Compressional Wave Speed Research Articles

Adjoint Waveform Tomography for Crustal and Upper Mantle Structure of the Middle East and Southwest Asia for Improved Waveform Simulations Using Openly Available Broadband Data

ABSTRACT We present a new model of radially anisotropic seismic wavespeeds for the crust and upper mantle of a broad region of the Middle East and Southwest Asia (MESWA) derived from adjoint waveform tomography. The new model enables fully 3D simulations of complete three-component waveforms and provides improved fits that were not possible with previous models. We inverted over 32,000 waveforms from 192 earthquakes recorded by over 1000 openly available broadband seismic stations from permanent and temporary networks in the region with highly uneven coverage. Inversion iterations proceeded from the period band 50–100 s in six stages and 54 total iterations reducing the minimum period to 30 s. Our final model, MESWA, improves waveform fits compared to the starting and other models for both the data used in the inversion and an independent validation set of 66 events. Restitution tests indicate that the model resolves features in the central part of the model to depths of about 150 km. The new model reveals tectonic features imaged by other studies and methods but in a new holistic model of anisotropic shear and compressional wavespeeds (VS and VP, respectively) covering a larger domain with smaller scale length and amplified features. Examples include low crustal VS in the Tethyan belt and low mantle VS following divergent (Gulf of Aden, Red Sea) and transform (Dead Sea fault) margins of the Arabian plate. Low VS is imaged below Cenozoic volcanic centers of the Mecca–Madina–Nafud Line, Arabian Peninsula, and the Türkiye–Iran border region. Elevated VS tracks Makran subduction under southeast Iran with near vertical dip. MESWA could be used as a starting model for further improvements, say, using waveforms from in-country seismic networks that are not currently openly available and/or smaller-scale studies targeting a shorter period. The model could be used to improve earthquake hazard studies and nuclear explosion monitoring.

Manifestations of the weak shear rigidity of marine sediments in amplitudes and travel times of acoustic normal modes and lateral waves

Compressional-to-shear wave conversion at interfaces within unconsolidated marine sediments and interference of shear waves within thin sediment layers have been found to make significant contributions to sound attenuation [O. A. Godin, J. Acoust. Soc. Am. 149, 3586–3598 (2021)] and produce unexpectedly strong perturbations in the normal mode group speed even when the shear speed is small compared to the compressional wave speed. This paper extends the previous analysis to the lateral waves and their applications to characterize the compressional wave speed and attenuation in the seabed. Shear wave-induced contributions to horizontal refraction of normal modes over horizontally inhomogeneous seabed are modeled to characterize the bearing and travel time errors that arise in the fluid-bottom approximation. The acoustic travel time and amplitude effects of the shear waves are found to accumulate at different rates with the propagation range depending on the stability of the shear wave interference within the seabed. The ways to distinguish between the manifestations in acoustic observables of the weak shear rigidity in the seabed and the other environmental uncertainties are discussed for range-independent, range-dependent, and horizontally inhomogeneous shallow-water waveguides. [Work supported by ONR.]

Shear and Scholte wave measurements on surficial intertidal mudflat sediments using medical sonoelastography techniques

In low-energy environments, the water–sediment interface often contains a dynamic fluffy transition layer (FTL) of suspended organic and inorganic matter, which is transformed by complex physical, biological, and chemical processes. The FTL mechanical properties are important for understanding carbon cycle dynamics and for robust sediment acoustic characterization, but erodibility and size make it difficult to study non-invasively and at sufficient resolution. In medical ultrasound sonoelastography, shear waves are excited within the body either using the acoustic radiation force (ARF) or via external vibration at the skin surface. Sub-micrometer displacements are measured remotely by subsequent ultrasound imaging at up to 10 kHz frame rate with approximately 0.5 μm spatial resolution. In this study, sediment cores were extracted from the intertidal mudflats from the Great Bay Estuary, USA. Compressional wave speed and attenuation measurements were performed using a core logger. Shear and Scholte waves were excited in the FTL using ARF- and external-vibration-based techniques and the motion detected with clinical ultrasound imaging arrays. Upward refraction of the shear wave was observed, and shear and Scholte wave speed and attenuation were measured. Finally, scanning electron microscopy was used to image the microstructure. Comparison between wave properties and microstructure will be discussed.

Estimation of sediment compressional and shear properties in the New England Mud Patch using acoustic pressure and particle velocity data

New England Mud Patch was the site of the Seabed Characterization Experiment in multiple years (2017, 2021, and 2022). This site is characterized by a layer of very fine grained surficial sediment layer over sand. Analysis of broadband data measured on the ocean bottom recorders (OBXs) and hydrophones will be discussed. OBX is a geophone–hydrophone system which measures three components (two orthogonal horizontal components and a vertical component) in addition to acoustic pressure. Five OBXs were deployed on the seabed and four hydrophones were configured as a tetrahedral array on the bottom mounted Geosled. Data from these sensors will be analyzed and different arrivals corresponding to compressional, shear and interface wave (Scholte and love waves) presence will be examined. Inversion schemes will be implemented to estimate the compressional and shear wave speeds and attenuation. A priori information from seismic surveys and sediment cores will be incorporated into the inversion schemes. Outputs of these inversions will be compared with inversion results from other investigators based on different inversion techniques and using data from the same location. [Work Supported by Office of Naval Research.]

In situ measurements of sediment shear wave speed from the New England Mud Patch and shelf break areas using the acoustic coring system

The acoustic coring system (ACS) is a probe-equipped gravity corer that provides in situ measurements of compressional and shear wave speed and attenuation. During an April 2022 coring survey, the ACS was deployed at 36 locations within the New England Mud Patch and New England Shelf Break areas. Data from these measurements will be presented to characterize the depth-dependent structure of the geoacoustic seabed properties as well as their spatial variability. The in situ measurements will be interpreted in the context of stratigraphic layering measured by a seismic survey. Historically, in situ shear speed measurements of the seabed have proved difficult to obtain. From the 2022 survey, depth-dependent (up to 4 meters) shear speed profiles at 800 and 1200 Hz were produced from a subset of the 36 deployments. These results, along with methods for analysis, challenges in the measurements, and supplemental laboratory experiments will be discussed. The results will also be compared to measurements from previous experiments. [Work sponsored by ONR.]

An estimate of absolute shear-wave speed in the Earth’s inner core

Observations of seismic body waves that traverse the Earth’s inner core (IC) as shear (J) waves are critical for understanding the IC shear properties, advancing our knowledge of the Earth’s internal structure and evolution. Here, we present several seismological observations of J phases detected in the earthquake late-coda correlation wavefield at periods of 15–50 s, notably via the correlation feature I-J, found to be independent of the Earth reference velocity model. Because I-J is unaffected by compressional wave speeds of the Earth’s inner core, outer core, and mantle, it represents an autonomous class of seismological measurements to benchmark the inner core properties. We estimate the absolute shear-wave speed in the IC to be 3.39 ± 0.02 km/s near the top and 3.54 ± 0.02 km/s in the center, lower than recently reported values. This is a 3.4 ± 0.5% reduction from the Preliminary Reference Earth Model (PREM), suggesting a less rigid IC than previously estimated from the normal mode data. Such a low shear-wave speed requires re-evaluating IC composition, including the abundance of light elements, the atomic properties and stable crystallographic phase of iron, and the IC solidification process.

Approximate Inversion of a Class of Generalized Radon Transforms

Generalized Radon transforms (GRT) serve, for instance, as linear models for seismic imaging in the acoustic regime. They occur when the corresponding inverse problem is linearized about a known background compression wave speed (Born approximation). The resulting GRT is completely determined by this background velocity. In this work, we present an implementation of approximate inversion formulas for this class of GRTs proposed and analyzed in [Grathwohl et al., Inverse Problems, 34 (2018), 014002] and [Grathwohl et al., Inverse Problems, 34 (2018), 114001], where we restrict ourselves to layered background velocities in two dimensions. In a series of numerical experiments, we intensively test our implementation, reproducing theoretical predictions. Further, we drive the validity of the linearization to its limits.

Geophysical‐Geochemical Modeling of Deep Crustal Compositions: Examples of Continental Crust in Typical Tectonic Settings and North China Craton

AbstractThe chemical composition of the deep continental crust is key to understanding the formation and evolution of the continental crust. Constraining the chemical composition of present‐day deep continental crust is, however, limited by indirect accessibility. This paper presents a modeling method for constraining deep crustal chemical structures from observed crustal seismic structures. We compiled a set of published composition models for the continental crust to construct functional relationships between seismic wave speed and major oxide content in the crust. Phase equilibria and compressional wave speeds (VP) for each composition model were calculated over a range of depths and temperatures of the deep crust. For conditions within the alpha(α)‐quartz stability field, robust functional relationships were obtained between VP and major oxide contents of the crust. Based on these relationships, observed VP of the deep crust can be inverted to chemical compositions for regions with given geotherms. We provide a MATLAB code for this process (CalcCrustComp). We apply this method to constrain compositions from deep crustal VP of global typical tectonic settings and the North China Craton (NCC). Our modeling results suggest that the lower crust in subduction‐related and rifting‐related tectonic settings may be more mafic than platforms/shields and orogens. The low VP signature in the deep crust of the NCC can be explained by intermediate crustal compositions, higher water contents, and/or higher temperatures. The chemical structure obtained by this method can serve as a reference model to further identify deep crustal features.

Broadband Geoacoustic Inversions for Seabed Characterization of the New England Mud Patch

Geoacoustic inversions using broadband acoustic data acquired during the Seabed Characterization Experiment 2017 conducted in the New England Mud Patch area in March 2017 are presented in this article. The primary goal of the data inversions is to estimate the compressional wave speed and density profiles of the sediment layers. Both linear perturbative and nonlinear Bayesian inversion methods are utilized. These two inversion methods share the same principle of solution schemes, i.e., to minimize the difference between the signal acquired (or derived quantities) at the receiver and the prediction by the optimized bottom model. The differences between the two approaches are in the type of data utilized for inversion and on the inversion procedure used to determine the bottom geoacoustic properties. The linear inversion method uses the time difference of modal arrival, while the Bayesian inversion uses the bandpass-filtered sound pressure waveform. One of the study objectives is for the Bayesian inversion to provide a reference solution to the inverse problem and determine the ability of the linear inversion method to provide comparable results as compared with a more exhaustive search used in the Bayesian method. Another objective is to use the verified linear inversion method that is computationally faster to explore different models of sediment layering structure and to estimate the most appropriate bottom model for the experimental area.

In situ measurements of compressional and shear wave speed from the New England Mud Patch and Shelf Break using the Acoustic Coring System

In situ measurements of geoacoustic properties provide direct characterization of the seabed at near ambient conditions. The Acoustic Coring System (ACS) is a gravity corer equipped with acoustic probes that obtain in-situ compressional wave (30–200 kHz) and shear wave (400–1200 Hz) measurements as the corer penetrates the seabed. During the April 2022 R/V Endeavor coring survey, the ACS was deployed at 36 locations within the New England Mud Patch and New England Shelf Break areas. Data from these measurements will be presented to characterize the depth-dependent structure of the geoacoustic seabed properties as well as their spatial variability. The in-situ measurements will be interpreted in the context of stratigraphic layering measured by a seismic survey. Depth-dependentprofiles of compressional speed from a subset of these deployments in the NEMP will be compared to profiles previously collected at nearby locations in 2016. In situ compressional wave records from both areas will be compared with ex-situ sediment core measurements, including data collected from core loggers and laboratory analyses. Finally, preliminary shear speed results will be discussed. [Work sponsored by ONR.]

Effects of weak shear rigidity of the seabed on sound propagation in a range-dependent ocean

The shear wave speed is often small compared to the compressional wave speed in unconsolidated marine sediments. Sediment stratification and especially mass density variation in the seabed enhance the effects of weak shear on acoustic normal modes in range-independent ocean and produce significant shear wave contributions to mode attenuation in shallow water [O. A. Godin, J. Acoust. Soc. Am., 149, 3586–3598 (2021)]. A distinctive feature of the shear-induced perturbations in the mode phase speed and attenuation is their rapid variation with sound frequency due to shear wave interference within a sediment layer. This paper investigates theoretically the shear wave-induced perturbations in normal mode phase, travel time, and attenuation when water depth and/or sediment layer thickness vary gradually with range. Frequency dependencies of the adiabatic normal mode travel time and attenuation are found to be highly sensitive to range dependence of the seabed layering. Gradual changes in the sediment layer thickness have an effect similar to frequency averaging or artificially increased shear wave attenuation. Implications of these findings for geoacoustic inversions will be discussed. Contributions of shear rigidity and cross-range gradients of the sediment layer thickness to horizontal refraction of normal modes will be also examined. [Work supported by ONR.]

Bayesian geoacoustic parameters inversion for multi-layer seabed in shallow sea using underwater acoustic field

Seabed geoacoustic parameters play an important role in underwater acoustic channel modeling. Traditional methods to determine these parameters, for example, drilling, are expensive and are being replaced by acoustic inverse technology. An inversion method based on Bayesian theory is presented to derive the structure and geoacoustic parameters of a layered seabed in a shallow sea. The seabed was considered a layered elastic medium. The objective of this research was to use the sound pressure detected by underwater acoustic sensors at different positions and to use nonlinear Bayesian inversion to estimate the geoacoustic parameters and their uncertainties in the multi-layer seabed. Specifically, the thickness, density, compression wave speed, shear wave speed, and the attenuation of these two wave speeds were determined. The maximum a posterior (MAP) model and posterior probability distribution of each parameter were estimated using the optimized simulated annealing (OSA) and Metropolis-Hastings sampling (MHS) methods. Model selection was carried out using the Bayesian information criterion (BIC) to determine the optimal model that thoroughly explained the experimental data for different parameterizations. The results showed that the OSA is much more capable of delivering high-accuracy results in multi-layer seabed models. The compression wave speed and shear wave speed were less uncertain than the other parameters, and the parameters in the upper layer had less uncertainty than those in the lower layer.

Effects of physical properties on the compression wave speed of seafloor sediment in the South China Sea: Comparisons between theoretical models and measured data

The compression wave speed and physical properties of seafloor sediments are significant in marine engineering and marine acoustics. Although most regression equations can be satisfactorily fitted to the data and provide a useful predictive method, there is little insight into the influence of physical properties on the compression wave speed of seafloor sediment in the South China Sea (SCS). To characterize the compression wave speed and physical properties of seafloor sediments, the compression wave speed, porosity, density, and mean grain size were measured and calculated for forty-two samples collected from the South China Sea. The results show that the RMS roughness of seafloor sediment in the South China Sea ranges from 0.2 to 15 μm, and the porosity and density are determined by the RMS roughness and mean grain size. The relationships between the compression wave speed and physical properties of seafloor sediments are consistent with the intergranular friction model. When we have the physical parameters but no compression wave speed, therefore, the intergranular friction model can be used to predict the compression wave speed of seafloor sediment. Compared with the Wood model curve, we also reveal that the intergranular friction of seafloor sediment is an important parameter in determining the compression wave speed.

Multimodule Deep Learning Scheme for Elastic Wave Inversion of Inhomogeneous Objects With High Contrasts

In elastic wave inverse scattering problems, the material-property reconstruction, e.g. distributions of mass density, compressional wave speed, and shear wave speed from a limited set of measurement data, has attracted considerable research interest. However, simultaneous inversion of multiple parameters endures high computation costs, and reconstructed compressional and shear speeds may become unrealistic if no physical constraint is imposed. Meanwhile, because large objects with high contrasts over the background medium tend to induce high nonlinearity during the inversion process, it is difficult to obtain high-quality high-contrast material properties. To overcome such difficulties, we have developed a multi-module deep learning scheme with physical constraint for multi-parameter elastic wave inversion of high-contrast objects in inhomogeneous media. This scheme consists of (1) a preliminary imaging module (PIM), in which a deep residual network (ResNet) is employed to convert the scattered field data into the preliminary inversion images, (2) an image-enhancement module (IEM), in which a U-Net is employed to further enhance the image quality, and (3) a convolutional neural network (CNN) that is employed as the physical constraint module (PCM) to allow elastic wave parameters to satisfy actual physical constraint. Numerical examples have demonstrated that the proposed scheme not only accurately achieves multi-parameter elastic wave inversion, but also has good generalizability. Finally, the scheme can be applied to complex objects with high contrasts in both noise-free and noisy environments.

An analysis of core–mantle boundary related seismic waves using full-waveform modelling and adjoint methods

SUMMARYThe core–mantle boundary (CMB) is the most abrupt internal discontinuity in the Earth, marking the solid–fluid boundary between mantle and outer core that strongly affects the dynamics of the Earth’s interior. However, good agreement between models of CMB topographic variations is still lacking. This is probably due to difficulties relating to observations on seismograms and to the lack of good models of lowermost mantle velocity structure. Using spectral-element synthetic seismograms and adjoint methods, we perform traveltime analyses of seismic waves interacting with the CMB. We focus on reflected and refracted P and S waves. We select some of the most important and routinely used seismic phases: ScS, SKS, SKKS, PcP, PKP, PKKP and PcS, given their path through mantle and core and their interaction with the CMB. These seismic waves have been widely deployed by seismologists trying to image CMB topography and lowermost mantle structure. To analyse the reliability of measuring their traveltimes to infer CMB topography, we perform experiments in two ways. First, we compute synthetic seismograms with a dominant period of T ≈ 11s, for computational efficiency, using existing models of CMB topography. We compare traveltime perturbations measured by cross-correlation on the synthetics to those predicted using ray theory. We find deviations from a perfect agreement between ray theoretical predictions of time shifts and those measured on synthetics with and without CMB topography. Second, we calculate Fréchet sensitivity kernels of traveltimes with respect to shear and compressional wave speeds. We also explicitly compute boundary sensitivities with respect to the CMB interface. We observe that the overall sensitivity of the traveltimes is mostly due to volumetric velocity structure and that imprints of CMB on traveltimes are less pronounced. Our study explains the observed difficulties relating to inferring CMB topography using traveltimes and provides a suite of finite frequency sensitivity kernels computed with the adjoint method. The kernels allow us to qualitatively explain the behaviour of measured traveltimes and understand the trade-off between velocity and CMB topography. They can also serve as reference of finite frequency effects on traveltimes of observed seismic phases. From our analyses we conclude that: i) traveltime anomalies measured on Swaves are more in accord with ray theoretical predictions, ii) PcP, PKP, ScS and SKS phases have more pronounced sensitivity to the boundary and iii) separating the greater effects of velocity from those due to the boundary structure is difficult, as they intricately affect the traveltime. We propose that jointly inverting for CMB topography and lowermost mantle velocity structure using full-waveform synthetics and adjoint sensitivity kernels can progress our understanding of deep Earth structure and finite frequency effects on observed waveforms.

In situ acoustic coring systems for investigating the acoustic and physical properties of mud

In situ measurements of compressional and shear wave speed and attenuation provide direct characterization of marine sediment acoustic properties at near ambient conditions, as opposed to measurements that are conducted on samples removed from the seabed. Probe-based acoustic measurement techniques utilize piezoelectric sources and receivers that penetrate the seabed to obtain spatially localized depth-dependent records of compressional and shear waves in the sediment. When such measurements are coupled with conventional coring techniques, sediment samples can be collected from the propagation path between probes, allowing for comparison between the in situ acoustic records and quantities obtained from ex situ core analyses, such as porosity, grain size, or organic matter. Two acoustic coring measurement systems have been developed to investigate the acoustic, physical, and biological properties of soft sediment—the acoustic coring system, a gravity-corer-based platform capable of penetrating the seabed up to several meters, and the acoustic multi-corer, a seabed lander that samples the upper tens of centimeters of sediment near the water-seabed interface. The design and operation of these measurement platforms will be described, along with their capabilities and limitations. Some representative results from field deployments of these systems will be presented. [Work sponsored by ONR.]

Seabed Characterization Experiment: Analysis of Broadband Data

Analysis of data acquired during the seabed characterization experiment conducted in the New England mud patch area in March 2017 is presented in this article. A particular feature of these data is the presence of the Airy phase in some of the modes. The mode dispersion data along with the Airy phase information are extracted using the time warping method. The mode dispersion data obtained from the analysis of the acoustic data are then used to estimate the compressional wave speed and density profiles of the sediment layers using linear inversion methods. The validity of these results is investigated, and their validity is demonstrated. The results are consistent with the results obtained by the analysis of similar data in which a nonlinear inversion method was used to estimate the sediment properties. These results further show that meaningful results can be obtained using the linearized inversion procedure.

Pacific Echo: A deep ocean collaborative experiment

This paper summarizes experiments carried out using towed line arrays in the Pacific Echo sea trials. Although John Preston was not a participant, the experiments are examples consistent with his efforts in promoting collaborative research at sea with horizontal arrays. The overall goal of Pacific Echo was to study the impact of thin sediment ocean bottom environments on sound propagation in deep water. The objective of the experiments described here was to study the evolution of young oceanic crust. The hypothesis was that the sound speed of young basalt increased with age of the crust. Sound speed was inferred from low frequency measurements of the reflection coefficient versus grazing angle at sites of increasing distance from the deep ocean spreading center. The experiments introduced a novel design for measuring the reflection coefficients using two ships and small explosive charges. The signals at the array were spatially filtered to resolve the specular reflections as the ships opened range on set courses. The data provided estimates of both the compressional and shear wave speeds of the basalt. Results from the broadside reflection technique showed that sound speed increased with crustal age and were consistent with measurements obtained from conventional seismic reflection surveys.

Velocity‐Porosity Relations in Carbonate and Siliciclastic Subduction Zone Input Materials

AbstractThe mechanical, physical, and frictional properties of incoming materials play an important role in subduction zone structure and slip behavior because these properties influence the strength of the accretionary wedge and megathrust plate boundary faults. Incoming sediment sections often show an increase in compressional wave speed (Vp) and a decrease in porosity with depth due to consolidation. These relations allow seismic‐velocity models to be used to elucidate properties and conditions at depth. However, variations in these properties are controlled by lithology and composition as well as cementation and diagenesis. We present an analysis of shipboard measurements of Vp and porosity on incoming sediment cores from International Ocean Discovery Program (IODP) expeditions at the Hikurangi Margin, Nankai Trough, Aleutian Trench, Middle America Trench, and Sunda Trench. Porosity for these samples ranges from 5% to 85% and Vp ranges from 1.5 to 6 km/s. Vp‐porosity relations developed by Erikson & Jarrad (1998), https://doi.org/10.1029/98JB02128 and Hoffman & Tobin (2004) https://10.2973/odp.proc.sr.190196.355.2004, with a critical porosity of ∼30%, can represent carbonate‐poor (<50 wt% CaCO3), mainly hemipelagic, incoming sediment regardless of the margin. But these relations tend to underestimate porosity in incoming sediments with carbonate content greater than 50 wt%, which appear to have a critical porosity of between 45% and 50%. This discrepancy will lead to inaccuracy in estimates of fluid budget and overpressure in subduction zones. The velocity‐porosity relation in carbonate sediments is non‐unique due to the complexity that results from the greater susceptibility of carbonate rocks to diagenetic processes.

Acoustic Radiation Force: A Review of Four Mechanisms for Biomedical Applications.

Radiation force is a universal phenomenon in any wave motion where the wave energy produces a static or transient force on the propagation medium. The theory of acoustic radiation force (ARF) dates back to the early 19th century. In recent years, there has been an increasing interest in the biomedical applications of ARF. Following a brief history of ARF, this article describes a concise theory of ARF under four physical mechanisms of radiation force generation in tissue-like media. These mechanisms are primarily based on the dissipation of acoustic energy of propagating waves, the reflection of the incident wave, gradients of the compressional wave speeds, and the spatial variations of energy density in standing acoustic waves. Examples describing some of the practical applications of ARF under each mechanism are presented. This article concludes with a discussion on selected ideas for potential future applications of ARF in biomedicine.