Imaging scattered seismic surface waves with nonuniform receiver arrays

The dispersion curves of surface waves have been widely used for the retrieval of subsurface structures. Because the surface-wave dispersion is not sensitive to Vp, classical dispersion inversion can only retrieve Vs structures. To retrieve Vp, the dispersion of guided-P waves, which belong to leaky waves, should be considered. Due to the difficulty in forward modeling, the quantitative analysis of the leaky-wave dispersion curves has been rarely reported. In this study, we first derive the sensitivity analysis method of the leaky-wave dispersion based on previously proposed the semi-analytical spectral element method. Then the quantitative sensitivity analysis of leaky-wave dispersion curves is carried out, which confirms the ability of guided-P wave dispersion to constrain the velocity structures. With the theoretical analysis, we propose a joint inversion method based on surface and guided-P wave dispersion curves, which can simultaneously retrieve Vp and Vs. To verify the effectiveness, the proposed joint inversion has been applied to different kinds of field data including ocean bottom seismometer data with active sources and the seismic data of Nevada in the United States in 2008. Both of the inversion tests show that the joint inversion of surface- and leaky-wave dispersion can effectively constrain the velocity structure of Vp and Vs at the same time, which helps to obtain more complete and accurate models than the traditional surface wave dispersion inversion.

The shallow subsurface can be extremely complex and challenging from a geological and geophysical point of view. Near-surface perturbations impact the imaging of the deeper events and therefore have to be corrected in the seismic reflection information. It is also known that solving the near surface is an important part of any depth imaging velocity modeling. Usually we use refracted waves for P-wave velocity model estimation. Refraction data provide the information about the deeper subsurface and gives directly Vp velocity. In areas with complex near surface estimation of Vp on refracted energy can be challenging: velocity inversions and hidden layers can produce ambiguity in the refraction solution. The near-surface model obtained through surface wave inversion gives geometric information about the near-surface layers, geology, velocity and attenuation distribution down to the investigation depth. Because the propagation properties of the surface waves are directly depend on elastic properties of the near surface, the dispersion of surface waves is the physical property used for its characterization. Presented example of the simultaneous joint inversion successfully shows that resulting compressional and shear depth velocity models provide significant improvement to support refraction statics and shallow velocity model building.

A scattering inversion experiment was conducted on exposed Panola granite located southeast of Atlanta, Georgia, in the Panola Mountain Research Watershed. The objective was to test a near-surface seismic technique for locating subsurface discontinuities, such as fractures, that can be strong scatterers of seismic waves. Shallow water-productive fractures in the crystalline and metamorphic rocks underlying expanding suburban and urban areas, like those in central Georgia, are important because they have the potential of significantly augmenting surface freshwater supplies. Sixteen vertical 100-Hz geophones were placed in an array with aperture of 15 m. A weight-drop source was deployed around the array at distances of 10 to 50 m. We successfully recorded seismic waves in the frequency range of 100 to 1,000 Hz traveling through the unweathered Panola granite. The lack of overburden suppressed the interference from dispersed surface waves and enhanced coupling of the high-frequency energy source to the granite. We calculated semblance coefficients for waves propagating across the array as a function of time, back-azimuth and apparent velocity and used these to locate the source of the scattered waves within a 150 × 150 m area extending 70-m below the surface. Zones that were strong scatterers of surface waves correlated with topographic irregularities on the rock outcrop and the location of a small creek. Scattered P- and S-waves identified a three-dimensional distribution of scatterers which outlines the downward extension of fractures below the stream and the contact of the granite with amphibolite biotite gneiss.

In this paper, a modified laser induced grating technique (LIG) has been utilized to generate narrow band surface waves in an epoxy-bonded copper-aluminum layered structure. A high performance optical interferometer system was utilized to detect the laser-generated surface waves. The dispersion of surface wave in an epoxy-bonded copper-aluminum specimen was measured and compared with the theoretical solution. An inverse algorithm based on the simplex method was then introduced to determine the bonding thickness as well as the elastic properties of the bonding layer. The inversion results demonstrated that the thickness in the microm range or the elastic properties of the bonding layer could be successfully determined.

In this study, we were interested in the dispersion of surface waves caused by the presence of a micrometric field of residual stress on the surface of an amorphous medium. We have shown that in relation to surface waves, a stressed structure like this is comparable to a layer on substrate type structure. The design and implementation of SAW-IDT MEMS sensors enabled quasi-monochromatic Rayleigh-type surface waves to be generated and the dispersion phenomenon to be studied over a wide range of frequencies for different superficial fields of residual stress. The thicknesses of the stressed cortical zones were estimated with good accuracy using an inverse method.

The scattering problem of acoustic plane waves from comb-like impedance gratings on a rigid surface has been investigated in this paper. A rigorous analytic approach for homogeneous plane-wave incidence is presented based on the periodicity of the grating structure, in which the problem was solved as a mixed boundary value problem and the scattered field was represented by the tangent velocity difference across a partition wall of the grating. A singular integral equation has been derived for the tangent velocity difference, which can directly be solved with the Gauss-Chebyshev procedure. The resulting solution consists of a series of Bloch-Floquet waves (plane bulk wave and surface wave modes) with explicit expressions for the expansion coefficients. When the grating period is much less than the incident wavelength (ka << 1), the grating structure is equivalent to a plane impedance surface and no surface waves can be excited with homogeneous plane-wave incidence. When the grating period is comparable to the incident wavelength, resonance phenomena are predicted under certain conditions and surface waves can form, even with homogeneous plane-wave incidence. The dispersion relation for surface waves has also been examined. The impedance effects of the grating on the reflection and diffraction waves as well as on the dispersion and formation of surface waves have been studied, with the acoustically hard grating being the special case of the general impedance grating.

The single-station Rayleigh surface wave group velocities and electrical resistivities are two data sets that we cooperatively employ to image the near surface (&lt; 40-m) anomaly structures. We numerically simulate the corresponding field measurements where the anomaly structures are assumed to have two-dimensional (2D) variations. The surface waves are represented by fundamental mode dispersion curves, and the electrical resistivities are assumed to be measured by using direct currents. We consider two types of anomaly structures, i.e., cavity and ore body. These two heterogeneities are easily distinguished from the surrounding geomaterial by their distinct physical properties. The cavity is characterized by low seismic velocity and high electrical resistivity, while the ore body is characterized by high seismic velocity and low electrical resistivity. The Rayleigh surface wave data is assumed to be collected throughout the classical common-shot gather. Multiple electrodes, multiple core cables, and multiple arrays are assumed to be used in the electrical survey. Both surface wave group velocities and electrical resistivities are shown to properly invert the anomalous structures in the subsurface. The surface wave group velocities have good horizontal resolution, while the corresponding vertical resolution is somewhat lower. The electrical resistivities have good resolution for shallow structures, but the resolution becomes somewhat reduced with increasing depth. Doi: 10.28991/HEF-2022-03-01-01 Full Text: PDF

3-D structure of the Rio Grande Rift from 1-D constrained joint inversion of receiver functions and surface wave dispersion

Potential (electrostatic) surface waves (SWs) in a semi-bounded plasma with degenerate electrons are studied using the quasi-classical mean-field kinetic model. The SW spectrum and the collisionless damping rate are obtained numerically for a wide range of wavelengths. In the limit of long wavelengths, the SW frequency ω approaches the cold-plasma limit ω=ωp/2 with ωp being the plasma frequency, while at short wavelengths, the SW spectrum asymptotically approaches the spectrum of zero-sound mode propagating along the boundary. It is shown that the surface waves in this system remain weakly damped at all wavelengths (in contrast to strongly damped surface waves in Maxwellian electron plasmas), and the damping rate nonmonotonically depends on the wavelength, with the maximum (yet small) damping occurring for surface waves with wavelength of ≈5πλF, where λF is the Thomas-Fermi length. The applicability of the used approximations and of the obtained results is discussed in detail.

Dispersive surface waves in anisotropic linear electromagnetoelasticity

We investigate the effects of methane clathrates on Titan’s thermal and seismic structure. The Dragonfly mission is planned to arrive at Titan in 2033 with a payload that includes a seismic package. The seismic instruments are tasked with recording seismic events and recovering the internal structure. Here, we explore whether differences in thermal and seismic profiles between a pure water ice shell and an ice shell with a clathrate lid could be detectable with seismic instrumentation. Due to their lower thermal conductivity, clathrates reduce the conductive lid thickness thus altering the thermal profile. The differences between seismic velocities of clathrates and pure water ice, coupled with changes in the thermal profile, indicate the clathrate lid will create lower seismic velocities, particularly for the upper 10 km of the surface ice shell. The differences in P and S velocity at the surface are 2.9% and 4.5%, respectively, and reach up to 8.4% (for both P and S) at a depth of 9.6 km. Due to changes in thermal profile, the seismic attenuation of the ice shell will change such that clathrates will suppress surface wave amplitudes relative to the pure water ice model. The clathrate lid will further create minor changes (≤2.0%) in the surface wave dispersion curves. Dragonfly, or other future seismic investigations, could provide evidence for or against the presence of a clathrate lid by constraining the thermal and seismic profile of Titan’s ice shell, by measuring the relative amplitudes of the surface to body waves, or by constraining the surface wave dispersion with high accuracy and precision.

In this presentation, we analytically derive the dispersion equation for surface waves traveling along reactive boundaries which are periodically modulated in time. In addition, we show numerical results for the dispersion curves and importantly uncover that time-varying boundaries generate band gaps that can be controlled by engineering the modulation spectrum. Furthermore, we also point out an interesting effect of field amplification related to the existence of such band gaps for surface waves. The effect of amplification does not require the synchronization of signal and pumping waves. This unique property is very promising to be applied in surface-wave communications from microwave to optical frequencies.

A matrix formalism developed by W. T. Thomson is used to obtain the phase velocity dispersion equations for elastic surface waves of Rayleigh and Love type on multilayered solid media. The method is used to compute phase and group velocities of Rayleigh waves for two assumed three-layer models and one two-layer model of the earth's crust in the continents. The computed group velocity curves are compared with published values of the group velocities at various frequencies of Rayleigh waves over continental paths. The scatter of the observed values is larger than the difference between the three computed curves. It is believed that not all of this scatter is due to observational errors, but probably represents a real horizontal heterogeneity of the continental crusts.

Nonlinear dispersion shallow water equations are derived, which describe propagation of long surface waves on a spherical surface with allowance for rotation of the Earth and mobility of the ocean bottom. Derivation of these equations is based on expanding the solution of hydrodynamic equations on a sphere in small parameters depending on the relative thickness of the water layer and dispersion of surface waves.

Charged dust grains can have a large effect on the dispersion characteristics of hydromagnetic Alfvén and magnetoacoustic waves propagating at frequencies well below the ion-cyclotron frequency, even if the proportion of the total charge on the dust (usually negative) is quite small in space and astrophysical plasmas, such as cometary atmospheres and interstellar molecular clouds. The dust introduces a cutoff-resonance pair. A number of effects of the dust are considered here. Wave energy propagating at oblique angles to the magnetic field in an increasing density gradient can be very efficiently damped by resonance absorption processes in a dusty plasma. It is shown that, as well as the usual Alfvén resonance, a low frequency dust-ion hybrid resonance also occurs. The effects of dust on the dispersion of surface waves in highly structured plasmas and damping due to dust charging may also play roles in such plasmas.