Geophysical Journal International

Saengduean, P; Snieder, R; Wakin, M B

2023 Geophysical Journal International

doi: 10.1093/gji/ggad328

SUMMARYSeismic interferometry (SI) is a technique that allows one to estimate the wavefields accounting for the wave propagation between seismometers, any of which can act as a virtual source (VS). Interferometry, particularly noise interferometry, has been applied to several geophysical disciplines such as passive monitoring and distributed acoustic sensing. In practice, one requires long recordings of seismic noise for noise interferometry. Additionally, one can have missing seismic interferometric traces because some receivers in seismic arrays may be absent or inoperative due to issues of receiver installation and malfunction. Thus, filling the gap of seismic interferometric profile requires wavefield reconstruction and regularization techniques. Compressive sensing (CS) is one such method that can reconstruct seismic interferometric wavefields and help mitigate the limitations by exploiting the sparsity of seismic waves. In our work, we use CS to reconstruct missing seismic interferometric wavefields. One can interpolate interferometric wavefields using correlograms provided by one VS. We call this method of reconstructing an individual VS gather single-source wavefield reconstruction. We propose an alternative technique called multi-source wavefield reconstruction, which applies CS to reconstruct multiple interferometric wavefields using a volume of VS gathers provided from all available VSs. Using numerical examples, we show that one can apply CS to recover interferometric wavefields resulting from interferometry of a linear seismic array. To exploit the sparsity of interferometric wavefields, we apply the Fourier and Curvelet transforms to the two reconstruction schemes. Using the signal-to-noise ratio (SNR) to compare reconstruction of interferometric wavefields, the Fourier multi-source method improves the recovery of interferometric wavefields by approximately 50 dB compared to the Fourier and Curvelet single-source wavefield reconstructions.

Ning, Ling; Xia, Jianghai; Dai, Tianyu; Zhang, Hao; Liu, Ya; Hong, Yu

2023 Geophysical Journal International

doi: 10.1093/gji/ggad343

SUMMARYStacking cross-correlations of time windows from continuous long-duration noise data is an effective solution to improve the quality of retrieved high-frequency (>1 Hz) surface waves and the accuracy of dispersion energy. The observation duration, however, is usually limited due to traffic control, making it difficult for ambient noise sources to fulfill the requirement of uniform distribution. Additionally, strong human-related noise sources exist near survey lines deployed along urban roads, which often act as interfering sources, such as local noise sources located in the non-stationary-phase zones. Local noise sources cause spurious arrivals in cross-correlations, degrade signal-to-noise ratio (SNR) of retrieved surface waves and distort their dispersion energy. To attenuate these adverse effects and improve the quality of surface waves retrieved from ultrashort noise data, we perform the eigendecomposition technique on the cross-spectral density matrix (CSDM) and apply a Wiener filter on the decomposed eigenvectors. The correct eigenvalues and the corresponding filtered eigenvectors are selected to reconstruct the CSDM related to stationary-phase sources based on the matched-field processing outputs. This procedure significantly suppresses the backpropagated signals and efficiently recovers surface waves by improving the contribution of the stationary-phase sources. We validate our scheme on a synthetic test and two practical applications and show that we obtain higher-SNR virtual shot gathers and higher-quality surface-wave dispersion images compared to seismic interferometry. Our scheme can be a new alternative technique to conduct passive seismic surveys in densely populated urban environments without being affected by local noise sources.

Hu, Jinyin; Phạm, Thanh-Son; Tkalčić, Hrvoje

2023 Geophysical Journal International

doi: 10.1093/gji/ggad348

SUMMARYDetermining the seismic moment tensor (MT) from the observed waveforms with available Earth's structure models is known as seismic waveform MT inversion. It remains challenging for small to moderate-size earthquakes at regional scales. First, because shallow isotropic (ISO) and compensated linear vector dipole (CLVD) components of MT radiate similar long-period waveforms at regional distances, an intrinsic ISO-CVLD ambiguity impedes resolving seismic sources at shallow depths within the Earth's crust. Secondly, regional scales usually bear 3-D structures; thus, inaccurate Earth's structure models can cause unreliable MT solutions but are rarely considered a theory error in the MT inversion. So far, only the error of the 1-D earth model (1-D structural error), apart from data errors, has been explicitly modelled in the source studies because of relatively inexpensive computation. Here, we utilize a hierarchical Bayesian MT inversion to address the above problems. Our approach takes advantage of affine-invariant ensemble samplers to explore the ISO-CLVD trade-off space thoroughly and effectively. Station-specific time-shifts are also searched for as free parameters to treat the structural errors along specific source–station paths (2-D structural errors). Synthetic experiments demonstrate the method's advantage in resolving the dominating ISO components. The explosive events conducted by the Democratic People's Republic of Korea (DPRK) are well-studied, and we use them to demonstrate highly similar source mechanisms, including dominating ISO and significant CLVD components. The recovered station-specific time-shifts from the blasts present a consistent pattern, which provides a better understanding of the azimuthal variation of Earth's 2-D structures surrounding the events’ location.

2023 Geophysical Journal International

doi: 10.1093/gji/ggad351

Ba, Jing; Fang, Zhijian; Fu, Li-Yun; Xu, Wenhao; Zhang, Lin

2023 Geophysical Journal International

doi: 10.1093/gji/ggad355

SUMMARYWave propagation in anelastic rocks is a relevant scientific topic in basic research with applications in exploration geophysics. The classical Biot theory laid the foundation for wave propagation in porous media composed of a solid frame and a saturating fluid, whose constitutive relations are linear. However, reservoir rocks may have a high-viscosity fluid, which exhibits a non-Newtonian (nN) behaviour. We develop a poroelasticity theory, where the fluid stress-strain relation is described with a Kelvin–Voigt mechanical model, thus incorporating viscoelasticity. First, we obtain the differential equations from first principles by defining the strain and kinetic energies and the dissipation function. We perform a plane-wave analysis to obtain the wave velocity and attenuation. The validity of the theory is demonstrated with three examples, namely, considering a porous solid saturated with a nN pore fluid, a nN fluid containing solid inclusions and a pure nN fluid. The analysis shows that the fluid may cause a negative velocity dispersion of the fast P(S)-wave velocities, that is velocity decreases with frequency. In acoustics, velocity increases with frequency (anomalous dispersion in optics). Furthermore, the fluid viscoelasticity has not a relevant effect on the wave responses observed in conventional field and laboratory tests. A comparison with previous theories supports the validity of the theory, which is useful to analyse wave propagation in a porous medium saturated with a fluid of high viscosity.

Hosseini, Samira; Thomas, Christine; Garnero, Edward J; Abreu, Rafael

2023 Geophysical Journal International

doi: 10.1093/gji/ggad357

SUMMARYStructure of the inner core is often measured through traveltime differences between waves that enter the inner core (PKPdf) and waves that travel through the outer core only (PKPab and PKPbc). Here we extend the method to converted waves PKSdf and SKPdf and compare results to PKP wave measurements. PKSdf and SKPdf have a very similar path to PKPdf and if velocity variations are present in the inner core, all three wave types should experience them equally. Since traveltime deviations can be due to velocity changes (either isotropic or anisotropy) as well as wave path deviations born from heterogeneity, we simultaneously investigate wave path directions and traveltimes of PKP, SKP and PKS waves for several source-array combinations. One of the path geometries is the anomalous polar corridor from South Sandwich to Alaska, which has strong traveltimes anomalies for PKPdf relative to more normal equatorial path geometries. Here we use array methods and determine slowness, traveltime and backazimuth deviations and compare them to synthetic data. We find that path deviations from theoretical values are present in all wave types and paths, but also that large scatter exists. Although some of the path deviations can be explained by mislocation vectors and crustal variations, our measurements argue that mantle structure has to be considered when investigating inner core anisotropy. Our polar path data show similar traveltime residuals as previously published, but we also find slowness residuals for this path. Interestingly, SKPdf and PKSdf for the South Sandwich to Alaska path show traveltime residuals that are partly opposite to those for PKPdf, possibly due to an interaction with a localized ultra-low velocity zone where waves enter the core.

Pertuz, Tatiana; Malehmir, Alireza

2023 Geophysical Journal International

doi: 10.1093/gji/ggad346

SUMMARYWe studied the benefits of a nine-component (9C) seismic survey over a landslide-prone area in southwest of Sweden to retrieve ultrahigh-resolution shear wave reflection images of the subsurface as well as crucial information on physical properties of the sediments. A complete, 1 m shot and receiver spacing, multicomponent 2-D seismic profile was acquired using three-component microelectromechanical-system-based landstreamer receivers, and a 5-kg sledgehammer strike in vertical and horizontal orientations as three-component seismic source. Given the rich number of shear wave reflections observed on all the 9C data, the processing work focused on their retrievals. It revealed three distinct reflections, two of which associated with coarse-grained materials and one with an extremely undulating bedrock surface. Given the extremely slow shear wave velocities on the order of 60–100 m s−1, we obtained ultrahigh-resolution shear wave sections avoiding temporal and spatial aliasing. Imaging results suggest vertical-source and horizontal-radial receiver (V–HR), and horizontal-transverse source–receiver orientations (HT–HT) provided the most optimum images of the subsurface. A non-hyperbolic algorithm was applied to the normal-moveout corrections justified by the traveltime differences of the bedrock reflection in different shear wave sections. The improved images by incorporating the anisotropy term suggest that the data set reveals moderate shear wave anisotropy along some portions of the profile. The Vp/Vs ratio obtained by using bedrock reflection in P- and S-wave sections suggests values ranging 10–16, which implies high water content. Areas with lower Vp/Vs coincides with greater anisotropic parameters and this can indicate disturbed clays or presence of sensitive clays.

Cheng, Guangsen; He, Chuanlin; Liang, Zhanyuan; Yin, Xingyao; Zhang, Xiaoyu; Zheng, Yi

2023 Geophysical Journal International

doi: 10.1093/gji/ggad349

SUMMARYAlthough subsurface media are usually assumed to be isotropic, anisotropy is ubiquitous in crustal rocks and leads to the variation of seismic response with direction. Transversely isotropic media with a vertical symmetry axis (VTI media) are widely found in the real world, such as in textured shale reservoirs. Plane-wave reflection coefficients (PRCs) in VTI media have been widely exploited in amplitude variation with offset (AVO) inversion to estimate the elastic and anisotropy parameters of subsurface media. However, the PRCs in VTI media meet some fundamental problems, especially at near-critical or post-critical incidence angles where the spherical-wave effect is significant. To consider the wave front curvature, a complex spherical-wave reflection coefficient (SRC) in VTI media is derived. To better understand the spherical-wave seismic response in VTI media, we investigate the dependence of the complex SRC on frequency, reflector depth and Thomsen anisotropy parameters ($\varepsilon $ and $\delta $). Based on a complex convolution model, a spherical-wave AVO inversion approach in VTI media is proposed to estimate the vertical (symmetry-axis) compressional and shear wave velocities (P and S waves), density and Thomsen anisotropy parameters from observed seismic data with different incidence angle and frequency components. Synthetic data with Gaussian random noise are used to verify the robustness of the spherical-wave AVO inversion approach in VTI media. Field data examples show that the proposed approach can produce reasonable inversion results that match well with the well-logging data.

Shen, Junzhu; Zhu, Tieyuan

2023 Geophysical Journal International

doi: 10.1093/gji/ggad352

SUMMARYExtreme weather events threaten life and property in populated areas. Timely and precise weather event monitoring and risk assessment are critical, but can be hampered by limited meteorological stations in cities. Recent studies have shown that seismic stations are sensitive to storm-induced noise. This study aims to investigate the sensitivity of distributed acoustic sensing (DAS), a technology capable of turning existing optical fibres into dense seismic sensors, for recording storm-induced seismic noise. We analyse 4-month continuous DAS recordings (June–September 2021) from a 4.2-km-long underground fibre-optic array in State College, PA. We calibrate the DAS data by comparing it to various meteorological data (rainfall and wind speed) from nearby weather stations. We first show DAS-recorded low-frequency wind-induced noise (0.5–8 Hz) probably caused by light poles swaying in the wind, as observed resonant frequencies agree with theoretical natural frequencies of nearby light poles. We find a strong linear correlation between DAS energy and wind speed. We further characterize rain-induced noise. Detailed observations from two rain events: a moderate rain and a heavy rain from Hurricane Ida, suggest that rain-induced noise is not generated by direct raindrops hitting the ground. Instead, the low-frequency noise (2–8 Hz) is attributed to the acoustic noise generated by water flow in stormwater drainage systems. Strong high-frequency noise up to 125 Hz is likely related to the rapid rainwater filling from the surface to the drainage system during heavy rain. We show linear relations between rain-induced DAS energy and rainfall rate, where the slopes of relations are related to the volume of rainwater, suggesting the influence of surface water and rainwater flow in the drainage system on DAS signals. Our results show the possibility of using DAS-equipped pre-existing telecom fibre-optic cables for sensing windstorms and rainstorms in urban areas and their interactions with urban infrastructures.

Aiman, Y A; Delorey, A A; Lu, Y; Bokelmann, G

2023 Geophysical Journal International

doi: 10.1093/gji/ggad353

SUMMARYThe orientation of SHmax is commonly estimated from in situ borehole breakouts and earthquake focal mechanisms. Borehole measurements are expensive, and therefore sparse, and earthquake measurements can only be made in regions with many well-characterized earthquakes. Here, we derive the stress-field orientation using stress-induced anisotropy in nonlinear elasticity. In this method, we measure the strain derivative of velocity as a function of azimuth. We use a natural pump-probe (NPP) approach which consists of measuring elastic wave speed using empirical Green’s functions (probe) at different points of the earth tidal strain cycle (pump). The approach is validated using a larger data set in the Northern Alpine Foreland region where the orientation of maximum horizontal compressive stress is known from borehole breakouts and drilling-induced fractures. The technique resolves NNW-SSW to N-S directed SHmax which is in good agreement with conventional methods and the recent crustal stress model. We confirm that the NPP method can be applied to dense large-scale seismic arrays. The technique is then applied to the Southern Alps to understand the contemporary stress pattern associated with the ongoing deformation due to counterclockwise rotation of the Adriatic plate with respect to the European plate. Our results explain why the two major faults in Northeastern Italy, the Giudicarie Fault and the Periadriatic Line (Pustertal–Gailtal Fault) are currently inactive, while the currently acting stress field allows faults in Slovenia to deform actively. We have demonstrated that the pump-probe method has the potential to fill in the measurement gap left by conventional approaches, both in terms of regional coverage and in depth.

Graf, M; Pail, R

2023 Geophysical Journal International

doi: 10.1093/gji/ggad356

SUMMARYIn this study, we develop a model that allows to combine gravimetric and geometric data. By the combination, we improve the spatial resolution of the resulting mass balance estimate compared to a purely gravimetric one. The equivalent ice or firn density of the changing ice volume is estimated within a mathematical inversion model, which includes geometric information about the volumetric change of the ice sheet and the resulting gravity change. This gravity change is computed from monthly GRACE gravity fields. They have a limited spatial resolution of a few 100 km, but allow direct conclusions about the true mass changes over Greenland. The ice-volume changes are described by a product by the Climate Change Initiative of European Space Agency, which is based on altimetry data. They have a very fine spatial resolution (down to a few km), but are not directly sensitive to mass changes. By combining both data sets in a common mathematical model, the advantages of both data types (direct sensitivity to mass versus high spatial resolution) are made use of. In this way, we improve the spatial resolution of mass balance estimates over Greenland. This leads to a map of mass trends, which has the same spatial resolution as the input map of geometric changes, but which is consistent with the input gravity fields. It will enable improving the localization of mass change signals of ice sheets and glaciers, which are usually rather small-scale. We compare our estimates to the results of complementary studies regarding the total mass loss of the Greenland ice sheet and its surrounding land surface. Our study leads to a value of $-213\pm 37\, \text{Gt}\,\text{a}^{-1}$ in the time span from 2011 to 2015. We also discuss the problem of separating the mass contribution of the Greenland ice sheet itself and its surrounding region.

Kuang, Wenhuan; Zhang, Jie; Zhang, Wei

2023 Geophysical Journal International

doi: 10.1093/gji/ggad350

SUMMARYMigration-based earthquake location methods may encounter the polarity reversal issue due to the non-explosive components of seismic sources, leading to an unfocused migration image. Such a problem usually makes it difficult to accurately retrieve the optimal location from the migrated source image. In this study, by taking advantage of the general pattern recognition ability of the convolutional neural network, we propose a novel deep-learning image condition (DLIC) to address this issue. The proposed DLIC measures the goodness of waveform alignments for both P and S waves, and it follows the geophysical principle of seismic imaging that the best-aligned waveforms represent fully a best-imaged source location. A synthetic test shows that the DLIC can effectively overcome the polarity reversal issues. Real data applications to southern California show that the DLIC can enhance the focusing of the migrated source image over the classic source scanning algorithm. Further tests show that the DLIC applies to continuous seismic data, to regions with few previously recorded earthquakes, and has the potential to locate small earthquakes. The proposed DLIC shall benefit the migration-based source location methods.

Chen, Shaolin; Shen, Jirong; Zhang, Jiao; Cheng, Shulin; Sun, Jie

2023 Geophysical Journal International

doi: 10.1093/gji/ggad358

SUMMARYNear-field seismoacoustic scattering must be considered across various domains, including marine seismic exploration, ocean acoustics and marine seismic engineering. This is a complex process due to the fluid–solid interaction between seawater and the seabed, particularly when the seabed is saturated with fluid. The interaction between sea fluid, saturated porous seabed and solid bedrock must also be considered. In this study, seawater and dry bedrock are treated as generalized saturated porous media with porosity of one and zero, respectively. The coupling between seawater, saturated seabed and dry bedrock can be analysed within a unified framework of generalized saturated porous media. Therefore, we proposed an efficient, unified method to address the challenges posed by near-field seismoacoustic scattering. This method comprises free field wave motion computation, which is used to provide input for scattering analysis. It also introduces a unified computational framework for modelling the wave propagation in the water-saturated seabed-bedrock system, and local transmitting boundary are used to account for the effect of an infinite domain. First, the differential equation of the generalized saturated porous media is discretized using lumped mass-based FEM, and the ordinary differential equation is integrated in time using an explicit scheme. Then, the equations for the motion of the nodes on the interface between two generalized saturated porous media with various porosity are derived. These equations are suitable for special cases such as fluid–solid interface, fluid-saturated porous media interface and saturated porous media-solid interface. To demonstrate the validity and feasibility of the proposed approach, a 1-D problem is considered, and the obtained response is verified using an analytical solution. Then, we compute the cases of a vertically incident plane P wave onto a 2-D basin-like fluid–solid structure, and compared the synthetic seismograms with results reported by other researchers. In this study, the findings of our proposed approach satisfy the continuity requirements at the interface and are consistent with those obtained using the reflection/transmission matrix method. Additionally, a 3-D site with basin-like terrain was analysed. The proposed approach treats the fluid, saturated porous media and solid in a unified framework, and has high efficiency due to lumped mass matrix-based explicit finite element and local transmitting artificial boundary. Furthermore, our approach can be easily implemented in parallel, making it suitable for solving large-scale seismoacoustic scattering problems.