km off‐axis. During three campaign‐style surveys, measurements of vertical seafloor motions were made at each of these benchmarks by precisely recording ambient seawater pressure as a proxy for seafloor depth with a mobile pressure recorder (MPR). The MPR was deployed using the manned submersible Alvin in 2008 and 2009 and the remotely operated vehicle Jason in 2011. The MPR observations are supplemented with data from a multiyear deployment of continuously recording bottom pressure recorders (BPRs) extending along this segment of the ridge that can record rapid changes in seafloor depth from seafloor eruptions and/or dike intrusions. These measurements show no diking events and up to 12 cm of volcanic inflation that occurred from December 2009 to October 2011 in the area of the 2005/6 eruption. These observations are fit with an inflating point source at a depth of 2.7 km and volume change of 2.3 × 106 m3/yr located on the ridge axis at approximately 9°51.166′ N, 407 m from our northernmost benchmark, suggesting that the magma chamber underlying this segment of the ridge is being recharged from a deeper source at a rate that is about half the long‐term inflation rate observed at Axial Seamount on the Juan de Fuca Ridge. These data represent the second location that active volcanic uplift has been measured on a mid‐ocean ridge segment, and the first on a nonhotspot influenced segment

Seismological imaging of ridge–arc interaction beneath the Eastern Lau Spreading Center from OBS ambient noise tomography

The Lau Basin displays large along-strike variations in ridge characters with the changing proximity of the adjacent subduction zone. The mechanism governing these changes is not well understood but one hypotheses relates them to interaction between the arc and back-arc magmatic systems. We present a 3D seismic velocity model of the shallow mantle beneath the Eastern Lau back-arc Spreading Center (ELSC) and the adjacent Tofua volcanic arc obtained from ambient noise tomography of ocean bottom seismograph data. Our seismic images reveal an asymmetric upper mantle low velocity zone (LVZ) beneath the ELSC. Two major trends are present as the ridge-to-arc distance increases: (1) the LVZ becomes increasingly offset from the ridge to the north, where crust is thinner and the ridge less magmatically active; (2) the LVZ becomes increasingly connected to a sub-arc low velocity zone to the south. The separation of the ridge and arc low velocity zones is spatially coincident with the abrupt transition in crustal composition and ridge morphology. Our results present the first mantle imaging confirmation of a direct connection between crustal properties and uppermost mantle processes at ELSC, and support the prediction that as ELSC migrates away from the arc, a changing mantle wedge flow pattern leads to the separation of the arc and ridge melting regions. Slab-derived water is cutoff from the ridge, resulting in abrupt changes in crustal lava composition and crustal porosity. The larger offset between mantle melt supply and the ridge along the northern ELSC may reduce melt extraction efficiency along the ridge, further decreasing the melt budget and leading to the observed flat and faulted ridge morphology, thinner crust and the lack of an axial melt lens

Removing Infragravity‐Wave‐Induced Noise from Ocean‐Bottom Seismographs (OBS) Data Deployed Offshore of Taiwan

Vertical ocean‐bottom seismograph (OBS) data at frequencies below 0.05 Hz are contaminated by noise induced by infragravity waves. We constructed the transfer function between pressure and velocity data from OBSs deployed in Taiwan waters to remove the wave pressure‐induced noise from seismic recordings. Data were analyzed from five portable broadband OBSs deployed each for 10 months at water depths from 1740 to 4600 m and from a cabled, shallow‐buried seismograph (EOS1) installed on the seafloor at 300 m depth. Removing long‐period noise from these OBS data improves the identification of teleseismic phases such as P, S, SS, Pdiff, and PKIKP that are otherwise ambiguous or unidentifiable. For EOS1, infragravity‐wave signals completely mask the P and S waveforms in the 10–50 s period band suitable for centroid moment tensor (CMT) solutions for most of the local events. Application of the transfer functions to predict and remove wave deformation yielded clean prominent P and S waveforms at these periods and aided in the CMT determination for small events jointly with land stations. The relative amplitudes of the wavenumber‐normalized transfer function for some of the OBSs are mostly determined by the thickness of the sediment at the OBS site

Report of a workshop on technical approaches to construction of a seafloor geomagnetic observatory

This report considers the technical issues on sensors, data recording and transmission, control and timing, power, and packaging associated with constricting a seafloor geomagnetic observatory. Existing technologies either already in use for oceanographic purposes or adapted from terrestral geomagnetic observatories could be applied to measure the vector magnetic field components and absolute intensity with minimal development. The major technical challenge arises in measuring absolute direction on the seafloor because terrestral techniques are not transferrable to the deep ocean. Two solutions to this problem were identified. The first requires the development of an instrument which measures the instantaneous declination and inclination of the magnetic field relative to a north-seeking gyroscope and the local vertical. The second is a straightforward extension of a precision acoustic method for determining absolute position on the seafloor.Funding was provided by the National Science Foundation under grant EAR94-21712 and the National Aeronautics and Space Administration

Hydrothermal circulation at the Cleft-Vance overlapping spreading center : results of a magnetometric resistivity survey

We report on a magnetometric resistivity sounding carried out in the overlapping spreading center between the Cleft and Vance segments of the Juan de Fuca Ridge. The data collected reveal a strong three dimensionality in the crustal electrical resistivity structure on wavelengths of a few kilometers. Areas of reduced crustal electrical resistivities, with values approaching that of seawater, are seen beneath the neovolcanic zones of both active spreading centers. We interpret these reduced resistivities as evidence of active hydrothermal circulation within the uppermost 1 km of hot, young oceanic crust

Fundus-controlled perimetry (microperimetry): Application as outcome measure in clinical trials

YesFundus-controlled perimetry (FCP, also called 'microperimetry') allows for spatially-resolved mapping of visual sensitivity and measurement of fixation stability, both in clinical practice as well as research. The accurate spatial characterization of visual function enabled by FCP can provide insightful information about disease severity and progression not reflected by best-corrected visual acuity in a large range of disorders. This is especially important for monitoring of retinal diseases that initially spare the central retina in earlier disease stages. Improved intra- and inter-session retest-variability through fundus-tracking and precise point-wise follow-up examinations even in patients with unstable fixation represent key advantages of these technique. The design of disease-specific test patterns and protocols reduces the burden of extensive and time-consuming FCP testing, permitting a more meaningful and focused application. Recent developments also allow for photoreceptor-specific testing through implementation of dark-adapted chromatic and photopic testing. A detailed understanding of the variety of available devices and test settings is a key prerequisite for the design and optimization of FCP protocols in future natural history studies and clinical trials. Accordingly, this review describes the theoretical and technical background of FCP, its prior application in clinical and research settings, data that qualify the application of FCP as an outcome measure in clinical trials as well as ongoing and future developments