Narrow Results

We developed a new method of an earthquake-resistant design to support conventional aseismic system using acoustic metamaterials. The device is an attenuator of a seismic wave that reduces the amplitude of the wave exponentially. Constructing a cylindrical shell-type waveguide composed of many Helmholtz resonators that creates a stop-band for the seismic frequency range, we convert the seismic wave into an attenuated one without touching the building that we want to protect. It is a mechanical way to convert the seismic energy into sound and heat.

This paper investigates the dynamic interaction between a lined tunnel and a hill under plane SV waves using the indirect boundary element method (IBEM), with the displacement and stress characteristics of the system presented in frequency domain. The IBEM has several unique advantages such as reducing calculation dimension, automatically satisfying the infinite radiation condition, etc. The numerical results indicated that the dynamic response of the tunnel–hill system is strongly dependent on incident wave characteristics, geometrical and material properties of the lined tunnel, as well as the topography of the hill. For a dimension ratio between the hill and tunnel of less than 10.0, the lined tunnel has large amplification or deamplification effect on the dynamic response of the hill. Correspondingly, the hill also greatly amplifies the displacement and stress concentration of the tunnel especially in the lower-frequency range, due to the complicated interference effect among the reflected waves and diffracted waves induced by the tunnel and hill. Also demonstrated is that the displacement and stress amplitude spectrums highly depend on the incident frequency and the space location, and there exist multiple peaks and troughs in the spectrum curve with the peaks usually appearing in the low-frequency range. Thus, for the seismic safety assessment of a hill slope or hill tunnel in practice, the dynamic interaction within the tunnel–hill system should be taken into consideration.

A seismic survey is perhaps the most common geophysical technique used to locate potential oil and natural gas deposits in the geologic structures. Thanks to the rapid development of modern high-performance computing systems, the computer simulation technology plays a crucial role in processing the field data. The precision of the full-waveform inversion (FWI) essentially depends on the quality of the direct problem solver. This paper introduces a new approach to the numerical simulation of wave processes in complex heterogeneous media. The linear elasticity theory is applied to simulate the dynamic behavior of curvilinear geological layers. In contrast to the conventional approach, the producing oil formation is described in the frame of a porous fluid-filled model. It allows us to explicitly take into account the porosity, oil density, and other physical parameters. The method of setting the physically correct contact conditions between the reservoir and the geological massif based on the transport equation solution for Riemann invariants was successfully implemented. The grid-characteristic method, previously thoroughly verified on acoustic and elastic problems, was adopted. The explicit time-stepping procedure was derived for a two-dimensional case with a method of splitting along coordinate axes. This method guarantees the preservation of the scheme approximation order. The potential application of the new method to a complex model based on the data from the famous Russian oil deposit — the Bazhen Formation — is demonstrated. The seismic responses were registered on the wave fields and synthetic seismograms. The novelty of this paper relates to a uniform approach to the wave propagation simulation in the heterogeneous medium containing contacting subdomains with different rheology types.

This paper considers numerical simulation of acoustic waves in heterogeneous two-dimensional (2D) media by the grid-characteristic method. We present a novel high-order compact numerical scheme that accurately handles discontinuous material parameters. We present one-dimensional and 2D formulations and discuss their programming implementation. The merits of this approach are evaluated in numerical experiments. It is verified empirically that the scheme demonstrates the second-order of convergence. Furthermore, we apply the scheme to the Sukhoi Log gold deposit model and the Marmousi model. The primary practical outcome of this research is that it provides a tool for precise simulation of wavefields in complex media with discontinuous parameters.

Impulse waves caused by a combination of earthquakes and landslides are a neglected problem in seismic research. In the Tibetan Plateau of China, where earthquakes and landslides are frequent and glacial lakes are widely distributed, even a small lake outburst could be catastrophic. However, minimal attention has been paid to the mechanism of impulse wave formation under the joint action of earthquakes and landslides. In this study, 120 large-scale shaking table experiments were conducted to reveal the formation regularity and characteristics of an impulse wave triggered by a combined earthquake and landslide. Several effective parameters were considered: still water depth, peak ground acceleration, impact velocity, and slide volume. Based on the experimental data, an empirical formula is proposed for the superposed height of an impulse wave triggered by a combined earthquake and landslide.

Elastic wave in the seabed caused by low frequency noise radiated from ship is called ship seismic wave and can be used to identify ship target. In order to obtain the propagation features of ship seismic wave in shallow sea with thick sediment, this paper introduces an algorithm for synthetic seismogram aroused by low frequency point sound source in shallow sea based on Biot’s wave theory for saturated porous media. Numerical calculation of synthetic seismogram at seafloor was carried out at a typical shallow sea environment with thick sediment. According to the results of numerical examples, the time series of seismic wave at seafloor is mostly composed of interface wave and leaky modes. The interface wave can propagate to far distance with small attenuation when the source frequency is very low. When the source frequency increases, the interface wave can no longer propagate to far distance like the leaky modes because the attenuation of sediment increases rapidly with frequency. The porosity and permeability of sediment in shallow sea have some influence on the dispersion characteristic of seismic wave at seafloor.

Several methods for handling sloping fluid–solid interfaces with the elastic parabolic equation are tested. A single-scattering approach that is modified for the fluid–solid case is accurate for some problems but breaks down when the contrast across the interface is sufficiently large and when there is a Scholte wave. An approximate condition for conserving energy breaks down when a Scholte wave propagates along a sloping interface but otherwise performs well for a large class of problems involving gradual slopes, a wide range of sediment parameters, and ice cover. An approach based on treating part of the fluid layer as a solid with low shear speed is developed and found to handle Scholte waves and a wide range of sediment parameters accurately, but this approach needs further development. The variable rotated parabolic equation is not effective for problems involving frequent or continuous changes in slope, but it provides a high level of accuracy for most of the test cases, which have regions of constant slope. Approaches based on a coordinate mapping and on using a film of solid material with low shear speed on the rises of the stair steps that approximate a sloping interface are also tested and found to produce accurate results for some cases.

After the parabolic equation method was initially applied to scalar wave propagation problems in ocean acoustics and seismology, it took more than a decade before there was any substantial progress in extending this approach to problems involving solid layers. Some of the key steps in the development of the elastic parabolic equation are discussed here. The first breakthrough came in 1985 with the discovery that changing to an unconventional set of dependent variables makes it possible to factor the operator in the elastic wave equation into a product of outgoing and incoming operators. This innovation, which included an approach for handling fluid-solid interfaces, was utilized in the first successful implementations of the elastic parabolic equation less than five years later. A series of papers during that period addressed the issues of accuracy and stability, which require special attention relative to the scalar case. During the 1990s, the self-starter made it possible to handle all types of waves, rotated rational approximations of the operator square root made it possible to handle relatively thin solid layers, and there was some progress in the accurate treatment of sloping interfaces. During the next decade, an improved formulation and approach for handling interfaces facilitated the treatment of piecewise continuous depth dependence and sloping interfaces. During the last 10 years, the accuracy of the elastic parabolic equation was improved and tested for problems involving sloping interfaces and boundaries, and this approach was applied to Arctic acoustics and other problems involving thin layers. After decades of development, the elastic parabolic equation has become a useful tool for a wide range of problems in seismology, seismo-acoustics, and Arctic acoustics, but possible directions for further work are discussed.

Seismic waves are mechanical vibrations that propagate through the Earth. They cannot propagate through vacuum. There are various types of seismic waves (e.g., body and surface waves); each type can be categorized into two subtypes based on the nature of particle motion during wave propagation. The two body wave types are P wave and S wave. The P wave is also known as primary wave or longitudinal wave. Particle motion or the oscillation of the medium during the propagation of this wave is in the direction of wave propagation; hence, it is similar to a sound wave. During wave propagation, the body experiences compression and dilatation, i.e., change in volume. This wave can propagate through both solid and fluid media and is the fastest of all seismic waves. The S wave is also known as shear wave or secondary wave. The medium oscillates in a direction perpendicular to the direction of propagation of wave front. The body experiences shearing motion that leads to a change in its shape, but no change in volume. As fluids cannot sustain shear, S waves cannot move through them. Hence, an S wave propagates only through solid media. In a given medium, its velocity is lower than that of a P wave; hence, it always arrives after the P wave, and that is why it is also called as secondary wave.

Surface waves develop due to interference of post-critical reflected P and S waves. Their maximum amplitude of vibration decreases exponentially with depth in the Earth. Hence, their propagation effect is maximum near the surface. That is why they are called surface wave. They travel with velocity less than that of S wave and arrive later than body waves. The two types of surface waves are known as Rayleigh wave and Love wave. A Rayleigh wave is generated by the constructive interference of a P wave and the vertical component of an S wave (also called SV wave). In a record of seismic wave called seismogram, normally this is the maximum amplitude at arrival. A Love wave is generated by the constructive interference of upgoing and downgoing components of an SH wave (where particle motion of the S wave is in the horizontal direction). Love waves travel faster than Rayleigh waves. There is another type of wave similar to the surface waves in nature that travels along an interface at deeper locations within the Earth called Stonley wave.

Apart from this, the Earth also experiences free oscillations — standing waves generated due to interference of long-period seismic waves. These waves cause the whole Earth to vibrate. There are two types of free oscillations: spheroidal and toroidal. Spheroidal oscillation is similar to the motion caused by a Rayleigh wave, and Toroidal motion is similar to the motion caused by a Love wave.

Composite foundation can be considered as an isolation base of the upper structure. Coulomb friction model was introduced to simulate the isolation of the composite foundation. Frequency analysis shows that the mode shapes were greatly affected by the composite foundation. It is difficult to take too many modes into account. So, numerical transient dynamic method was employed to analyze the effects of the existence of composite foundation on a steel transmission tower. Two seismic waves, with the same peak acceleration 0.4g, were chosen for the analysis of the tower with composite foundation and with fixed end. The results show that composite foundation can apparently change the frequency characteristic of the tower and decrease the responses of the steel tower.