Ambient Wavefield Research Articles

Observations from the seafloor: ultra-low-frequency ambient ocean-bottom nodal seismology at the Amendment field

SUMMARY Large-scale ocean-bottom node (OBN) arrays of 1000s of multicomponent instruments deployed over 1000s of square kilometres have been used successfully for active-source seismic exploration activities including full waveform inversion (FWI) at exploration frequencies above about 2.0 Hz. The analysis of concurrently recorded lower-frequency ambient wavefield data, though, is only just beginning. A key long-term objective of such ambient wavefield analyses is to exploit the sensitivity of sub-2.0 Hz energy to build long-wavelength initial elastic models, thus facilitating FWI applications. However, doing so requires a more detailed understanding of ambient wavefield information recorded on the seafloor, the types, frequency structure and effective source distribution of recorded surface-wave modes, the near-seafloor elastic model structure, and the sensitivity of recorded wave modes to subsurface model structure. To this end, we present a wavefield analysis of low- and ultra-low-frequency ambient data (defined as <1.0 and <0.1 Hz, respectively) acquired on 2712 OBN stations in the Amendment Phase 1 survey covering 2750 km2 of the Gulf of Mexico. After applying ambient data conditioning prior to cross-correlation and seismic cross-coherence interferometry workflows, we demonstrate that the resulting virtual shot gather (VSG) volumes contain evidence for surface-wave and guided P-wave mode propagation between the 0.01 and 1.0 Hz that remains coherent to distances of at least 80 km. Evidence for surface-wave scattering from near-surface salt-body structure between 0.35 and 0.85 Hz is also present in a wide spatial distribution of VSG data. Finally, the interferometric VSG volumes clearly show waveform repetition at 20 s intervals in sub-0.3 Hz surface-wave arrivals, a periodicity consistent with the mean active-source shot interval. This suggests that the dominant contribution of surface-wave energy acquired in this VSG frequency band is likely predominantly related to air-gun excitation rather than by naturally occurring energy sources. Overall, these observations may have important consequences for the early stages of initial model building for elastic FWI analysis.

Examining 22 Years of Ambient Seismic Wavefield at Mount St. Helens

Abstract An increase in seismic activity precedes most volcanic eruptions. Whereas event-based forecasting approaches have been successful, some eruptions remain unanticipated, resulting in casualties and damage. Our study leverages the recent advancements in ambient field seismology. We explore features extracted from continuous ambient fields using traditional methods, for example, peak ground velocity, peak ground acceleration, root mean square, root median square, real-time seismic amplitude measurement, and novel methods (displacement seismic amplitude ratio and spectral width). In addition, we explore unsupervised learning of higher order wavelet features using scattering networks. We find that combining all the methods was necessary to disentangle the effects of seismic sources from structural changes at Mount St. Helens. Although the ambient wavefield-based approach does not yield additional or more significant precursory signals than event-based methods at Mount St. Helens, our study demonstrates that the ambient wavefield provides supplementary information, mainly about structural changes and complements traditional methods. The ambient seismic wavefield offers additional insights into long-lasting processes. We find enhanced wave attenuation correlating with geochemical measurements. We interpret this as ongoing structural changes, such as dome growth or the evolution of the volcanic conduit system. On annual and decadal timescales, we interpret seasonal seismic attenuation in the shallow subsurface as groundwater fluctuations, corroborated by observations at the nearby Spirit Lake level. This multimethod approach at Mount St. Helens sheds light on a volcanic system’s underlying dynamics and structure.

Detecting ice on plate-like structures via flow-induced random vibration: a dispersion curve shift identification approach

This paper proposes an ice detection framework for thin plate structures using flow-induced random guided waves measured by two passive sensors. Its principle is based on the fact that the ice accretion shifts the dispersion curve of guided wave, which can be extracted from the cross-correlation of ambient wave fields at two receivers. More specifically, the group velocity under various frequencies is identified from the prominent peak of the cross-correlation with a band-pass filter, which signifies the travel time of a narrow-band guided wave, and this forms a reconstruction of the dispersion curve. The ice thickness is then estimated by minimizing the weighted error between the reconstructed dispersion curve and its theoretical model solved from the Rayleigh–Lamb equations for the two-layer (ice-plate) structure. The weights are assigned to various frequencies according to the global sensitivity of the group velocity versus the ice accretion. The proposed method is assessed by a laboratory experiment where the ice accretion on an aluminum plate is carried out in an air-cooling environment and the random vibration is excited by spraying air jet onto the plate surface. Experimental results show that the ice thickness can be accurately estimated if it is of the same order or even lower than the plate thickness. The proposed method has the potential to realize the long-range real-time detection of aircraft icing conditions where the passive sensors can be tiny, light, free of energy supply, and mounted on the internal surface of fuselage skin without any effect on aircraft aerodynamics.

Towards the systematic reconnaissance of seismic signals from glaciers and ice sheets – Part 2: Unsupervised learning for source process characterization

Abstract. Given the high number and diversity of events in a typical cryoseismic dataset, in particular those recorded on ice sheet margins, it is desirable to use a semi-automated method of grouping similar events for reconnaissance and ongoing analysis. We present a workflow for employing semi-unsupervised cluster analysis to inform investigations of the processes occurring in glaciers and ice sheets. In this demonstration study, we make use of a seismic event catalogue previously compiled for the Whillans Ice Stream, for the 2010–2011 austral summer (outlined in Part 1, Latto et al., 2024). We address the challenges of seismic event analysis for a complex wave field by clustering similar seismic events into groups using characteristic temporal, spectral, and polarization attributes of seismic time series with the k-means++ algorithm. This provides the basis for a reconnaissance analysis of a seismic wave field that contains local events (from the ice stream) set in an ambient wave field that itself contains a diversity of signals (mostly from the Ross Ice Shelf). As one result, we find that two clusters include stick-slip events that diverge in terms of length and initiation locality (i.e., central sticky spot and/or the grounding line). We also identify a swarm of high-frequency signals on 16–17 January 2011 that are potentially associated with a surface melt event from the Ross Ice Shelf. Used together with the event detection presented in Part 1, the semi-automated workflow could readily be generalized to other locations and, as a possible benchmark procedure, could enable the monitoring of remote glaciers over time and comparisons between locations.

Urban subsurface exploration improved by denoising of virtual shot gathers from distributed acoustic sensing ambient noise

SUMMARY Ambient noise tomography on the basis of distributed acoustic sensing (DAS) deployed on existing telecommunication networks provides an opportunity to image the urban subsurface at regional scales and high-resolution. This capability has important implications in the assessment of the urban subsurface’s potential for sustainable and safe utilization, such as geothermal development. However, extracting coherent seismic signals from the DAS ambient wavefield in urban environments at low cost remains a challenge. One obstacle is the presence of complex sources of noise in urban environments, which may not be homogeneously distributed. Consequently, long recordings are required for the calculation of high-quality virtual shot gathers, which necessitates significant time and computational cost. In this paper, we present the analysis of 15 d of DAS data recorded on a pre-existing fibre optic cable (dark fibres), running along an 11-km-long major road in urban Berlin (Germany), hosting heavy traffic including vehicles and trains. To retrieve virtual shot gathers, we apply interferometric analysis based on the cross-correlation approach where we exclude low-quality virtual shot gathers to increase the signal-to-noise ratio of the stacked gathers. Moreover, we modify the conventional ambient noise interferometry workflow by incorporating a coherence-based enhancement approach designed for wavefield data recorded with large-N arrays. We then conduct multichannel analysis of surface waves to retrieve 1-D velocity models for two exemplary fibre subsegments, and compare the results of the conventional and modified workflows. The resulting 1-D velocity models correspond well with available lithology information. The modified workflow yields improved dispersion spectra, particularly in the low-frequency band (<1 Hz) of the signal. This leads to an increased investigation depth along with lower uncertainties in the inversion result. Additionally, these improved results were achieved using significantly less data than required using conventional approaches, thus opening the opportunity for shortening required acquisition times and accordingly lowering costs.

3-D body-wave tomography from the seismic ambient noise recorded by a dense array in the Dehdasht area, Iran

SUMMARY The strong attenuation of the thick sedimentary layers in the Dehdasht region, Iran, renders active seismic exploration surveys difficult. The imaging of the existent limestone reservoirs is limited to the shallow subsurface due to the strong attenuation of seismic waves. Here, we discuss a different approach to imaging the subsurface using body waves extracted from the cross-correlation of the seismic ambient wavefield. We discuss the technical challenges to extracting clear P-wave arrivals from the seismic ambient wavefield of a dense 3-component seismic array deployed in the Dehdasht basin. We invert the data for the 3-D P-wave velocity structure and compare the velocity model with results from the 2-D active seismic surveys in the area. The results show the potential of using body waves extracted from the seismic ambient wavefield for imaging purposes in highly attenuating areas.

Fiber-Optic Observation of Volcanic Tremor through Floating Ice Sheet Resonance

AbstractEntirely covered by the Vatnajökull ice cap, Grímsvötn is among Iceland’s largest and most hazardous volcanoes. Here we demonstrate that fiber-optic sensing technology deployed on a natural floating ice resonator can detect volcanic tremor at the level of few nanostrain/s, thereby enabling a new mode of subglacial volcano monitoring under harsh conditions. A 12.5 km long fiber-optic cable deployed on Grímsvötn in May 2021 revealed a high level of local earthquake activity, superimposed onto nearly monochromatic oscillations of the caldera. High data quality combined with dense spatial sampling identify these oscillations as flexural gravity wave resonance of the ice sheet that floats atop a subglacial lake. Although being affected by the ambient wavefield, the time–frequency characteristics of observed caldera resonance require the presence of an additional persistent driving force with temporal variations over several days, that is most plausibly explained in terms of low-frequency volcanic tremor. In addition to demonstrating the logistical feasibility of installing a large, high-quality fiber-optic sensing network in a subarctic environment, our experiment shows that ice sheet resonance may act as a natural amplifier of otherwise undetectable (volcanic) signals. This suggests that similar resonators might be used in a targeted fashion to improve monitoring of ice-covered volcanic systems.

The Effect of Porous Media on Wave-Induced Sloshing in a Floating Tank

Placing porous media in a water tank can change the dynamic characteristics of the sloshing fluid. Its extra damping effect can mitigate sloshing and, thereby, protect the integrity of a liquefied natural gas tank. In addition, the out-of-phase sloshing force enables the water tank to serve as a dynamic vibration absorber for floating structures in the ocean environment. The influence of porous media on wave-induced sloshing fluid in a floating tank and the associated interaction with the substructure in the ambient wave field are the focus of this study. The numerical coupling algorithm includes the potential-based Eulerian–Lagrangian method for fluid simulation and the Newmark time-integration method for rigid-body dynamics. An equivalent mechanical model for the sloshing fluid in a rectangular tank subject to pitch motion is proposed and validated. In this approach, the degrees of freedom modeling of the sloshing fluid can be reduced so the numerical computation is fast and inexpensive. The results of the linear mechanical model and the nonlinear Eulerian–Lagrangian method are correlated. The dynamic interaction between the sloshing fluid and floating body is characterized. The effectiveness of the added porous media in controlling the vibration and mitigating the sloshing response is confirmed through frequency response analysis.

Seismic Monitoring of Permafrost in Svalbard, Arctic Norway

AbstractWe analyze data from passive and active seismic experiments conducted in the Adventdalen valley of Svalbard in the Norwegian Arctic. Our objective is to characterize the ambient wavefield of the region and to investigate permafrost dynamics through estimates of seismic velocity variations. We are motivated by a need for early geophysical detection of potentially hazardous changes to permafrost stability. We draw upon several data sources to constrain various aspects of seismic wave propagation in Adventdalen. We use f-k analysis of five years of continuous data from the Spitsbergen seismic array (SPITS) to demonstrate that ambient seismic noise on Svalbard consists of continuously present body waves and intermittent surface waves appearing at regular intervals. A change in wavefield direction accompanies the sudden onset of surface waves when the average temperature rises above the freezing point, suggesting a cryogenic origin. This hypothesis is supported further by our analysis of records from a temporary broadband network, which indicates that the background wavefield is dominated by icequakes. Synthetic Green’s functions calculated from a 3D velocity model match well with empirical Green’s functions constructed from the recorded ambient seismic noise. We use a shallow shear-wave velocity model, obtained from active seismic measurements, to estimate the maximum depth of Rayleigh wave sensitivity to changes in shear velocity to be in the 50–100 m range. We extract seasonal variations in seismic velocities from ambient noise cross-correlation functions computed over three years of SPITS data. We attribute relative velocity variations to changes in the ice content of the shallow (2–4 m depth) permafrost, which is sensitive to seasonal temperature changes. A linear decreasing trend in seismic velocity is observed over the years, most likely due to permafrost warming.

Spatial autocorrelation method for reliable measurements of two-station dispersion curves in heterogeneous ambient noise wavefields

SUMMARYThe microtremor survey method (MSM) is used to estimate S-wave velocity profiles from microtremors or ambient noise. Although array-based MSM analyses are usually used for shallow exploration purposes because of their robustness, the extraction of numerous phase-velocity dispersion curves by two-station microtremor analysis is attractive because those dispersion curves can be used to construct high-resolution phase-velocity maps by solving a least-squares problem. However, in exploration studies (>1 Hz), the reliability of two-station microtremor analysis can be affected by short data acquisition times and heterogeneous noise distributions mainly caused by anthropogenic noises. In this study, we propose a new approach to estimate surface wave dispersion curves between station pairs considering a heterogeneous ambient noise distribution based on the spatial autocorrelation method. We first estimated azimuthal variations of noise energy from the complex coherencies between all station pairs in a receiver array, and then estimated dispersion curves between station pairs. Our field example demonstrates that modelling the azimuthal noise energy distribution allows us to use not only the real parts of complex coherencies, but also the imaginary parts, which are usually neglected when assuming a homogeneous noise field. The simultaneous use of the real and imaginary parts of complex coherencies improves the reliability and continuity of phase-velocity estimations between station pairs. Because the stability of phase-velocity estimations depends on the azimuths between station pairs, we carefully selected between-station azimuths that produce stable phase velocities. Selected phase velocities at 8 Hz can be used to construct high-resolution phase-velocity maps with least-squares inversion. Because our approach does not require a regular receiver interval for two-station analysis, it allows for more flexible seismic array geometries. This is particularly important for MSM analyses in urban areas, where limited space is available to install seismic stations. We conclude that our proposed approach is effective in reconstructing high-resolution shallow structures in heterogeneous ambient noise fields.

The origin of secondary microseism Love waves

The interaction of ocean surface waves produces pressure fluctuations at the seafloor capable of generating seismic waves in the solid Earth. The accepted mechanism satisfactorily explains secondary microseisms of the Rayleigh type, but it does not justify the presence of transversely polarized Love waves, nevertheless widely observed. An explanation for two-thirds of the worldwide ambient wave field has been wanting for over a century. Using numerical simulations of global-scale seismic wave propagation at unprecedented high frequency, here we explain the origin of secondary microseism Love waves. A small fraction of those is generated by boundary force-splitting at bathymetric inclines, but the majority is generated by the interaction of the seismic wave field with three-dimensional heterogeneity within the Earth. We present evidence for an ergodic model that explains observed seismic wave partitioning, a requirement for full-wave field ambient-noise tomography to account for realistic source distributions.

Teleseismic earthquake wavefields observed on the Ross Ice Shelf

AbstractObservations of teleseismic earthquakes using broadband seismometers on the Ross Ice Shelf (RIS) must contend with environmental and structural processes that do not exist for land-sited seismometers. Important considerations are: (1) a broadband, multi-mode ambient wavefield excited by ocean gravity wave interactions with the ice shelf; (2) body wave reverberations produced by seismic impedance contrasts at the ice/water and water/seafloor interfaces and (3) decoupling of the solid Earth horizontal wavefield by the sub-shelf water column. We analyze seasonal and geographic variations in signal-to-noise ratios for teleseismic P-wave (0.5–2.0 s), S-wave (10–15 s) and surface wave (13–25 s) arrivals relative to the RIS noise field. We use ice and water layer reverberations generated by teleseismic P-waves to accurately estimate the sub-station thicknesses of these layers. We present observations consistent with the theoretically predicted transition of the water column from compressible to incompressible mechanics, relevant for vertically incident solid Earth waves with periods longer than 3 s. Finally, we observe symmetric-mode Lamb waves generated by teleseismic S-waves incident on the grounding zones. Despite their complexity, we conclude that teleseismic coda can be utilized for passive imaging of sub-shelf Earth structure, although longer deployments relative to conventional land-sited seismometers will be necessary to acquire adequate data.

On PSI Interactions in Internal Gravity Wave Fields and the Decay of Baroclinic Tides

AbstractBarotropic tidal oscillations over seafloor topography generate baroclinic tides that may be damped in turn via nonlinear triad interactions with internal gravity waves, fueling the ambient wave field. We derive the kinetic equations for this tidal damping and the energy transfer to the ambient wave field and compute damping times and energy transfer rates for the M2 tide and a Garrett–Munk-like ambient wave field. We show that parametric subharmonic instability (PSI) interactions are important, where the tide interacts resonantly with two background waves, each of half the tidal frequency. PSI is restricted to the latitude belt 28.8°N/S and yields under typical conditions damping times of about 20 days for tides with low vertical wavenumber. Damping times decrease with equivalent mode number j roughly as 1/j2. Outside the critical latitudes PSI is not possible, and damping times are from one to two orders of magnitude larger. The energy transfer to the ambient wave field is concentrated at half the tidal frequency ω at all latitudes within the critical latitude belt. Outside, the transfer is much smaller and peaks at ω + f and N. An estimation of the tidal spectral transfer on the global scale is hampered by insufficient knowledge of the baroclinic energy distribution over the vertical modes. Using results from a numerical circulation model with tidal forcing, we find an energy transfer from the tide to the ambient wave field of typically 0.3 TW, about half of what is currently proposed for the conversion of barotropic to baroclinic energy.

The 2018 Geothermal Reservoir Stimulation in Espoo/Helsinki, Southern Finland: Seismic Network Anatomy and Data Features

AbstractA seismic network was installed in Helsinki, Finland to monitor the response to an ∼6-kilometer-deep geothermal stimulation experiment in 2018. We present initial results of multiple induced earthquake seismogram and ambient wavefield analyses. The used data are from parts of the borehole network deployed by the operating St1 Deep Heat Company, from surface broadband sensors and 100 geophones installed by the Institute of Seismology, University of Helsinki, and from Finnish National Seismic Network stations. Records collected in the urban environment contain many signals associated with anthropogenic activity. This results in time- and frequency-dependent variations of the signal-to-noise ratio of earthquake records from a 260-meter-deep borehole sensor compared to the combined signals of 24 collocated surface array sensors. Manual relocations of ∼500 events indicate three distinct zones of induced earthquake activity that are consistent with the three clusters of seismicity identified by the company. The fault-plane solutions of 14 selected ML 0.6–1.8 events indicate a dominant reverse-faulting style, and the associated SH radiation patterns appear to control the first-order features of the macroseismic report distribution. Beamforming of earthquake data from six arrays suggests heterogeneous medium properties, in particular between the injection site and two arrays to the west and southwest. Ambient-noise cross-correlation functions reconstruct regional surface-wave propagation and path-dependent body-wave propagation. A 1D inversion of the weakly dispersive surface waves reveals average shear-wave velocities around 3.3 km/s below 20 m depth. Consistent features observed in relative velocity change time series and in temporal variations of a proxy for wavefield partitioning likely reflect the medium response to the stimulation. The resolution properties of the obtained data can inform future monitoring strategies and network designs around natural laboratories.

Seasonal and spatial variations in the ocean-coupled ambient wavefield of the Ross Ice Shelf

AbstractThe Ross Ice Shelf (RIS) is host to a broadband, multimode seismic wavefield that is excited in response to atmospheric, oceanic and solid Earth source processes. A 34-station broadband seismographic network installed on the RIS from late 2014 through early 2017 produced continuous vibrational observations of Earth's largest ice shelf at both floating and grounded locations. We characterize temporal and spatial variations in broadband ambient wavefield power, with a focus on period bands associated with primary (10–20 s) and secondary (5–10 s) microseism signals, and an oceanic source process near the ice front (0.4–4.0 s). Horizontal component signals on floating stations overwhelmingly reflect oceanic excitations year-round due to near-complete isolation from solid Earth shear waves. The spectrum at all periods is shown to be strongly modulated by the concentration of sea ice near the ice shelf front. Contiguous and extensive sea ice damps ocean wave coupling sufficiently so that wintertime background levels can approach or surpass those of land-sited stations in Antarctica.

Bear Encounters with Seismic Stations in Alaska and Northwestern Canada

ABSTRACTA typical seismic experiment involves installing 10–50 seismometers for 2–3 yr to record distant and local earthquakes, along with Earth’s ambient noise wavefield. The choice of the region is governed by scientific questions that may be addressed with newly recorded seismic data. In most experiments, not all stations record data for the full expected duration. Data loss may arise from defective equipment, improperly installed equipment, vandalism or theft, inadequate power sources, environmental disruptions (e.g., snow covering solar panels and causing power outage), and many other reasons. In remote regions of Alaska and northwestern Canada, bears are a particular threat to seismic stations. Here, we document three recent projects (Southern Alaska Lithosphere and Mantle Observation Network, Fault Locations and Alaska Tectonics from Seismicity, and Mackenzie Mountains EarthScope Project) in which bears were regular visitors to remote seismic stations. For these projects, there were documented bear encounters at 56 out of 88 remote stations and 6 out of 85 nonremote stations. Considering bear‐disrupted sites—such as dug‐up cables or outages—there were 29 cases at remote stations and one case at nonremote stations. We also compile bear encounters with permanent stations within the Alaska Seismic Network, as well as stations of the Alaska Transportable Array. For these two networks, the stations are designed with fiberglass huts that house and protect equipment. Data losses at these networks because of bears are minor (<5%), though evidence suggests they are regularly visited by bears, and data disruptions are exclusively at remote stations. The primary goal of this study is to formally document the impacts of bears on seismic stations in Alaska and northwestern Canada. We propose that the threat of damage from bears to a station increases with the remoteness of the site and the density of bears, and it decreases with the strength and security of materials used. We suggest that low‐power electric fences be considered for seismic stations—especially for temporary experiments—to protect the equipment and to protect the bears. With the goal of 100% data returns, future seismic experiments in remote regions of bear country should carefully consider the impacts of bears.

Simulating the wave-induced response of a submerged wave-energy converter using a non-hydrostatic wave-flow model

Abstract With the increasing interest in wave energy, and when moving towards commercial-scale wave-energy projects, a detailed understanding of the interactions between single and arrays of wave-energy converters (WECs) with the ambient wave and flow field becomes imperative for both design and operational purposes, as well as assessment of their environmental impacts. This work presents a new numerical approach to simulate the nonlinear evolution of the waves and their interactions with a submerged wave-energy converter at the scale of a realistic coastal region. The numerical approach is based on the non-hydrostatic framework, and implemented in the open-source SWASH model, which provides an efficient tool to simulate the nonlinear evolution of waves over realistic coastal bathymetries. Here, we present a numerical extension to the non-hydrostatic approach to account for interactions between waves and a submerged point absorber, and to capture the response of such a wave energy device. Model results are compared with an analytical solution based on potential flow theory, a CFD simulation, and experimental data to validate its capabilities in simulating the wave-WEC interactions for both linear and nonlinear wave conditions. Overall, the results of this validation demonstrate that the model captures the wave-structure interactions and the body response with satisfactory accuracy. Notably, the results also indicate that a coarse vertical resolution was sufficient to capture these dynamics, making the model sufficiently computationally efficient to simulate the interaction of waves and WECs over large scales. As a consequence, this new modelling approach should provide a promising new alternative to simulate the interactions between nonlinear wave fields and submerged point absorbers at the scale of a realistic coastal region.

A Seismic Shift in Scalable Acquisition Demands New Processing: Fiber-Optic Seismic Signal Retrieval in Urban Areas with Unsupervised Learning for Coherent Noise Removal

With the development of fiber-optic seismic acquisition systems, dense seismic monitoring of the near surface in urban areas is quickly becoming much easier than ever before. We provide a case study illustrating the use of data from a new type of deployment, a fiber-optic array in existing telecommunications conduits underneath the Stanford University campus in California. We perform cross correlations of strain-rate measurements of the ambient wavefield to extract signals that mimic the response of the array to virtual active seismic sources at any receiver location. Now that we can so easily use our telecommunications infrastructure for continuous, dense, urban seismic acquisition, data collected in such a manner will go to waste unless we significantly automate ambient noise processing. We analyze 37 days of ambient noise; introduce an exploratory data analysis tool that clusters a week of noise to help us quickly find coherent, repeating noises that inhibit reliable extraction of useful signals; and design filters to remove nearby car recordings from the next 30 days of noise. We review a metric to quantify convergence of the cross correlations and use that metric extended throughout the array to support filtering out recordings of cars near the array.

Statistical redundancy of instantaneous phases: theory and application to the seismic ambient wavefield

AbstractIn order to detect possible signal redundancies in the ambient seismic wavefield, we develop a new method based on pairwise comparisons among a set of synchronous time-series. This approach is based on instantaneous phase coherence statistics. The first and second moments of the pairwise phase coherence distribution are used to characterize the phase randomness. For perfect phase randomness, the theoretical values of the mean and variance are equal to 0 and $\sqrt{1-2/\pi }$, respectively. As a consequence, any deviation from these values indicates the presence of a redundant phase in the raw continuous signal. A previously detected microseismic source in the Gulf of Guinea is used to illustrate one of the possible ways of handling phase coherence statistics. The proposed approach allows us to properly localize this persistent source, and to quantify its contribution to the overall seismic ambient wavefield. The strength of the phase coherence statistics relies in its ability to quantify the redundancy of a given phase among a set of time-series with various useful applications in seismic noise-based studies (tomography and/or source characterization).

In situ observations of velocity changes in response to tidal deformation from analysis of the high‐frequency ambient wavefield

AbstractWe report systematic seismic velocity variations in response to tidal deformation. Measurements are made on correlation functions of the ambient seismic wavefield at 2–8 Hz recorded by a dense array at the site of the Piñon Flat Observatory, Southern California. The key observation is the dependence of the response on the component of wave motion and coda lapse time τ. Measurements on the vertical correlation component indicate reduced wave speeds during periods of volumetric compression, whereas data from horizontal components show the opposite behavior, compatible with previous observations. These effects are amplified by the directional sensitivities of the different surface wave types constituting the early coda of vertical and horizontal correlation components to the anisotropic behavior of the compliant layer. The decrease of the velocity (volumetric) strain sensitivity Sθ with τ indicates that this response is constrained to shallow depths. The observed velocity dependence on strain implies nonlinear behavior, but conclusions regarding elasticity are more ambiguous. The anisotropic response is possibly associated with inelastic dilatancy of the unconsolidated, low‐velocity material above the granitic basement. However, equal polarity of vertical component velocity changes and deformation in the vertical direction indicate that a nonlinear Poisson effect is similarly compatible with the observed response pattern. Peak relative velocity changes at small τ are 0.03%, which translates into an absolute velocity strain sensitivity of Sθ≈5 × 103 and a stress sensitivity of 0.5 MPa−1. The potentially evolving velocity strain sensitivity of crustal and fault zone materials can be studied with the method introduced here.