Seismic Wavelength Research Articles

Mathematical modelling for seismic affected zoning of tunnel cavity section under SH wave incidence and shaking table verification

Tunnel portals are particularly vulnerable during seismic events due to the influence of adjacent slope geometry and fractured rock formations. This study evaluates the seismic response of tunnel portals and determines an appropriate fortification range. Ray theory is employed to calculate the displacement field of a single-sided slope subjected to incident SH waves, while the tunnel entrance is modeled as an elastic foundation beam. An analytical expression for hoop strain is derived, accounting for the interaction between the tunnel and the surrounding rock, as well as the attenuation of seismic waves. A parametric analysis is also performed to examine the impact of slope angle, seismic wavelength, frequency, and foundation stiffness on the tunnel response. Shaking table tests are conducted to validate the theoretical results and reveal failure patterns in both the slope and tunnel. The findings show that the spandrel and arch foot exhibit the highest hoop strain responses, identifying these positions as critical points of seismic vulnerability. The seismic-affected zones of the tunnel portal are classified into three distinct regions based on the distribution of peak hoop strain. The recommended fortification range for the tunnel portal extends up to 7.5 times the tunnel span, effectively encompassing the areas of peak strain to mitigate potential damage.

Pseudo-static Winkler springs for longitudinal underground structures subjected to shear waves

ABSTRACT Shear waves in underground longitudinal structures impose deformations that need to be accounted for in structural design. Simplified approaches for such structures typically consider the Winkler foundation model, assuming Euler-Bernoulli beam theory, which neglects shearing-induced distortions, especially significant when shear waves are a dominant component of the seismic motion. In order to overcome these limitations, Timoshenko beam models have been proposed in the literature. These approaches however depend on an appropriate determination of the ground springs. Existing analytical formulations often assume plane-strain conditions, inadequate for representing low frequencies and thus are not directly applicable for pseudo-static interaction analyses. The present paper develops analytical solutions to determine transverse and rotation Winkler springs for structures subjected shear waves. The proposed springs overcome the drawbacks of plane-strain models and can be construed as a generalisation of them. The springs are obtained as a function of the seismic wavelength and the ground-structure stiffness contrast. Results obtained are validated against solutions from the literature and numerical results from a full 3D finite-element model. A non-dimensional parametric study is also presented, that allow an expedited evaluation of ground springs for practical applications.

Strain field reconstruction from helical-winding fiber distributed acoustic sensing and its application in anisotropic elastic reverse time migration

Optical fiber-based distributed acoustic sensing (DAS) technology has been a popular seismic acquisition tool due to its easy deployment, wide bandwidth, and dense sampling. However, the sensitivity of straight optical fiber to only single-axis strain presents challenges in fully characterizing multicomponent seismic wavefields, making it difficult to use these data in elastic reverse time migration (ERTM). The helical-winding fiber receives projecting signals projected onto the fiber from all seismic strain field components and has the potential to reconstruct those strain components for ERTM imaging. Here, we give detailed mathematical principles of helical fiber-based DAS with crucial parameters such as pitch angle, gauge length, and rotating angle. At least six points of DAS responses are required in one or several winding periods to rebuild the strain fields within the seismic wavelength. The projecting matrix of conventional regular helical-winding fiber is singular and ill conditioned, which results in computation challenges for the inverse of the Hessian matrix for strain component reconstruction. To tackle this problem, we develop a nonregular variant pitch-angle winding configuration for helical fiber. Our winding design is validated using the rank and condition number of the projecting matrix, which is proven to be an important tool in the reconstruction of the original seismic strains. The recovered strain components from the DAS response are then used to backward propagate the receiver wavefields in ERTM with an efficient P/S decoupled approach. To summarize, we develop a novel winding design of helical fiber to recover the strain fields and then develop an efficient 3D anisotropic P/S wave-mode decomposition method for generating vector P and S wavefields during their propagation. Both methods are applied to build an anisotropic DAS-ERTM workflow for producing PP and PS images. Two synthetic examples demonstrate the effectiveness of our approach.

Spatial Heterogeneity of Pore Structure in the Crustal Section of the Samail Ophiolite: Implications for High VP/VS Anomalies in Subducting Oceanic Crust

AbstractSeismic surveys along subduction zones have identified anomalously high ratio of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust that are possibly due to the presence of pore water. Such interpretations postulate that the pore structure is homogeneous at the scale of the seismic wavelength. Here we present the first statistical evidence of a heterogeneous pore structure in oceanic crust at scales larger than laboratory samples. The spatial correlation of measured bulk density profiles of the crustal section of the Samail ophiolite suggests that the pore structure is heterogeneous at scales smaller than ∼1 m. Wave‐induced fluid flow cannot follow the loading during the seismic wave propagation at this estimated heterogeneity, which implies that fluid‐filled microscopic pores and cracks have a limited impact on the observed high VP/VS anomalies in the subducting oceanic crust. Large‐scale cracks may therefore play an important role in shaping these anomalies.

Regional stratigraphically discordant dolomite mapping based on a dolomitization mechanism on the Arabian carbonate shelf

Dolomite mapping, as a first step of carbonate diagenetic modeling, is critical to understanding dolomitization mechanisms and predicting reservoir quality. Stratigraphically discordant dolomite bodies within the Upper Jurassic intervals have long been studied on the Arabian shelf. Our dolomitization mechanisms find that faults/fractures serve as migration conduits and determine dolomite distribution by controlling fluid flow. We establish an innovative approach based on the diagenetic mechanisms and comprehensive characterization of fracture systems at multiple scales to delineate dolomite in the study area. The complex fracture networks in the subsurface are categorized into macroscale, mesoscale, and microscale fractures. Macroscale fractures, i.e., faults, are much greater than the seismic wavelength and are easily observed in seismic sections due to their obvious seismic discontinuities. Mesoscale fractures are slightly greater than or equal to the seismic wavelength and are recognized using seismic attributes. Microscale fractures are significantly smaller than the seismic wavelength and are observed mainly on core samples and thin sections. The dolomite mapping workflow includes three steps: (1) calibrate the seismic response of multiscale fractures with borehole image logs and core interpretations, (2) implement multiscale fracture characterization using multiple seismic attributes, and (3) interpret dolomite bodies based on fracture characterization and log identification. The mapping results show that massive dolomite bodies are heterogeneously developed and distributed mainly in the northeastern part of the study area, with a southward decrease in dolomite content. The application demonstrates that multiscale fracture systems play critical roles in massive dolomitization, and multiscale fracture prediction provides an innovative solution to delineate complex dolomitization systems and the combination of reservoir and seal in diagenetic traps.

Analysis of the acoustic seismic energy flux propagated through synthetic models formed by different structures

AbstractSeismic‐acoustic wavefield propagations on synthetic 2D models formed by different structural backgrounds are analysed in four test cases. These test cases were carried out employing staggered‐grid finite differences and absorbing boundaries of the convolutional perfectly matched layers type, which allowed analysing in a time of 7.0 s: (a) the seismic‐acoustic wavefield behaviour, (b) the kinetic seismic energy flux and potential seismic energy flux and (c) the energy differences (ΔKEF/ΔPEF) due to each type of proposed structure. In the first two test cases, a relationship was observed between average kinetic seismic energy flux/potential seismic energy flux values and the different structural densities ε of each synthetic model, which were formed by a similar type of structure (fractures). The average kinetic seismic energy flux values increased when ε decreased, whereas the average potential seismic energy flux values decreased when ε also decreased. The largest ΔKEF/ΔPEF values were observed for those models with more structures, reaching up to 0.2 × 108/0.21 × 108 J/m2 in the first test case and up to 1.95 × 108/1.91 × 108 J/m2 in the second test case. Meanwhile, for those models with less number of structures was 0.04 × 108/0 J/m2 in test case 1 and 0.26 × 108/0.61 × 108 J/m2 in test case 2. In the third test case, a relationship was observed between the average potential seismic energy flux values and the ε values, but not with the average kinetic seismic energy flux values, the synthetic models of which were formed by different types of structures (lobes and fractures under different shapes and sizes). The kinetic seismic energy fluxes were more affected by structures closer to each other than by those further apart. Meanwhile, the potential seismic energy fluxes were more affected by larger structures, maintaining a relationship with the seismic frequency and wavelength. The largest ΔKEF/ΔPEF values reached up to 3.42 × 108/3.24 × 108 J/m2 in model with the highest ε value, whereas up to 0.49 × 108/0.67 × 108 J/m2 in the model with the lowest ε value. In the fourth test case, similar types of structures (fractures) were analysed under different orientations in each model. The kinetic seismic energy fluxes/potential seismic energy fluxes were higher in the model formed by better‐oriented structures or parallel with respect to the propagation direction of the seismic‐acoustic wavefield. These energy differences between each test case show how not only the number of structures in a model and the ε values influenced the seismic‐acoustic wavefield and the kinetic seismic energy flux/potential seismic energy flux, but also the structural aspects, the intrinsic elastic properties (e.g. the density used in structures), location (separation between structures) and orientation of structures.

Amplification effect of near-field ground motion around deep tunnels based on finite fracturing seismic source model

Dynamic failure of rock masses around deep tunnels, such as fault-slip rockburst and seismic-induced collapse, can pose a significant threat to tunnel construction safety. One of the most significant factors that control the accuracy of its risk assessment is the estimation of the ground motion around a tunnel caused by seismicity events. In general, the characteristic parameters of ground motion are estimated in terms of empirical scaling laws. However, these scaling laws make it difficult to accurately estimate the near-field ground motion parameters because the roles of control factors, such as tunnel geometry, damage zone distribution, and seismic source parameters, are not considered. For this, the finite fracturing seismic source model (FFSSM) proposed in this study is used to simulate the near-field ground motion characteristics around deep tunnels. Then, the amplification effects of ground motion caused by the interaction between seismic waves and deep tunnels and corresponding control factors are studied. The control effects of four factors on the near-field ground motion amplification effect are analyzed, including the main seismic source wavelength, tunnel span, tunnel shape, and range of damage zones. An empirical formula for the maximum amplification factor ( α m ) of the near-field ground motion around deep tunnels is proposed, which consists of four control factors, i.e. the wavelength control factor ( F λ ), tunnel span factor ( F D ), tunnel shape factor ( F s ) and excavation damage factor ( F d ). This empirical formula provides an easy approach for accurately estimating the ground motion parameters in seismicity-prone regimes and the rock support design of deep tunnels under dynamic loads.

Interpretation method of tight sandstone based on seismic forward modeling: a case study of Permian in Southeast Ordos Basin

Based on Permian data and the reflection coefficient convolved 0° and 90° phase ricker wavelets form different phases of synthetic seismic sections. This paper theoretically discusses three aspects: one is Permian sandstone thickness and seismic amplitude, the second is tight sandstone (tuning) thickness and seismic wave frequency, and the last is tight sandstone and seismic wave phase. The result shows that when sandstone thickness is greater than seismic wavelength λ/4, the sandstone thickness can be determined through the time difference between the sandstone’s top and bottom Peaks and troughs. When sandstone thickness is less than λ/4, only the seismic reflection amplitude of the top surface of sandstone was used to determine the thickness of sandstone. In the 0° phase seismic model, peaks were located on the thin layer’s upper part and trough in the lower half, with no correspondence between the polar and lithology. In the 90° phase synthesis record, the thin sandstone layer roughly corresponds to the seismic reflection trough. 0° and 90° phase synthesis have the exact vertical resolution; the frequency variation can reflect sand body distribution. The optimal frequency makes the thickest layer of sandstone reach amplitude-frequency tuning (tuning frequency).

Undulating sediments of the Cape Fear submarine landslide system, offshore U.S. Atlantic margin: Sediment waves versus creep deformation

We interpret a region of undulatory sediments adjacent to a major headwall of the Cape Fear submarine landslide system offshore of North Carolina, USA, as sediment waves rather than creep or fault-related deformation. The wave package extends 19 km upslope from the S4 landslide headwall and thickens upslope from approximately 250 to 450 m. The field of undulating sediments displays the continuity of seismic horizons, upslope-migrating crests, downslope thinning, and wave heights and lengths of approximately 26 m and approximately 1 km, respectively, which are consistent with sediment wavefields. The Western Boundary Undercurrent formed contourites on the nearby Blake Ridge and it is possible that these sediment undulations were deposited via similar mechanisms. Ocean Drilling Program (ODP) cores near the sediment undulation field suggest that the turbidity currents also may have played a role in wave formation. Although most of the 19 km long field comprises unaltered sediment waves, we observe an approximately 5 km long zone adjacent to the landslide scarp that expresses evidence of faults that offset and deform the sediment wave strata. We interpret this deformation as the result of reduction in stress following the removal of the landslide mass. Given that the Cape Fear system has generated several episodes of potentially tsunamigenic slope failure, the future stability of the system is pertinent. Redefining these undulatory sediments as sediment waves eliminates a major slope instability mechanism of the system and is important for understanding the future slope stability hazards of Cape Fear. Our analysis highlights the importance of understanding sediment waves in hybrid submarine landslide-sediment wave systems. Geological feature: Cape Fear submarine landslide sediment waves Seismic appearance: Continuous undulating horizons Alternative interpretations: Downslope creep or faults Features with similar appearance: Extensional slope failure Formation: Alongslope and downslope currents Age: Quaternary Location: Cape Fear submarine landslide complex, offshore North Carolina Seismic data: High resolution multichannel seismic data Analysis tools: Multichannel seismic data, multibeam bathymetry, sub-bottom Chirp

Resolving Continental Magma Reservoirs With 3D Surface Wave Tomography

AbstractSurface wave tomography is widely used to improve our understanding of continental magma reservoirs that may be capable of fueling explosive volcanic eruptions. However, traditional surface wave tomography based on inversions for phase velocity maps and locally 1D shear velocity may have difficulty resolving strong 3D low‐velocity anomalies associated with crustal magma reservoirs. Here, we perform synthetic tomography experiments based on 3D seismic waveform simulations to understand how the limitations of surface wave tomography could affect interpretations of tomography in volcanic settings. We focus our modeling on the Yellowstone volcanic system, one of the largest and most thoroughly studied continental magmatic systems, and explore scenarios in which the maximum shear velocity anomaly associated with the crustal magma reservoir ranges between −10% and −66%. We find that even with the well‐instrumented setting near Yellowstone, the recovered shear velocity anomalies in the mid‐to‐upper crust are severely diminished due to the small spatial scale of the reservoir with respect to the seismic wavelengths that sample it. In particular, recovered VS anomalies could be reduced by a factor of two or more, implying that the inferred melt fraction of large‐scale continental magma reservoirs may be considerably underestimated.

Longitudinal vibrations of underground pipelines of finite length with account for the node weight on the ends

The paper presents an analysis of the dynamic response of an underground main pipe under the action of longitudinal wave, propagating in soil along the pipe. The outer surface of the pipeline is in contact with soil along the pipeline axis according to the elastic- viscous law, and the ends of the pipeline are connected to massive nodes by elastic elements. Using the Fourier method, an analytical solution to the problem of longitudinal vibration of an underground pipeline, pliantly fixed by nodes at its ends, is obtained. The problem of longitudinal vibration of an underground pipeline with visco-elastically fixed nodes at the ends is solved numerically. Longitudinal wave in soil is taken as a traveling sine wave. The paper presents a comparative analysis of the results for some values of seismic wavelengths, the ratio of the wave propagation velocity in soil and in the pipeline, the coefficients of elastic and viscous interaction. The viscosity coefficient of interaction at the "pipe-soil" system contact leads to attenuation of the wave front in the underground pipeline. For soils, the values of the viscous interaction coefficient of more than 100 kN⋅s/m2, may lead to complete attenuation of the bursts at the wave front in the pipeline.

Simplified Analytical Solutions for Deep Tunnels Subjected to vertically incident shear wave with arbitrary vibration direction

A circular tunnel in a homogeneous infinite ground subjected to a vertically incident shear wave with an arbitrary vibration direction in the horizontal plane is considered in this study. A simplified analytical solution with explicit formulations for the deformation and stress of the tunnel lining is presented here. The superposition of two solutions under SH and SV waves are derived first. In the analytical solutions, the tunnel lining is taken as a thick-walled cylinder, which enables more precise forecasts and is much closer to practical engineering than the thin shell model adopted in other previous solutions. Moreover, the analytical solutions consider the slippage effect at the ground-lining interface by introducing a spring-type flexibility coefficient into the interaction force-displacement relationship. The results of the analytical solutions are verified by a comparison with the results of the dynamic numerical solutions via a 3-D ground-lining interaction model under earthquakes; the results show good agreement. By using the analytical solutions, the effects of the vibration direction, ground condition, lining thickness and flexibility coefficient on the seismic response of the tunnel are investigated. Additionally, the appropriate range of seismic wavelengths for the analytical solutions is discussed. The proposed solutions can be applied to the seismic analysis and design of tunnel structures.

An integrated long‐wavelength statics method applied to seismic processing of Tibetan permafrost

ABSTRACTPermafrost in high‐altitude regions complicates near‐surface modelling and exacerbates static effects during seismic processing and interpretation, especially in the form of long‐wavelength issues in cases where it extends laterally beyond the length of the seismic wavelength. The long‐wavelength component of static corrections (statics) for permafrost when solved using a single conventional method is prone to seismic imaging artefacts due to limited field layouts and unmet preconditions. The nonlinearity between the velocities and the thickness from low‐velocity near surface to high‐velocity bedrocks ulteriorly complicates near‐surface modelling for long‐wavelength statics by first‐arrival inversion. Combining the advantages of as much long‐wavelength characteristics as possible associated with the permafrost and rugged topography, we estimate an integrated long‐wavelength statics method including tomographic inversion and refraction residual statics. The integrated method can approximate linearized near‐surface modelling through multi‐step processing. An example from the Qiangtang Basin (central Qinghai–Tibet Plateau) suggests that the integrated method is capable of gradual refining complex near‐surface modelling and long‐wavelength statics and eliminates upward‐stretching abnormalities caused by Tibetan permafrost.

Extraction of diffractions from seismic data using convolutional U-net and transfer learning

Diffraction images can be used for modeling reservoir heterogeneities at or below the seismic wavelength scale. However, the extraction of diffractions is challenging because their amplitude is weaker than that of overlapping reflections. Recently, deep-learning (DL) approaches have been used as a powerful tool for diffraction extraction. Most DL approaches use a classification algorithm that classifies pixels in the seismic data as diffraction, reflection, noise, or diffraction with reflection and takes whole values for the classified diffraction pixels. Thus, these DL methods cannot extract diffraction energy from pixels for which diffractions are masked by reflections. We have developed a DL-based diffraction extraction method that preserves the amplitude and phase characteristics of diffractions. Through the systematic generation of a training data set using synthetic modeling based on t-distributed stochastic neighbor embedding analysis, this technique extracts not only faint diffractions but also diffraction tails overlapped by strong reflection events. We also determine that the DL model pretrained with a basic synthetic data set can be applied to seismic field data through transfer learning. Because the diffractions extracted by our method preserve the amplitude and phase, they can be used for velocity model building and high-resolution diffraction imaging.

Active ocean–continent transform margins: seismic investigation of the Cayman Trough-Swan Island ridge-transform intersection

SUMMARY The southern boundary of the Cayman Trough in the Caribbean is marked by the Swan Islands transform fault (SITF), which also represents the ocean–continent transition of the Honduras continental margin. This is one of the few places globally where a transform continental margin is currently active. The CAYSEIS experiment acquired an ∼165-km-long seismic refraction and gravity profile (P01) running across this transform margin, and along the ridge-axis of the Mid-Cayman Spreading Centre (MCSC) to the north. This profile reveals not only the crustal structure of an actively evolving transform continental margin, that juxtaposes Mesozoic-age continental crust to the south against zero-age ultraslow spread oceanic crust to the north, but also the nature of the crust and uppermost mantle beneath the ridge-transform intersection (RTI). The traveltimes of arrivals recorded by ocean-bottom seismographs (OBSs) deployed along-profile have been inverse and forward modelled, in combination with gravity modelling, to reveal an ∼25-km-thick continental crust that has been continuously thinned over a distance of ∼65 km to ∼10 km adjacent to the SITF, where it is juxtaposed against ∼3–4-km-thick oceanic crust. This thinning is primarily accommodated within the lower crust. Since Moho reflections are only sparsely observed, and, even then, only by a few OBSs located on the continental margin, the 7.5 km s–1 velocity contour is used as a proxy to locate the crust–mantle boundary along-profile. Along the MCSC, the crust–mantle boundary appears to be a transition zone, at least at the seismic wavelengths used for CAYSEIS data acquisition. Although the traveltime inversion only directly constrains the upper crust at the SITF, gravity modelling suggests that it is underlain by a higher density (>3000 kg m–3) region spanning the width (∼15 km) of its bathymetric expression, that may reflect a broad region of metasomatism, mantle hydration or melt-depleted lithospheric mantle. At the MCSC ridge-axis to the north, the oceanic crust appears to be forming in zones, where each zone is defined by the volume of its magma supply. The ridge tip adjacent to the SITF is currently in a magma rich phase of accretion. However, there is no evidence for melt leakage into the transform zone. The width and crustal structure of the SITF suggests its motion is currently predominantly orthogonal to spreading. Comparison to CAYSEIS Profile P04, located to the west and running across-margin and through 10 Ma MCSC oceanic crust, suggests that, at about this time, motion along the SITF had a left-lateral transtensional component, that accounts for its apparently broad seabed appearance westwards.

High‐Precision Earthquake Location Using Source‐Specific Station Terms and Inter‐Event Waveform Similarity

AbstractEarthquake monitoring and many seismological studies depend on earthquake locations from phase arrival‐times. We present an extended, arrival‐time earthquake location procedure (NLL‐SSST‐coherence) which approaches the precision of differential‐timing based, relative location methods. NLL‐SSST‐coherence is based on the probabilistic, global‐search NonLinLoc (NLL) location algorithm which defines a probability density function (PDF) in 3D space for hypocenter location and is highly robust to outlier data. NLL‐SSST‐coherence location first reduces velocity model error through iteratively generated, smooth, source‐specific, station travel‐time corrections (SSST). Next, arrival‐time error is reduced by consolidating location information across events based on inter‐event waveform coherency. If the waveforms at a station for multiple events are very similar (have high coherency) up to a given frequency, then the distance separating these “multiplet” events is small relative to the seismic wavelength at that frequency. NLL‐coherence relocation for a target event is a stack over 3D space of the NLL‐SSST location PDF for the event and the PDF's for other multiplet events, each weighted by its waveform coherency with the target. NLL‐coherence relocation requires waveforms from only one or a few seismic stations, enabling precise relocation with sparse networks, for foreshocks and early aftershocks of significant events before installation of temporary stations, and for older data sets with few waveform data. We show the behavior and performance of NLL‐SSST‐coherence with synthetic and ground‐truth tests, and through application and comparison to relative locations for California earthquake sequences with dense and sparse station coverage.

Sensitivity Analysis of FWI Applied to OVSP Synthetic Data for Fault Detection and Characterization in Crystalline Rocks

We have performed several sensitivity studies to assess the ability of the Full Wave Inversion method to detect, delineate and characterize faults in a crystalline geothermal reservoir from OVSP data. The distant goal is to apply the method to the Soultz-sous-Forêts site (France). Our approach consists of performing synthetic Full Wave 2D Inversion experiments using offset vertical seismic and comparing the estimated fields provided by the inversion, i.e., the estimated underground images, to the initial reference model including the fault target. We first tuned the inversion algorithmic parameters in order to adapt the FWI software, originally dedicated to a sedimentary context, to a crystalline context. In a second step, we studied the sensitivity of the FWI fault imaging results as a function of the acquisition geometry parameters, namely, the number of shots, the intershot distance, the maximum offset and also the antenna length and well deviation. From this study, we suggest rules to design the acquisition geometry in order to improve the fault detection, delineation and characterization. In a third step, we studied the sensitivity of the FWI fault imaging results as a function of the fault or the fault zone characteristics, namely, the fault dip, thickness and the contrast of physical parameters between the fault materials and the surrounding fresh rocks. We have shown that a fault with high dip, between 60 and 90° as thin as 10 m (i.e. lower than a tenth of the seismic wavelength of 120 m for Vp and 70 m for Vs) can be imaged by FWI, even in the presence of additive gaussian noise. In summary, for a crystalline geological context, and dealing with acceptable S/N ratio data, the FWI show a high potential for accurately detecting, delineating and characterizing the fault zones.

Seismoelectric and Electroseismic Modeling in Stratified Porous Media With a Shallow or Ground Surface Source

AbstractFor a shallow or ground surface source and receiver at the same level or close depth, it is very difficult or computationally inefficient to simulate seismoelectric or electroseismic wave‐fields in stratified porous media by current reflectivity methods, such as the Luco‐Apsel‐Chen generalized reflection and transmission method (LAC GRTM). In this work, the peak‐trough averaging method which has been proved effective and efficient in dealing with this kind of computational problem is adopted to update the seismoelectric and electroseismic modeling algorithm based on LAC GRTM. After thoroughly verifying the accuracy and computational efficiency of the updated algorithm, we utilize it to numerically investigate both the electroseismic and seismoelectric couplings. Snapshots of electroseismic wave‐fields indicate evanescent electroseismic conversion, a reverse process of evanescent seismoelectric conversion, dominates at relatively larger ratios of seismic wavelength to interface depth, whereas the interfacial radiation electroseismic conversion is more prominent for the opposite situation. Our seismoelectric modeling results demonstrate that electric signals can arrive at the ground surface a few milliseconds earlier than their causative seismic signals due to evanescent seismoelectric conversion. This is the first modeling result considering source‐receiver geometries on the surface capable of explaining similar phenomena reported in geophysical field observations of seismically induced electrokinetic effects over a long history. The updated algorithm offers an accurate and efficient tool for forward modeling and will benefit interpretations of field observations as well as future inversion studies.

Insights from synthetic seismogram modelling study of oceanic lower crust and Moho Transition Zone

The Moho discontinuity is one of the fundamental boundaries on Earth, separating the crust from the upper mantle. Arrival times of seismic reflection and refraction data are used to estimate the depth of the Moho, but the nature of the Moho remains illusive. We present a synthetic seismogram modelling study for a suite of possible velocity models of the oceanic lower crust and Moho transition zone (MTZ). For modelling, we have used a 45-km long multichannel streamer (MCS) geometry, to get insight about waveforms and amplitudes of seismic phases like the crustal refraction (Pg), Moho reflection (PmP) and mantle refraction (Pn), and the interactions between these phases. We find that the nature of PmP reflection not only depends on the velocity structure of the MTZ but also on the velocity structure of the lower crust and upper mantle. The triplication resulting from these three seismic arrivals and its lateral extent mainly depend on the velocity structure in the lower crust. Apart from the velocity in the crust, the critical distance for the PmP arrival depends on the velocity gradient in the MTZ. The amplitude versus offset analysis of the wide-angle PmP arrivals indicates that there are two broad high amplitude peaks for the MTZ thickness less than or equal to the dominant seismic wavelength and only one large narrow peak for a thicker MTZ. Consequently, strong PmP arrivals for a thick MTZ lie in a narrow offset range around the critical offset whereas they lie over a large offset range for a sharp Moho. We also have modelled the synthetic seismograms for velocity structures from the Oman and the Bay of Islands ophiolites. Our modelling results indicate that the quantitative analysis of amplitudes and waveforms of ultra-long offset MCS data could provide detailed information about the lower crust and MTZ.

Comparison of 2-D and 3-D full waveform inversion imaging using wide-angle seismic data from the Deep Galicia Margin

SUMMARY Full waveform inversion (FWI) is a data-fitting technique capable of generating high-resolution velocity models with a resolution down to half the seismic wavelength. FWI is applied typically to densely sampled seismic data. In this study, we applied FWI to 3-D wide-angle seismic data acquired using sparsely spaced ocean bottom seismometers (OBSs) from the Deep Galicia Margin west of Iberia. Our data set samples the S-reflector, a low-angle detachment present in this area. Here we highlight differences between 2-D, 2.5-D and 3-D-FWI performances using a real sparsely spaced data set. We performed 3-D FWI in the time domain and compared the results with 2-D and 2.5-D FWI results from a profile through the 3-D model. When overlaid on multichannel seismic images, the 3-D FWI results constrain better the complex faulting within the pre- and syn-rift sediments and crystalline crust compared to the 2-D result. Furthermore, we estimate variable serpentinization of the upper mantle below the S-reflector along the profile using 3-D FWI, reaching a maximum of 45 per cent. Differences in the data residuals of the 2-D, 2.5-D and 3-D inversions suggest that 2-D inversion can be prone to overfitting when using a sparse data set. To validate our results, we performed tests to recover the anomalies introduced by the inversions in the final models using synthetic data sets. Based on our comparison of the velocity models, we conclude that the use of 3-D data can partially mitigate the problem of receiver sparsity in FWI.