Wave Speed Profile Research Articles

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.

Bistable traveling waves in a delayed competitive system

<p style='text-indent:20px;'>This paper is concerned with the existence, uniqueness and asymptotic stability of traveling wave solutions in a delayed competitive system with spatial diffusion. When the interspecific competition is strong, the corresponding functional differential system has two stable and two unstable steady states. Firstly, we establish the existence of bistable traveling wave solutions by the theory of monotone semiflows. With the help of squeezing technique based on comparison principle, the asymptotic stability of traveling wave solutions is proved, which further implies the uniqueness of wave speed and wave profile in the sense of phase shift.</p>

Numerical wave propagation aided by deep learning

We propose a deep learning approach for wave propagation in media with multiscale wave speed, using a second-order linear wave equation model. We use neural networks to enhance the accuracy of a given inaccurate coarse solver, which under-resolves a class of multiscale wave media and wave fields of interest. Our approach involves generating training data by the given computationally efficient coarse solver and another sufficiently accurate solver, applied to a class of wave media (described by their wave speed profiles) and initial wave fields. We find that the trained neural networks can approximate the nonlinear dependence of the propagation on the wave speed as long as the causality is appropriately sampled in training data. We combine the neural-network-enhanced coarse solver with the parareal algorithm and demonstrate that the coupled approach improves the stability of parareal algorithms for wave propagation and improves the accuracy of the enhanced coarse solvers.

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.

Wave speed and attenuation profiles in a stratified marine sediment: Geo-acoustic modeling of seabed layering using the viscous grain shearing theory.

In the viscous grain shearing (VGS) theory of wave propagation in an unconsolidated sediment, the dispersion formulas for the phase speed and attenuation of the compressional and shear wave involve two parameters, the compressional modulus, γp, and the shear modulus, γs, which depend on the radius of the circle of contact between contiguous grains in the granular medium. The radius of contact itself depends on the overburden pressure, and hence depth, in the sediment. Based on these observations, the VGS theory is extended to create a geo-acoustic model of a horizontally stratified sediment in which each layer has a uniform porosity, bulk density, and mean grain size, all of which are assumed known from geological survey data. In a given layer, the overburden pressure consists of the contributions from all the higher layers. From the overburden pressure, the compressional and shear moduli are expressed as functions of depth throughout the layer, thereby allowing the frequency dependent phase speed and attenuation profiles of both types of wave to be computed from the VGS dispersion formulas. To illustrate the VGS geo-acoustic modeling technique, two examples are discussed, one of which represents the mud overlying sand sediment at the New England Mud Patch.

Radial earth models revisited

SUMMARYThe current set of reference models for the radial variation of Earth structure have been in use for several decades, and provide a good representation of many aspects of the seismic wavefield. Nevertheless, strong constraints from the differential times between pairs of SmKS phases indicate the need to modify the P wave speed profile in the upper part of the outer core. In order to incorporate such a change and maintain the representation of the full suite of seismic phases compensatory adjustments have to be made, dominantly in the mantle. Using multi-objective optimization, a new preferred radial model ek137 has been generated that provides a good representation of the traveltimes of all core phases. An adiabatic profile can be maintained through most of the outer core, but departures are needed at the base, as in the ak135 model. The latest estimates for inner core shear wave speed are included in ek137.

Estimation of three-dimensional water column sound speed profiles and sediment compressional wave speed and density profiles using a distributed network of buoys.

Broadband data acquired during the Modal Mapping Experiment (MOMAX) V experiment are used to invert simultaneously for the three-dimensional (3D) water column sound speed profiles and the compressional wave speed and density profiles of the seabed in shallow waters off the coast of New Jersey. Linear Frequency Modulation sweep signals in the band 50-300 Hz are transmitted from a nearly stationary source at several discrete positions to a set of freely drifting receivers. Mode travel times are estimated from the signals acquired by the drifting buoys, and these are then used as input data in an inversion algorithm that estimates the acoustic properties of the water column and sediments. The resulting 3D compressional wave speed profiles in the seabed are generally consistent with the one-dimensional profile obtained during the narrowband component of MOMAX V, as well as the results from other experiments in the same area. The validity of the inversion results has also been assessed by the ability of the inverted model to predict the fields measured during the narrowband experiments.

In situ measurements of compressional and shear wave properties during gravity coring operations

Sediment cores provide valuable insight about the physical properties of the seabed, and laboratory measurements of sediment wave speed from cores are often considered “ground truth.” However, sound-speed estimates obtained from cores can be inaccurate due to changes in pressure, temperature, and mechanical properties of the sediment caused by the removal of the core from the seabed and its subsequent transport to the laboratory. To address these issues, the Acoustic Coring System (ACS) was developed. The system uses sets of transducers mounted below the penetrating tip of a sediment corer to make in situ measurements of geoacoustic properties as the corer penetrates the seabed. Compressional wave measurements are obtained with rod-mounted piezoelectric cylinders, and shear wave measurements are obtained with bender elements mounted in flat blades. The ACS was deployed during the environmental survey for ONR Seabed Characterization Experiment (SBCEX) in 2016 and in support of the ONR Canada Basin Acoustic Propagation Experiment (CANAPE) in 2017. In both cases, the system provided measurements of the sediment sound-speed profile as a function of depth within the seabed. On-going work focused on the measurement of shear wave speed profiles and compressional and shear wave attenuation will also be presented. [Work supported by ONR.]Sediment cores provide valuable insight about the physical properties of the seabed, and laboratory measurements of sediment wave speed from cores are often considered “ground truth.” However, sound-speed estimates obtained from cores can be inaccurate due to changes in pressure, temperature, and mechanical properties of the sediment caused by the removal of the core from the seabed and its subsequent transport to the laboratory. To address these issues, the Acoustic Coring System (ACS) was developed. The system uses sets of transducers mounted below the penetrating tip of a sediment corer to make in situ measurements of geoacoustic properties as the corer penetrates the seabed. Compressional wave measurements are obtained with rod-mounted piezoelectric cylinders, and shear wave measurements are obtained with bender elements mounted in flat blades. The ACS was deployed during the environmental survey for ONR Seabed Characterization Experiment (SBCEX) in 2016 and in support of the ONR Canada Basin Acoustic...

Analytical Study of the Head-On Collision Process between Hydroelastic Solitary Waves in the Presence of a Uniform Current

The present study discusses an analytical simulation of the head-on collision between a pair of hydroelastic solitary waves propagating in the opposite directions in the presence of a uniform current. An infinite thin elastic plate is floating on the surface of water. The mathematical modeling of the thin elastic plate is based on the Euler–Bernoulli beam model. The resulting kinematic and dynamic boundary conditions are highly nonlinear, which are solved analytically with the help of a singular perturbation method. The Poincaré–Lighthill–Kuo method is applied to obtain the solution of the nonlinear partial differential equations. The resulting solutions are presented separately for the left- and right-going waves. The behavior of all the emerging parameters are presented mathematically and discussed graphically for the phase shift, maximum run-up amplitude, distortion profile, wave speed, and solitary wave profile. It is found that the presence of a current strongly affects the wavelength and wave speed of both solitary waves. A graphical comparison with pure-gravity waves is also presented as a particular case of our study.

RSNA/QIBA efforts to standardize shear wave speed as a biomarker for liver fibrosis staging

Commercial shear elasticity systems have been available to clinicians for several years for characterizing liver stiffness. The Radiological Society of North America (RSNA) established a Quantitative Imaging Biomarker Alliance (QIBA) committee on Ultrasound Shear Wave Speed to help achieve better consistency between the shear wave speed and elastic modulus values reported by different manufacturer systems. An inter-laboratory study of shear wave speed estimation was conducted at academic, industry and clinical laboratories using commercial ultrasound and magnetic resonance elastography systems in elastic (Phase I) and viscoelastic (Phase II) homogeneous, isotropic tissue-mimicking phantoms. In the elastic phantoms, shear wave speeds from different systems were within ±5% of the grand mean of all system measurements. Different appraisers and replicate measurements at a given site were not sources of significant variability in shear wave speed measurements. In the viscoelastic phantoms, shear wave speeds from different systems were within ±15% of the grand mean of all systems, with the stiffest phantoms at the deepest focal depth (7 cm) having the greatest measurement variance. Magnetic resonance elastography measurements at 140 Hz in the viscoelastic phantoms were consistent to within ±5% of thosemade with the ultrasound shear elasticity systems, while measurements made at 60 Hz had a mean bias of -2.6% relative to ultrasound systems. A method for reporting group shear wave velocities using both displacement and velocity data, along with shear wave phase velocity analysis, has been developed as a common calibration approach for the phantoms. An open-source repository of research scanner shear wave sequences and processing code has been created to allow manufacturers to have a calibration standard to assess compliance with the QIBA US shear wave speed profile and to achieve more consistent results to facilitate more rapid clinical adoption of this technology to evaluate liver fibrosis.

Characterization of soft marine sediments using vector sensors

Horizontally and vertically polarized interface waves with phase speeds of 45–375 m/s were observed in the North Sea using a horizontal linear array of three-component seismometers [H. Dong et al., J. Acoust. Soc. Am. 134, 176–184 (2013)]. The waves were generated by a low-frequency shear wave source on the seafloor, which operated in the 2–60 Hz band. Dispersion curves of the interface waves exhibit linear dependences between the logarithm of phase speed and logarithm of frequency, with distinct slopes at large and small phase speeds. This suggests a geoacoustic model consisting of two sediment layers with power-law dependences of the shear wave speed on depth below seafloor [O. A. Godin and D. M. F. Chapman, J. Acoust. Soc. Am. 110, 1890–1907 (2001)]. In this paper, dispersion curves of interface waves are inverted for the shear wave speed profile in the sediments. Observations of lateral wave arrivals are used to corroborate the geoacoustic model suggested by the surface wave dispersion and constrain c...

Finite element modeling of Rayleigh wave propagation in a laboratory sand tank

Rayleigh waves were measured in a laboratory sand tank using an array of accelerometers. This tank allows for the measurement of interface waves for various sediment and soil arrangements in a well-controlled laboratory setting. Rayleigh waves were excited by simple impulsive sources including dropped balls and hammer impacts. In this study, a finite element modeling tool developed by Sandia National Laboratories was used to simulate the sand tank during these excitations. Different modeling scenarios were investigated. Time series of the Rayleigh wave arrivals were modeled at the sensor locations using time domain analysis. A calibrated bender element system was also developed to provide direct measurements of the shear wave speed profile at selected depths. These data were used to develop the material property input parameters for the finite element analysis. The simulated time series at the sensor locations are compared to the accelerometer data. [Work supported by Army Research Laboratory and BOEM.]

A comparison between directly measured and inferred wave speeds from an acoustic propagation experiment in Currituck Sound.

An acoustic propagation experiment was conducted in Currituck Sound to characterize low-frequency propagation in a very-shallow-water estuarine environment. The water column properties were homogeneous over the study area, and the emphasis of this work is on understanding the propagation effects induced by the estuarine bed. During the experiment, low-frequency sound propagation measurements of waterborne sound and interface waves were acquired, and direct measurements of the compressional and shear wave properties were obtained at high frequencies. The propagation data consist of signals from a Combustive Sound Source recorded on bottom mounted geophones and a vertical line array of hydrophones. A statistical inference method was applied to obtain an estimate of the sediment compressional and shear wave speed profiles as a function of depth within the estuarine bed. The direct measurements were obtained in situ by inserting probes 30 cm into the sediment. Sediment acoustics models were fit to the high-frequency in situ data to enable comparison with the inferred low-frequency wave speeds. Overall, good agreement was found between the directly measured and inferred wave speeds for both the compressional and shear wave data.

Head-on Collision Between Two Hydroelastic Solitary Waves in Shallow Water

A comprehensive theoretical study on the head-on collision between two solitary waves in a thin elastic plate floating on an inviscid fluid of finite depth is investigated analytically by means of a singular perturbation method. The effects of plate compression are also taken into account. The Poincare–Lighthill–Kuo method has been used to derive the solution up to the fourth order of the resulting nonlinear differential equation, which in principle gives the asymptotic series solution. It is found that after collision, both the hydroelastic solitary waves preserves their original shape and positions. However a collision does have imprints on colliding waves with non-uniform phase shift up to the third order which creates tilting in the wave profile. Maximum run-up amplitude, wave speed, phase shift and distortion profile have also been calculated and plotted for two colliding solitary waves.

Numerical simulation of seismic wave field in graded geological media containing multiple cavities

In this study, we develop an efficient boundary integral equation method for estimation of seismic motion in a graded medium with multiple cavities under antiplane strain conditions. This inhomogeneous and heterogeneous medium is subjected to either time-harmonic incident shear seismic waves or to body waves radiating from a point seismic source. Three different types of soil material gradient are considered: (i) density and shear modulus vary proportionally as quadratic functions of depth, but the wave velocity remains constant; (ii) the soil material is viscoelastic, with a shear modulus and density that vary with respect to the spatial coordinates in an arbitrary fashion, so that the wave velocity is both frequency and position-dependent and (iii) the soil material has position-dependent shear modulus and constant density, yielding a linear profile for the wave velocity. Three different, frequency-dependent boundary integral equation schemes are respectively developed for the aforementioned three types of graded soil materials based on: (i) Green's function for the quadratically graded elastic half-plane; (ii) a fundamental solution for the viscoelastic full-plane with position-dependent wave speed profiles and (iii) a fundamental solution for an elastic full-plane with a linearly varying wave speed profile. Next, a number of cases involving geological media with position-dependent material properties and any number of cavities of various shapes and geometry are solved in the frequency domain. The numerical results reveal the dependency of the wave fields and zones of stress concentration on the following key factors: (i) type and properties of the soil material gradient; (ii) type and characteristics of the applied seismic load; (iii) shape, position and number of cavities and (iv) interaction phenomena between the cavities and the free surface.

Field measurements of interface waves and geoacoustic characterization of near surface sediment and soil

There is a need for rapid and nondestructive sensing of near-surface shear properties of the ground and seafloor. Our approach is to measure interface wave dispersion and invert these measurements to extract a shear wave speed profile. Field measurements of interface waves using geophones and accelerometers will be presented. A laser Doppler vibrometer (LDV) will also be incorporated in the suite of sensors for the measurement of wave properties. Data from these sensors will be compared to understand the coupling of the sensors into the soil or sediment and its effect on the measurements. Geoacoustic and geotechnical properties will be estimated using the interface wave data. The uncertainty and resolution of the estimates in different sediment/soil types will be explored. The Interface Wave Testing Facility at the University of Rhode Island will also be used to understand the surface wave propagation characteristics in different type of sediments/soils. [Work supported by Army Research Office and the Office of Naval Research.]

A comparative study between the IGN-2 equations and the fully nonlinear, weakly dispersive Boussinesq equations

This work focuses on the comparison between the fully nonlinear, and weakly dispersive Boussinesq equations and the irrotational Green–Naghdi Level 2 (IGN-2) equations for incompressible and inviscid fluid and irrotational flow resulting from waves in finite water depth. We provide in this paper for the first time a comprehensive comparison of the GN/IGN and Boussinesq wave theories for strongly nonlinear and weakly dispersive waves. The Boussinesq approach results in many different theories depending on the different orders of approximation of nonlinearity and dispersion used in the corresponding perturbation series. The fully nonlinear, weakly dispersive Boussinesq equations have been widely used in problems involving coastal regions and harbors because it is more efficient than fully nonlinear, strongly dispersive Boussinesq equations. A competing theory is the strongly nonlinear IGN-2 wave theory. Since both sets of equations have comparable complexity, it is appropriate to compare the results of these equations with known numerically exact solutions to determine the advantages of each approach. In this study, we use the same iterative algorithm to obtain the steady solutions of the IGN-2 and Boussinesq equations for periodic waves. The steady solutions of the IGN-2 and Boussinesq equations are studied for four different wavelengths that cover a range of applicability of the theories. We show in this study that the Boussinesq equations give more accurate results for the velocity distribution at the wave-crest transection. The IGN-2 equations give more accurate results for the velocity distribution at the wave-trough transection. Moreover, it is shown that the IGN-2 equations give more accurate results on the wave speed and wave profile, and that they can treat accurately waves higher than the ones that the Boussinesq equations can treat. The calculations show that both of these sets of equations are very good for strongly nonlinear wave simulations and the limitations of each are different.

Modeling explosion generated Scholte waves in sandy sediments with power law dependent shear wave speed.

Experimental measurements of Scholte waves from underwater explosions collected off the coast of Virginia Beach, VA in shallow water are presented. It is shown here that the dispersion of these explosion-generated Scholte waves traveling in the sandy seabed can be modeled using a power-law dependent shear wave speed profile and an empirical source model that determines the pressure time-series at 1 m from the source as a function of TNT-equivalent charge weight.

Laboratory and field measurements of Rayleigh waves

There is a need for rapid and nondestructive sensing of near-surface shear properties of the ground and seafloor. Our approach on land is to measure Rayleigh wave dispersion and invert these measurements to extract a shear wave speed profile. A portable surface wave inversion system has been developed that employs an array of six accelerometers to measure Rayleigh waves excited by simple impulsive sources in the laboratory and in the field. To verify the shear wave speed estimated by the Rayleigh waves, a calibrated bender element system was also developed to provide independent measurements of the shear wave speed profile at selected depths. The bender element system was placed in a test tank initially filled with mason sand and the Rayleigh wave inversion system was deployed on the surface of the sand in the tank. The estimated shear wave speeds were then compared to the results of the bender element system as well measurements by a dynamic cone penetrometer and laser vibrometer. The approach allows for the measurement of shear speeds in for various sediment and soil arrangements in the well-controlled laboratory setting as well in the field. [Work supported by Army Research Office and ONR.]

Outer core compositional layering and constraints on core liquid transport properties

A variety of studies of the Earth's outer core report wave speeds near the top of the core slightly lower than reference models for core properties. One interpretation of the slower wavespeed profile is that it could represent a change in the core's light element concentration with depth in the core. I explore the consequences of this idea by interpreting the velocity profile as arising from diffusion gradients imposed in the outer core by various mechanisms. In order to estimate relative diffusion rates for light elements in liquid iron I also examine theories for transport properties of high pressure metallic liquids that are based on hard-sphere models. From the seismic wavespeed profile, an effective diffusivity may be obtained, which ranges from 0.1 to 10×10−7 m2s−1 depending on the particular boundary condition or initial condition chosen. The upper bound of the range is higher than expected from high pressure experiments and models of diffusivity in liquid metals for all elements except H. The lower bound is within the uncertainty of theoretical predictions and experimental determinations given the range of expected outer core temperatures if diffusion involves low Z elements. Plausible agreement arises from a class of models that represent diffusion out of a compositionally different layer existing from the time of the formation of the Earth. If the wavespeed profile in the core is diffusive in nature, the data suggest that it is an original feature of the core.