Ambient Noise Recordings Research Articles

Structural Health Monitoring of a High-Rise Building Using Ambient Noise Recordings and a Regional Earthquake Record

Abstract Resonance frequency and seismic-wave velocity monitoring are both important for structural health monitoring (SHM). However, the limited number of vibration sensors or seismometers in most buildings necessitates the study of the sensitivity of monitoring indicators to ensure effective monitoring while reducing data acquisition costs and processing time. This study compares the changes in resonance frequency and seismic-wave velocity under ambient noise and a regional seismic event. Resonance frequency is calculated by power spectral density, whereas seismic-wave velocity is estimated using impulse response function and moving-window cross-spectrum methods. The results indicate that relative resonance frequency changes are more suitable for overall SHM due to its higher sensitivity and lower instrument requirements. Moreover, the time–frequency analysis method provides higher resolution results in resonance frequency during seismic events, a precision that seismic velocity methods cannot achieve.

Detection and characterization of ship underwater radiated narrowband noise

Underwater radiated noise (URN) emitted by ships is a high-intensity noise that interferes with acoustic transmissions. It is composed of broadband cavitation noise, tonal noise and its harmonics that can be observed up to several tens of kHz. The in-situ detection of ship noise and the identification of its tonal components are therefore useful for adaptive underwater communication techniques and for channel sensing. To this end, we present a detection scheme to identify the presence of a vessel and identifying its narrowband components. For presence detection, we develop a Convolution Neural Network (CNN) model whose input is a Detection Envelope Modulation On Noise (DEMON) analysis for a given observation window. The model is trained with our collected recordings of ship data and ambient noise. After identifying the ship’s URN, our tonal detector relies on the stability and stationarity of the ship’s tonal lines, as opposed to the randomness of the ambient noise. Cross-correlation and spectral entropy are used as detection metrics. To reduce the sensitivity to the tested environment, the detection thresholds are set adaptively. The results show a favorable trade-off between precision and recall compared to benchmark methods. We share our database of URN of labeled ships.

Anisotropic Reversible‐Jump McMC Shear‐Velocity Tomography of the Eastern Alpine Crust

AbstractThe eastern Alpine crust has been shaped by the continental collision of the European and Adriatic plates beginning at 35 Ma and was affected by a major reorganization after 20 Ma. To better understand how the eastern Alpine surface structures link with deep seated processes, we analyze the depth‐dependent seismic anisotropy based on Rayleigh wave propagation. Ambient noise recordings are evaluated to extract Rayleigh wave phase dispersion measurements. These are inverted in a two step approach for the azimuthally anisotropic shear velocity structure. Both steps are performed with a reversible jump Markov chain Monte Carlo (rj‐McMC) approach that estimates data errors and propagates the modeled uncertainties from the phase velocity maps into the depth inversion. A two layer structure of azimuthal anisotropy is imaged in the Alpine crust, with an orogen‐parallel upper crust and approximately orogen‐perpendicular layer in the lower crust and the uppermost mantle. In the upper layer, the anisotropy tends to follow major fault lines and may thus be an apparent, structurally driven anisotropy. The main foliation and fold axis orientations might contribute to the anisotropy. In the lower crust, the N‐S orientation of the fast axis is mostly confined to regions north of the Periadriatic Fault and may be related to European subduction. Outside the orogen, no clearly layered structure is identified. The anisotropy pattern in the northern Alpine foreland is found to be similar compared to SKS studies which is an indication of very homogeneous fast axis directions throughout the crust and the upper mantle.

An Open-Access Data Set of Active-Source and Passive-Wavefield DAS and Nodal Seismometer Measurements at the Newberry Florida Site

Abstract This article documents a comprehensive subsurface imaging experiment using seismic waves in a well-studied outdoor laboratory at Newberry, Florida, which is known for significant spatial variability, karstic voids, and underground anomalies. The experiment used approximately two kilometers of distributed acoustic sensing (DAS) fiber-optic cable, forming a dense 2D array of 1920 horizontal-component channels, and a 2D array of 144 SmartSolo three-component nodal seismometers, to sense active-source and passive-wavefield seismic waves. The active-source data were generated using a powerful, triaxial vibroseis shaker truck (T-Rex) and impact sources (accelerated weight drop and an eight-pound sledgehammer) that were simultaneously recorded by both the DAS and nodal seismometers. The vibroseis truck was used to excite the ground in three directions (two horizontal and one vertical) at 260 locations inside and outside the instrumented array, whereas the impact sources were used at 268 locations within the instrumented array. The passive-wavefield data recorded using the nodal seismometers comprised 48 hr of ambient noise collected over a period of four days in four 12-hour time blocks, whereas the passive wavefield data collected using DAS consisted of four hours of ambient noise recordings. This article aims to provide a comprehensive overview of the testing site, experiment layout, the DAS and nodal seismometer acquisition parameters, and implemented raw data processing steps. Although potential use cases, such as surface-wave testing, full-waveform inversion, and ambient noise tomography, are discussed relative to example data, the focus of this article is on documenting this unique data set and presenting its initial data quality rather than on generating subsurface imaging results. The raw and processed data, along with detailed documentation of the experiment and Python tools to aid in visualizing the DAS data set, have been made publicly available.

New constraints on the shear wave velocity structure of the Ivrea geophysical body from seismic ambient noise tomography (Ivrea-Verbano Zone, Alps)

SUMMARY We performed seismic ambient noise tomography to investigate the shallow crustal structure around the Ivrea geophysical body (IGB) in the Ivrea-Verbano Zone (IVZ). We achieved higher resolution with respect to previous tomographic works covering the Western Alps, by processing seismic data collected by both permanent and temporary seismic networks (61 broad-band seismic stations in total). This included IvreaArray, a temporary, passive seismic experiment designed to investigate the IVZ crustal structure. Starting from continuous seismic ambient noise recordings, we measured and inverted the dispersion of the group velocity of surface Rayleigh waves (fundamental mode) in the period range 4–25 s. We obtained a new, 3-D vS model of the IVZ crust via the stochastic neighbourhood algorithm (NA), with the highest resolution between 3 to 40 km depth. The fast and shallow shear wave velocity anomaly associated with the IGB presents velocities of 3.6 km s−1 directly at the surface, in remarkable agreement with the location of the exposed lower-to-middle crustal and mantle outcrops. This suggests a continuity between the surface geological observations and the subsurface geophysical anomalies. The fast IGB structure reaches vS of 4 km s−1 at 20–25 km depth, at the boundary between the European and Adriatic tectonic plates, and in correspondence with the earlier identified Moho jump in the same area. The interpretation of a very shallow reaching IGB is further supported by the comparison of our new results with recent geophysical investigations, based on receiver functions and gravity anomaly data. By combining the new geophysical constraints and the geological observations at the surface, we provide a new structural interpretation of the IGB, which features lower crustal and mantle rocks at upper crustal depths. The comparison of the obtained vS values with the physical properties from laboratory analysis of local rock samples suggests that the bulk of the IGB consists of a combination of mantle peridotite, ultramafic and lower crustal rocks, bound in a heterogeneous structure. These new findings, based on vS tomography, corroborate the recent interpretation for which the Balmuccia peridotite outcrops are continuously linked to the IGB structure beneath. The new outcomes contribute to a multidisciplinary framework for the interpretation of the forthcoming results of the scientific drilling project DIVE. DIVE aims at probing the lower continental crust and its transition to the mantle, with two ongoing and one future boreholes (down to 4 km depth) in the IVZ area, providing new, complementary information on rock structure and composition across scales. In this framework, we constrain the upper crustal IGB geometries and lithology based on new evidence for vS, connecting prior crustal knowledge to recent active seismic investigations.

Soil characterization of Babol City using single-station ambient seismic noise method

The interrelation between the local site conditions and the severity of destruction during earthquakes has increased the necessity for more investigations on soil characterization in recent years. In residential areas, where active seismic methods are problematic for defining soil properties, passive seismic methods have attracted considerable attention due to their cost-effective and non-invasive features. Therefore, we have utilized 35 single-station ambient seismic noise recordings in the seismically vulnerable city of Babol, Iran, to evaluate the site response (including azimuthal variation and resonance frequency), estimate the seismic vulnerability index (Kg), and classify the soil based on the Vs30 of the region. To address the aim, we extract Rayleigh wave ellipticities from ambient seismic noise using their polarization features and employ them to define the azimuthal variations, resonance frequencies, and Kg values of the region. In each station, the ellipticity curve is inverted with the support of supplementary geotechnical information to obtain a shear velocity profile. The inversion results have good consistency with borehole and downhole test information.The study identifies a low resonance frequency of 0.9 Hz with a NE directivity for the majority of the city and a Vs30 range of 180 m/s to 350 m/s. Moreover, the seismic bedrock of Babol City is represented by a 3D Vs model at a depth of 60 m. Our results reveal that the city is vulnerable to seismic hazards due to the thick and low-velocity deposits responsible for high amplification affecting the study region, which combines with a high seismic vulnerability index, particularly in the southern part. This work has applications for future structural design, urban development, and seismic hazard reduction.

Modified Frequency-Bessel Transform Method for Dispersion Imaging of Surface Wave from Six-Component Ambient Noise Recordings

Surface wave analysis methods based on ambient noise have attracted considerable attention within the geophysical and civil engineering communities, particularly in light of the limitations associated with traditional seismic surveys in densely populated urban areas. Conventionally, translational seismic sensors have been employed for the observation of ambient noise. Nevertheless, recent advancements in rotational sensor technology have facilitated the direct measurement of ground motion induced by seismic waves in a rotational manner. Through the integration of these rotational sensors with traditional inertial seismometers, a novel six-component seismic sensor has been developed. This innovative sensor enables the local observation of six degrees of freedom pertaining to ground motion, encompassing three orthogonal components of translational motion and three orthogonal components of rotational motion. In order to demonstrate the seismological significance of rotational ground-motion data, we applied the modified frequency-Bessel transform method (MFJ) to six-component ambient noise recordings for surface wave analysis. To underscore the practical utility of the MFJ method, a real-world case study is presented, illustrating its effectiveness in quantifying Rayleigh and Love wave dispersion from six-component ambient noise recordings.

Detecting gas upwelling hazards in coastal areas through integration of active and passive electrical and seismic methods (Fiumicino, Central Italy)

The accurate location of gas upwelling flows is still an open problem for non-invasive imaging techniques in populated areas. Gas blowouts of deep origin may represent a serious threat to human health in urban areas and should be correctly imaged with high-resolution for assessing the related hazards. In this work, we propose an integration of active (electrical resistivity tomography and high-resolution sub-bottom profiling complemented with the multibeam bathymetry) and passive (self-potential and ambient noise recordings) geophysical methods to image gas upwelling flows in the coastal area of Fiumicino (Central Italy), where the gas presence is well-documented by previous works. We demonstrate that merging seismic sub-bottom profiling and electrical resistivity tomography has enormous diagnostic potential for gas detection, since they combine the high resolution needed to correctly image the subsurface and the interfaces between different media with the high diagnostic capability of electrical methods to detect anomalies associated with the gas emissions. Passive seismic methods complement the analysis enabling an estimation of the shear-wave velocity through array measurements. Finally, the reconstruction of the natural electrical sources, inferred from the inversion of self-potential data, confirms the location of the near-surface gas upwelling flows assessed through the resistivity model. This work demonstrates that the integration of high-resolution active and passive seismic and electrical methods can be an effective choice for the accurate location of risk-prone areas by imaging the near surface gas pathways where borehole drilling is strongly limited if not forbidden.

Transdimensional ambient-noise surface wave tomography of the Reykjanes Peninsula, SW Iceland

SUMMARY Ambient noise seismic tomography has proven to be an effective tool for subsurface imaging, particularly in volcanic regions such as the Reykjanes Peninsula (RP), SW Iceland, where ambient seismic noise is ideal with isotropic illumination. The primary purpose of this study is to obtain a reliable shear wave velocity model of the RP, to get a better understanding of the subsurface structure of the RP and how it relates to other geoscientific results. This is the first tomographic model of the RP which is based on both on- and off-shore seismic stations. We use the ambient seismic noise data and apply a novel algorithm called one-step 3-D transdimensional tomography. The main geological structures in the study area (i.e. covered by seismic stations) are the four NE–SW trending volcanic systems, orientated highly oblique to the plate spreading on the RP. These are from west to east; Reykjanes, Eldvörp-Svartsengi, Fagradalsfjall and Krýsuvík, of which all except Fagradalsfjall host a known high-temperature geothermal field. Using surface waves retrieved from ambient noise recordings, we recovered a 3-D model of shear wave velocity. We observe low-velocity anomalies below these known high-temperature fields. The observed low-velocity anomalies below Reykjanes and Eldvörp-Svartsengi are significant but relatively small. The low-velocity anomaly observed below Krýsuvík is both larger and stronger, oriented near-perpendicular to the volcanic system, and coinciding well with a previously found low-resistivity anomaly. A low-velocity anomaly in the depth range of 5–8 km extends horizontally along the whole RP, but below the high-temperature fields, the onset of the velocity decrease is shallower, at around 3 km depth. This is in good agreement with the brittle–ductile transition zone on the RP. In considerably greater detail, our results confirm previous tomographic models obtained in the area. This study demonstrates the potential of the entirely data-driven, one-step 3-D transdimensional ambient noise tomography as a routine tomography tool and a complementary seismological tool for geothermal exploration, providing an enhanced understanding of the upper crustal structure of the RP.

Monitoring performance of slopes via ambient seismic noise recordings: Case study in a colluvium deposit

Long-term monitoring of slopes is of significance for engineering geology research and geo-disaster prevention. There is a growing need to develop fast, nondestructive and affordable techniques that can detect gradual and cyclic changes inside slopes. For this purpose, we propose a fast and low-cost computational framework based on processing the single-point ambient seismic noise recordings by the horizontal-to-vertical spectral ratio (HVSR) method. To test the efficiency of the proposed framework, we conduct a demonstration study in a road-cut slope in colluvium deposits in Southwest China. First, we carry out short-term ambient seismic noise surveys on the slope, delineate the shear wave velocity (Vs) structure of the slope, verify and establish this structure as the reference Vs model for the slope. Then we conduct the long-term monitoring of the ambient seismic noise on two profiles of the slope, calculate the HVSR curves and observe the time-dependent variations of the predominant peaks that reflect temporal changes of subsurface interfaces. Finally, we perform the Vs inversion to investigate changes in the Vs structure with rainfall. Through the monitoring, we identify the rainfall-induced slope failure and discover that both predominant frequency and shallow subsurface Vs of slope are negatively correlated with rainfall. The HVSR calculation, the predominant peak identification and the Vs inversion can all be implemented in minutes, which is much faster than the array-based surface wave method. The theoretical analysis and the demonstration application show that the framework we proposed in this study has great potential for monitoring changes in performance of slopes.

Vs30 Mapping of the Greater Montreal Region Using Multiple Data Sources

The metropolitan community of Montreal (MMC) is located in Eastern Canada and included in the western Quebec seismic zone characterized by shallow crustal earthquakes and moderate seismicity. Most of the urbanized areas are settled close to the Saint-Lawrence River and its tributaries and within the region, delimiting the extension of the clay deposits from the Champlain Sea. The influence of these recent and soft deposits on seismic waves has been observed after the 1988 M5.8 Saguenay earthquake and has proven to be crucial in seismic hazard analysis. The shear-wave velocity Vs averaged over the 30 m of soil, abbreviated Vs30, is one of the most used parameters to characterize the site condition and its influence on seismic waves. Since 2000, a site condition model has been developed for the municipalities of Montreal and Laval, combining seismic and borehole data for risk mitigation purposes. The paper presents an extended version of the Vs30 mapping for the entire region of the MMC, which accounts for half of the population of Quebec, including additional ambient noise recordings, recently updated borehole datasets, geological vector map and unpublished seismic refraction data to derive Vs profiles. The estimated Vs30 values for thousands of sites are then interpolated on a regular grid of 0.01 degrees using the inverse distance weighted interpolation approach. Regions with the lowest estimated Vs30 values where site amplification could be expected on seismic waves are in the Northeastern part and in the Southwest of the MMC. The map expresses in terms of site classes is compared with intensity values derived from citizen observations after recent felt. In general, the highest reported intensity values are found in regions with the lowest Vs30 values on the map. Areas where this rule does not apply, should be investigated further. This site condition model can be used in seismic hazard and risk analysis.

Characteristic evaluation of passive seismic ellipticity method in urban shield tunnel

In recent years, engineering examples have shown that traditional surface-wave detection methods are easily limited in areas with small space and heavy traffic. The horizontal-to-vertical spectral ratio survey method has become increasingly prevalent in site measurements and is favored by geophysicists for its ability to detect near-surface velocity structures with greater sensitivity than phase velocities. Based on the phase difference method and polarization filtering, we proposed a passive seismic ellipticity method. The objective of this method is to extract the Rayleigh waves from ambient noise recordings and the analysis of ellipticity dispersion characteristic. This method was applied to an actual case study. Specifically, the velocity dispersion profile of the site is obtained using the active-source surface-wave method. Then, we compared the results obtained by the passive seismic ellipticity method with those obtained by the active-source surface-wave method and the field data, to evaluate the accuracy of results. The results demonstrate that the proposed method can effectively detect the location of subsurface shield tunnels and has the characteristics of simple data processing steps, less interference, and high stability of results. This demonstrates the necessity of using the phase difference method to select segments dominated by Rayleigh waves in ambient noise recordings and applying polarization filtering before calculating ellipticity. The paper provides an actual case study for the application and promotion of the passive seismic ellipticity method and has significant practical value and reference significance.

Seismic anisotropy and shear-wave splitting: Achievements and perspectives: foreword

This special issue of Annals of Geophysics “Seismic anisotropy and shear-wave splitting: Achievements and perspectives” originates from a session (S10) of the 37th General assembly of the European Seismological commission ESC 2021 Conference which was planned to take place on 21 September 2021, in Corfu Greece, but due to the Covid19 pandemic was Virtual. The main theme of the session and of this special issue was the crucial role of seismic anisotropy in investigating the Earth’s interior from the upper crust to the inner core. Shear-wave splitting, one of the most effective ways to study seismic anisotropy, can identify the properties and the geodynamics of the upper mantle, and identify the presence of fluid-saturated microcracks, oriented according to the stress regime, in the upper crust. Azimuthal anisotropy and radial anisotropy can be assessed from earthquake or ambient noise recordings to detect the seismic layered features and to rebuild the 3D seismic structure

In situ estimation of effective rock elastic moduli by seismic ambient vibrations

In this study, we performed a non-invasive ambient noise investigation of unsaturated rock structures in the Bohemian Paradise (Bohemian Cretaceous Basin, Czech Republic). Our study focused on two key topics: 1) An in situ elastic moduli estimate of competent, horizontally deposited sandstone layers using ambient noise array measurements. Recordings were processed using an f-k array analysis, from which frequency-dependent Love and Rayleigh wave dispersion curves, as well as Rayleigh wave ellipticity, were retrieved. Data were inverted for the P- and S-wave velocity profiles, from which Young's and shear moduli were successfully estimated. 2) A study of the local response of the Kapelník rock tower. We analysed a dataset of ambient noise recordings obtained from the top of the tower and its foot. Information regarding tower oscillation frequencies and directions, together with amplification ratios, were retrieved from a particle motion polarisation analysis and from site-to-reference spectral ratios. Euler-Bernoulli beam theory was also employed for interpreting measured data using elastic moduli estimated from noise array measurements.

Inter-Story Drift Ratio Detection of High-Rise Buildings Based on Ambient Noise Recordings

The inter-story drift ratio (IDR) is a crucial parameter in structural health monitoring to judge the safety, stability, and serviceability of buildings. Real-time, continuous, and widely applicable detection on IDRs is essential. However, the current methods present some challenges in conducting such detection, including adverse effects from weather, the requirement for large amounts of space, and fragile instruments. This study proposes an alternative method to overcome these defects to measure IDRs and evaluate the structural conditions using ambient noise recordings. Ambient noise is a random and continuous wave signal with various sources and is modified by its propagating medium. Taking the Zhonghe Building on the campus of Tongji University, Shanghai, China, as an example, 24 three-component seismometers were deployed to capture and record the ambient noise continuously from 20 November 2021 to 9 December 2021. Using analysis of the polarization parameters of ambient noise during the building’s most dangerous time and ordinary time, a deflection curve, IDRs, and harmful IDRs of the Zhonghe Building during the most dangerous time were calculated. The computed maximum drift was 0.06 m, the maximum IDR was 1.140×10−3, and the maximum harmful IDR was −2.573×10−4. These results were compared with the relevant specifications in China, and it was found that the structure was in good condition. The study proposes an alternative method to measure IDRs with high applicability and continuity in real time and underscores the need for further research to achieve a localized and real-time structural health monitoring system.

Mapping crustal structure across southern Australia using seismic ambient noise tomography

The rocks of southern Australia record over three billion years of Earth’s evolution, but the basement geology is veiled by sediment. Geophysical data are needed to unveil the geology. The 2018–2022 Lake Eyre Basin and 2020–2022 AusArray SA seismic arrays expand seismic coverage in South Australia. In conjunction with permanent and preceding temporary arrays, we extracted Rayleigh wave dispersion data from ambient noise recordings at a total of 501 seismic stations spanning the transition from Precambrian to Phanerozoic Australia. These data were used to develop Rayleigh wave phase velocity maps of southern Australia at periods 3–20s, and in turn, a shear wave velocity model to 20km depth. At upper-crustal depths, low velocity structure tracks Phanerozoic sedimentary accumulations. The Moyston Fault, regarded as the boundary between the Delamerian and Lachlan Orogens, has an intermittent expression in the shear velocity model: it has no obvious expression at depths shallower than ∼10km, but in the mid-crust is marked by a velocity contrast in southern Victoria and a velocity contrast tracing the southern edge of the Darling Basin in western New South Wales. An arcuate velocity contrast characterising the western edge of deep, sediment-filled troughs of the Darling Basin is a candidate for the transition from Precambrian to Phanerozoic crust. Fluid derived from neotectonic metamorphic devolatilization and/or remnant hydrated mantle is our preferred hypothesis for explaining seismicity and coincident seismic and conductivity anomalies in the mid-to-lower crust beneath the Ikara–Flinders Ranges. Our model suggests that the Olympic Dam and Carrapateena IOCG deposits reside above the margin of a mid-crustal low seismic velocity zone. We surmise that this might reflect past metalliferous fluid movement associated with the Olympic Cu-Au Province, akin to low reflectivity and low resistivity zones evident in 2D reflection seismic and magnetotelluric profiles, respectively.

Geotechnical site characterization with 3D ambient noise tomography

We develop a new 3D ambient noise tomography (3D ANT) method for geotechnical site characterization. It requires recording ambient noise fields using a 2D surface array of geophones, from which experimental crosscorrelation functions (CCFs) are then extracted and directly inverted to obtain an S-wave velocity ([Formula: see text]) structure. The method consists of a forward simulation using 3D P-SV elastic wave equations to compute the synthetic CCF and an adjoint-state inversion to match the synthetic CCFs to the experimental CCFs for extraction of [Formula: see text]. The main advantage of the presented method, as compared with conventional passive-source seismic methods using characteristics of Green’s function (GF), is that it does not require equal energy on both sides of each receiver pair or far-field wavefields to retrieve the true GF. Instead, the source power spectrum density is inverted during the analysis and incorporated into the forward simulation of the synthetic CCFs to account for source energy distribution. After testing on synthetic data, the 3D ANT method is applied to 3 h of ambient noise recordings at the Garner Valley Downhole Array (GVDA) site in California, using a surface array of 196 geophones placed on a 14 × 14 grid with 5 m spacing. The inverted 3D [Formula: see text] model is found to be consistent with previous invasive and noninvasive geotechnical characterization efforts at the GVDA site.

Surface-wave dispersion curves extraction method from ambient noise based on U-net++ and density clustering algorithm

It is crucial to accurately and rapidly extract dispersion curves of surface waves from ambient noise recordings for inverting the subsurface shear (S) wave velocity structures. Conventional manual picking methods are labor-intensive and affected by subjective factors related to seismologists. The existing deep learning methods can automatically and efficiently extract dispersion curves from dispersion spectrograms with a single branch. However, extracting dispersion curves from phase-velocity dispersion spectrograms with multiple branches remains challenging due to complex label-making processes and extensive label-preparation times. Notably, phase velocities typically display a positive correlation with periods and are slightly higher than group velocities. Given the significance of this empirical relationship, we present an intelligent surface-wave dispersion curves extraction method based on U-net++ and density clustering algorithm. Initially, guided by domain knowledge that dispersion curves are smooth, a global searching method is employed to automatically label group-velocity dispersion curves from group-velocity dispersion spectrograms. Subsequently, the group-velocity dispersion curves undergo transformation into probability images of the curves using a Gaussian function. The U-net++ then nonlinearly converts group-velocity dispersion spectrograms into probability images of group-velocity dispersion curves within a high-dimensional space. Following this, the Density-Based Spatial Clustering of Applications with Noise (DBSCAN) algorithm is applied to obtain multi-mode phase-velocity dispersion curves from phase-velocity dispersion spectrograms. We then remove invalid dispersion values in the multi-mode phase-velocity dispersion curves based on the empirical relationship between phase and group velocities, ultimately obtaining the phase-velocity dispersion curve corresponding to the group-velocity dispersion curve. Our proposed method has been tested using synthetic data and 3D real-world data collected near Lake Chao in Anhui Province. The test results show that our approach effectively extracts both group-velocity and phase-velocity dispersion curves. Inverted S-wave velocity structures based on extracted dispersion curves in real-world data can characterize the Tan-Lu fault zone, demonstrating the effectiveness of our proposed method.

Efficient wave type fingerprinting and filtering by six-component polarization analysis

SUMMARYWe present a technique to automatically classify the wave type of seismic phases that are recorded on a single six-component recording station (measuring both three components of translational and rotational ground motion) at the Earth’s surface. We make use of the fact that each wave type leaves a unique ’fingerprint’ in the six-component motion of the sensor (i.e. the motion is unique for each wave type). This fingerprint can be extracted by performing an eigenanalysis of the data covariance matrix, similar to conventional three-component polarization analysis. To assign a wave type to the fingerprint extracted from the data, we compare it to analytically derived six-component polarization models that are valid for pure-state plane wave arrivals. For efficient classification, we make use of the supervised machine learning method of support vector machines that is trained using data-independent, analytically derived six-component polarization models. This enables the rapid classification of seismic phases in a fully automated fashion, even for large data volumes, such as encountered in land-seismic exploration or ambient noise seismology. Once the wave-type is known, additional wave parameters (velocity, directionality and ellipticity) can be directly extracted from the six-component polarization states without the need to resort to expensive optimization algorithms. We illustrate the benefits of our approach on various real and synthetic data examples for applications such as automated phase picking, aliased ground-roll suppression in land-seismic exploration and the rapid close-to real-time extraction of surface wave dispersion curves from single-station recordings of ambient noise. Additionally, we argue that an initial step of wave type classification is necessary in order to successfully apply the common technique of extracting phase velocities from combined measurements of rotational and translational motion.

Shear wave velocity structure at the Fukushima forearc region based on H/V analysis of ambient noise recordings by ocean bottom seismometers

SUMMARYThis study presents the shear wave velocity (VS) structures of sedimentary sequences and a section of the upper crustal layer in the Fukushima forearc region of the Japan Trench subduction zone, which were obtained by analysing the horizontal-to-vertical (H/V) spectral ratios of ambient vibration records. The H/V curves were derived using 31 d of continuous seismic data from 3 broad-band and 16 short-period ocean bottom seismometer (OBS) stations. Using the broad-band data, H/V ratios from 0.01 to 10 Hz were derived, but the ratios below 0.1 Hz frequencies were unusually large and temporally unstable. Characterization of seismic noise energy from ∼1 yr of seismic data of three broad-band OBSs revealed variable and elevated energy conditions below 0.1 Hz due to typical long-period oceanic noise; we link these observations with the unstable H/V ratios below this frequency. Therefore, H/V analysis was performed in the frequency range of 0.1–10 Hz for both broad-band and short-period OBSs to obtain subsurface VS profiles. For the forward calculation of the H/V ratios in the inversion process, we used the recently developed ‘hvgeneralized’ method, which is based on the diffuse field assumption, and accounts for the water layer on top of stratified media. Moreover, available prior geological and geophysical information was utilized during the inversion of the H/V curves. We found that subsurface VS ranged from approximately 30 m s−1 at the seabed to approximately 4900 m s−1 at 7000 m below the sea floor (mbsf). Starting with the best model candidate at each OBS location, the effect of the water layer on the H/V curve in the deep ocean was investigated by comparing synthetic H/V curves with and without the water layer. The synthetic H/V analysis revealed that the water layer had a significant effect on H/V amplitudes at higher frequencies (>1 Hz), whereas comparatively little effect was observed at lower frequencies (<1 Hz). This study provides an empirical basis for H/V analysis using OBS data to determine VS down to several kilometres of sedimentary sequences to the upper crust with high-resolution.