Low-frequency Seismic Waves Research Articles

Shaking table test of acceleration response of surrounding rock of the 3D cross tunnel under earthquake

Taking the 3D cross-engineering of Caomeigou No.1 Tunnel and Pandaoling Tunnel as an example, a shaking table test was carried out to study the effect of tunnel spatial position on seismic wave propagation characteristics and acceleration response of surrounding rock under earthquake seismic excitation. Based on the influence of the spatial position of the tunnel, the characteristic form of the surrounding rock between the cross section and non-cross section is divided, and the accelerometer layout scheme is designed. Based on statistical probability, the ratio of peak ground velocity to peak ground acceleration (PGA) was introduced to quantitatively characterize the characteristics of seismic wave propagation spectrum. Furthermore, the SPECTR calculation was used to obtain the regional difference in the seismic wave propagation potential damage potential displacement parameter (P d ). Under the influence of the spatial location of the 3D cross tunnel, the peak acceleration and motion duration of the seismic wave are mainly reflected in the variation of the section along the elevation direction. Low-frequency (≤20 Hz) seismic waves have a greater impact on the tunnel structure, and peak ground velocity/peak ground acceleration ratio has a positive correlation with the peak input energy of ground motion. The ultra-small net-spacing cross tunnel has a spatially distributed coupling deformation effect. The crown of the upper-span tunnel is highly sensitive to earthquakes and becomes a weak link in seismic design. These results help us provide a theoretical basis for the seismic design of the cross-tunnel.

Attenuation Zones of Combined Periodic Foundations with Different Combination Patterns

One-dimensional periodic foundations with single-size unit cells are difficult to possess attenuation zones with both low frequencies and wide bandwidths. Combined one-dimensional periodic foundations are proposed by combing unit cells with different sizes in tandem in this paper. Attenuation zones of combined one-dimensional periodic foundations with different combination patterns are compared. The present study is useful for the design of one-dimensional periodic foundations to isolate low frequency seismic waves.

Seismic Optimization of High Cantilever Multianchor Pile Strengthening Soil Slopes against Earthquakes

To explore the optimal seismic performance of multianchor pile, we carried out a series of shaking table tests. Based on the special form of multianchor piles’ reinforcement, we put forward the optimal design scheme of using EPS foam as damping layers and energy-dissipation springs for improving the self-coordinating devices of anchor head. By measuring acceleration and dynamic soil-pressure response under different intensities of vibration, we analyzed the correlation between acceleration caused by seismic wave action and damage characterized by time-domain and spectral characteristics of dynamic soil-pressure. We discuss in detail the relationship between frequency and specific period of dynamic soil-pressure and acceleration. We then used the SPECTR program to calculate the energy spectrum. Under the reciprocating action of seismic waves of different intensities, our slope model showed the continuous effect of spatial coupling deformation leading to regional damage and failure. Furthermore, the spatial distribution for amplitude of acceleration and dynamic soil-pressure showed the outstanding response of lateral amplitude of pile structures without optimization. The energy-spectrum distribution of acceleration seismic input was orderly, while the dynamic soil-pressure distribution of piles was disordered. Low-frequency (≤10 Hz) seismic waves have a great influence on these structures. The difference of acceleration hysteresis along the elevations was mainly caused by the propagation stage after the main earthquake. The correlation between dynamic soil-pressure and acceleration response in each group before the pile occurred in the same earthquake area was very weak, showing a low correlation. The optimization effect of optimized structures is related to the position of the shock-absorbing layer. Under high acceleration, multianchor piles easily cause bulge failures or shear failures at the positions of sliding surfaces. These results are helpful for improvements to reliably optimize designs in pile structure dynamic parameters.

Shaking Table Tests for Seismic Response of Oblique Overlapped Tunnel

To study the dynamic response and spectrum characteristics of the three-dimensional crossing tunnel under the action of seismic load, we established a 1/50 downscale model based on a typical of the oblique overlapped tunnel and conducted a series of shaking table tests. Through examining the recorded dynamic responses (acceleration and dynamic strain measured at different locations in model tunnels), we found that the seismic response of the crown was the largest at the central section, and the invert of the tunnels was exactly opposite to the crown, which presented a “parabolic” distribution, and we inferred that the damage within the model may be mainly concentrated on the crown of the tunnels. Additionally, the dynamic strain showed obvious nonlinear and nonstationary characteristics under the action of different degrees of seismic intensities. Different from a single tunnel, the acceleration superposition effect appears in the cross section of two tunnels because of the spatial effect of overlapping tunnels, resulting in the obvious seismic response in the cross section. Meanwhile, we also found that the 1st dominant frequency (0.1–6.26 Hz) seismic wave played a leading role in the process of tunnel slope failure. Furthermore, the analysis of the acceleration response spectrum also showed that the surrounding rock mass has an amplification effect on low-frequency seismic waves. These results help us better understand the features of the dynamic responses and also provide evidence to reinforce the overlapped tunnels against earthquakes.

Research and microscopic analysis of seepage characteristics of sandstone under low-frequency cyclic loading

The change of rock permeability properties under cyclic load is of great significance to the safety of underground engineering. However, there are few studies on triaxial seepage tests under different cyclic loads, and most of them have not carried out microscopic mechanism analysis. Therefore, in this paper, the seepage test of intact sandstone under different cyclic loads was carried out under low confining pressure. The test used sine waves to simulate low-frequency seismic waves, highlighting the permeability of underground water-bearing engineering rock mass with vibration frequency and amplitude under low-frequency source disturbance Variations and test results show that in the sandstone triaxial stress-strain curve, from the compacted section, the elastic section to the expansion section, the permeability of the rock gradually increases, and the sensitivity to cyclic load also increases. At the same time, the damage threshold of the rock gradually decreases, and the rock will break even in the elastic section. Under low confining pressure, the loading amplitude has a greater impact on rock permeability than the loading frequency under low-frequency cyclic loading. The loading frequency and loading amplitude can be comprehensively characterized by the average loading rate. Then, using nuclear magnetic imaging technology to change the microstructure of the rock sample before and after the test, the analysis shows that the effect of different cyclic load parameters on the permeability of the rock sample is actually obtained through the effect of force on pores and cracks. For dense rocks, The increase in permeability caused by cracks is even greater.

A digital low-frequency geophone based on 4th-order sigma-delta modulator and single-coil velocity feedback

Widely deployed conventional moving-coil geophones lack the responses needed at low frequencies for detecting long-period seismic waves, and rely heavily on the A/D conversion chips for data acquisition. This work reports on a digital low-frequency geophone based on the PID force balancing technique and a single loop 4th-order sigma-delta modulator. The system is studied by Simulink modeling and is designed to satisfy demanding bandwidth and stability requirements. In addition, the hardware implementation adopts a readout interface for a geophone that consists of two parts: the analog part picks off the coil voltage while driving a control balancing force; and the digital part performs a real-time time DF integrators filtering in an FPGA for high-order noise shaping. The experimental results confirm the design concept of the proposed digital geophone. The balancing loop extends the low-frequency response of a commercial 4.5 Hz geophone down to 0.16 Hz. Within a 200 Hz bandwidth, the sigma-delta modulator achieves an equivalent acceleration noise floor of 4.3 ng/√Hz @ 1.0 Hz and shapes the in-band quantization noise to a level below other noises in the system. This investigation shows its potential to detect low-frequency seismic waves for efficient and high precision deep Earth exploration and hazard predictions.

The modelling and prediction of dynamic responses of slender continua deployed in tall structures under long-period seismic excitations

Tall building structures are susceptible to large sway motions when subjected to earthquake excitations. They are particularly affected by long period earthquake ground motions. These low frequency seismic waves resonate with the fundamental mode of the building structure which in turn causes resonance interactions with long slender continua deployed in modular non-structural installations, such as lifts. Damage due to large resonance motions of suspension/compensating ropes and cables during earthquake are one of the most common modes of failure in high-rise lift installations. In this paper an analytical model to predict the dynamic responses of a suspension rope system installed in a tall host structure under seismic conditions is presented. The model is then used to predict the dynamic performance of the system under earthquake excitations. The predictions can then be used to develop suitable mitigating strategies and protective measures to minimize the earthquake damage.

On the mechanical coupling of a fiber optic cable used for distributed acoustic/vibration sensing applications—a theoretical consideration

In recent years, fiber optic cables are increasingly used for the acquisition of dynamic strain changes for seismic surveys. When considering seismic amplitudes, one of the first questions arising is the mechanical coupling between optical fiber and the surrounding medium. Here we analyse the interaction of ground movement with a typical telecom-grade fiber optic cable from an existing telecommunication network deployed in a sand filled trench at the surface. Within the cable, the optical fiber is embedded in a gel-filled plastic tube. We apply Hooke’s law to calculate the stress needed to strain the optical fiber throughout the cable structure. In case the stress magnitude at the cable–sand interface as well as the gel–optical fiber interface is below the yield strength of the respective material, sand and gel, it can be regarded as an elastic medium. Hence, a multilayer radial symmetric model can be used to calculate the coupling of the optical fiber with the surrounding medium. We show that the transfer function has a dB lower cut-off wavelength of about 22 m. The magnitude response of this telecom-grade fiber optic cable is therefore almost perfect at typical low frequency seismic waves. The approach presented here can be applied to various cable designs to estimate the strain transfer between ground movement and an optical fiber.

Numerical Simulation of Gravity Waves Excited in the Ocean by Low-Frequency Surface Seismic Waves Based on GPS Recordings

A numerical experiment for reproducing the generation of free gravity waves in the ocean by low-frequency surface seismic waves passing across the bottom is described. The dynamics of the bottom movement is reconstructed based on the real GPS data recorded during the disastrous Tohoku earthquake of March 11, 2011. Results of the numerical simulation show that horizontal movements of underwater slopes play a key role in the generation of free gravity waves.

Source process with heterogeneous rupture velocity for the 2011 Tohoku-Oki earthquake based on 1-Hz GPS data

A rupture model with varying rupture front expansion velocity for the March 11, 2011, Tohoku-Oki earthquake was obtained by the joint inversion of high-rate Global Positioning System (GPS) data and ocean bottom GPS/acoustic (OB-GPS) data. The inverted rupture velocity with a complex distribution gradually increases near the hypocenter and shows rapid rupture expansion at the shallowest part of the fault. The entire rupture process, which lasted 160 s, can be divided into three energy release stages, based on the moment rate function. The preferred slip model, showing a compatible relationship with aftershocks, has a primary asperity concentrated from the hypocenter to the trench and a small asperity located on the southern fault. Source time functions for subfaults and temporal rupture images suggest that repeated slips occurred in the primary rupture, which is consistent with that from seismic waveforms. Our estimated maximum slip and total seismic moment are ~65 m and 4.2 × 1022 Nm (Mw 9.0), respectively. The large slip, stress drop, and rupture velocity are all concentrated at shallow depths, which indicates that the shallow part of the fault radiated high-frequency as well as low-frequency seismic waves.

Modeling of low-frequency seismic waves in a shallow sea using the staggered grid difference method

Elastic waves in the seabed generated by low-frequency noise radiating from ships are known as ship seismic waves and can be used to detect and identify ships. To obtain the propagation characteristics of ship seismic waves, an algorithm for calculating seismic waves at the seafloor is presented based on the staggered-grid finite difference method. The accuracy of the algorithm was tested by comparison with analytical solutions. Numerical simulation of seismic waves generated by a low-frequency point sound source in a typical shallow sea environment was carried out. Using various source frequencies and locations in the numerical simulation, we show that the seismic waves in the near field are composed mostly of transmitted S-waves and interface waves while transmitted P-waves are weak near the seafloor. However, in the far field, the wave components of the seismic wave are mainly normal modes and interface waves, with the latter being relatively strong in the waveforms. As the source frequency decreases, the normal modes become smaller and the interface waves dominate the time series of the seismic waves.

Investigation of Seismic Response Caused by Explosion of the Chelyabinsk Meteorite

A model of the origin and evolution of low-frequency seismic waves in the surface layer of the ground surface is proposed. Seismic waves are initiated by the acoustic wave from the explosion of the Chelyabinsk meteorite. As part of the software package the propagation of acoustic waves is first calculated and then matched with next seismic calculation as a seismic source. The simulation results are compared with real data obtained at the seismic station in Obninsk during the Chelyabinsk event.

Viscoelastic characteristics of low-frequency seismic wave attenuation in porous media

Mesoscopic fluid flow is the major cause of wave attenuation and velocity dispersion at seismic frequencies in porous rocks. The Johnson model provides solutions for the frequency-dependent quality factor and phase velocity in partially saturated porous media with pore patches of arbitrary shapes. We use the Johnson model to derive approximations for the quality factor Q at the high and low frequency limit, and obtain the approximate equation for Qmin based on geophysical and geometric parameters. A more accurate equation for Qmin is obtained after correcting for the linear errors between the exact and approximate Q values. The complexity of the pore patch shape affects the maximum attenuation of Qmin and the transition frequency ftr; furthermore, the effect on ftr is stronger than that on Qmin. Numerical solutions to Biot’s equation are computationally intensive; thus, we build an equivalent viscoelastic model on the basis of the Zener model, which well approximates the wave attenuation and dispersion in porous rocks in the seismic band.

Data Acquisition System Connect to CMG-3T Low-Frequency Seismic Pendulum

Aiming at the low-frequency performance of the lowfrequency seismic pendulum CMG -3T, we design a data acquisition system can acquire the low-frequency seismic waves, at the same time we accomplish the process of adjusting the CMG-3T. This paper gives designing scheme include the low noise design of the power module, the design of the connector to RS232 which can send command to adjusting the mass of CMG-3T,the design of voltage attenuation network and no-source low-pass filter network of data acquisition system, the last is the design of software for adjusting the CMG-3T.

Dynamic behavior of railway embankment slope subjected to seismic excitation

To reveal the dynamic behavior of a railway embankment slope subjected to seismic excitation, a shaking table model test was performed on a 1:8 scale embankment slope. Different types of seismic wave of differing amplitudes were applied to study the dynamic behavior of the embankment slope, and white noise excitations were interspersed among the seismic waves to observe the changes of dynamic characteristics of the embankment slope. Residual deformation behaviors of the embankment slope were also investigated. The results of the tests show that the natural frequency of the embankment slope exhibits a decreasing trend and that the damping ratio exhibits an increasing trend. The embankment slope exhibits a significant amplification effect on the input acceleration, and the acceleration response differs greatly when subjected to different seismic excitations of differing spectrum characteristics. The filler of the embankment slope affects the changes of the spectrum characteristics of the seismic wave. The filler performs a filtering effect on high-frequency seismic waves and amplifies the energy of low-frequency seismic waves, especially when the frequency is close to the natural frequency of the embankment slope. A bidirectional excitation creates a greater acceleration response than a unidirectional excitation does. The seismic residual deformation of the embankment slope occurs under the seismic subsidence mode.

Evidence for remotely triggered microearthquakes during salt cavern collapse

SUMMARY Microseismicity is a good indicator of spatio-temporal evolution of physical properties of rocks prior to catastrophic events like volcanic eruptions or landslides and may be triggered by a number of causes including dynamic characteristics of processes in play or/and external forces. We show evidence of triggered microseismicity observed in the vicinity of an underground salt cavern prone to collapse by a remote M∼ 7.2 earthquake, which occurred ∼12 000 km away. High-dynamic range broad-band records reveal the strong time-correlation between a dramatic change in the rate of local high-frequency microseismicity and the passage of low-frequency seismic waves, including body, Love and Rayleigh surface waves. Pressure was lowered in the cavern by pumping operations of brine out of the cavern. We demonstrate the near critical state of the cavern before the collapse by means of 2-D axisymmetric elastic finite-element simulations. On this basis, we show that the increment of stress necessary for the failure of the Dolomite layer, which ensures the stability of the whole system, is of the same order of magnitude as the maximum dynamic stress magnitude observed during the passage of the earthquakes waves. This suggests that the stress oscillations due to the seismic waves correlated with the recorded microearthquakes induced damage of the overburden, which eventually led to the collapse of the salt cavern. We show that the contribution of Rayleigh waves is the most efficient to trigger microseismicity at periods close to the natural fundamental frequency of the cavern system found at about 10–20 s by investigating the impulse response of the cavern + overburden + brine system.

Dislocation Damping and Anisotropic Seismic Wave Attenuation in Earth's Upper Mantle

Crystal defects form during tectonic deformation and are reactivated by the shear stress associated with passing seismic waves. Although these defects, known as dislocations, potentially contribute to the attenuation of seismic waves in Earth's upper mantle, evidence for dislocation damping from laboratory studies has been circumstantial. We experimentally determined the shear modulus and associated strain-energy dissipation in pre-deformed synthetic olivine aggregates under high pressures and temperatures. Enhanced high-temperature background dissipation occurred in specimens pre-deformed by dislocation creep in either compression or torsion, the enhancement being greater for prior deformation in torsion. These observations suggest the possibility of anisotropic attenuation in relatively coarse-grained rocks where olivine is or was deformed at relatively high stress by dislocation creep in Earth's upper mantle.

Seismic wave propagation through surface basalts – implications for coal seismic surveys

Seismic reflection surveying is one of the most widely used and effective techniques for coal seam structure delineation and risk mitigation for underground longwall mining. However, the ability of the method can be compromised by the presence of volcanic cover. This problem arises within parts of the Bowen and Sydney Basins of Australia and seismic surveying can be unsuccessful. As a consequence, such areas are less attractive for coal mining. Techniques to improve the success of seismic surveying over basalt flows are needed.In this paper, we use elastic wave-equation-based forward modelling techniques to investigate the effects and characteristics of seismic wave propagation under different settings involving changes in basalt properties, its thickness, lateral extent, relative position to the shot position and various forms of inhomogeneity. The modelling results suggests that: 1) basalts with high impedance contrasts and multiple flows generate strong multiples and weak reflectors; 2) thin basalts have less effect than thick basalts; 3) partial basalt cover has less effect than full basalt cover; 4) low frequency seismic waves (especially at large offsets) have better penetration through the basalt than high frequency waves; and 5) the deeper the coal seams are below basalts of limited extent, the less influence the basalts will have on the wave propagation. In addition to providing insights into the issues that arise when seismic surveying under basalts, these observations suggest that careful management of seismic noise and the acquisition of long-offset seismic data with low-frequency geophones have the potential to improve the seismic results.

New algorithms of emission tomography for passive seismic monitoring of a producing hydrocarbon deposit: Part II. Results of real data processing

The new suggested algorithms of seismic emission tomography and those used earlier made it possible to dispose of the effects caused by a strong industrial noise, to study the structure of the upper part of the Earth’s crust and to monitor the seismic processes that take place within the oil deposit being developed in West Siberia. An efficient use of the industrial background seismic illumination allowed three-dimensional imaging of the fault zone. From the recorded seismic noise at the time of arrival of the high-amplitude low-frequency seismic wave, a localized soliton-like object floating upwards on the rupture with the velocity of 10 m/s is identified. In the depth range from 1 to 6 km a vertical filament-like seismic source that is hypothesized to be associated with the vortex geological structure, namely, the zone of torsional strains, is revealed. The position of this source in a plane remains stable within 9 days, although the bursts in the amplitude of both industrial and natural seismic radiation affect the depth distribution of the radiation and result in the appearance of additional seismic sources.

Seismic-induced accelerations detected by two parallel gravity meters in continuous recording with a high sampling rate at Etna volcano

We analyse a microgravity data set acquired from two spring LaCoste & Romberg gravity meters operated in parallel at the same site on Etna volcano (Italy) for about two months (August – September 2005). The high sampling rate acquisition (2Hz) allowed the correlation of short-lasting gravity fluctuations with seismic events. After characterizing the oscillation behavior of the meters, through the study of spectral content and the background noise level of both sequences, we recognized fluctuations in the gravity data, spanning a range of periods from 1 second to about 30 seconds dominated by components with a period of about 15 ÷ 25 seconds, during time intervals encompassing both local seismic events and large worldwide earthquakes. The data analyses demonstrate that observed earthquake-induced gravity fluctuations have some differences due to diverse spectral content of the earthquakes. When local seismic events which present high frequency content excite the meters, the correlation between the two gravity signals is poor (factor < 0.3). Vice versa, when large worldwide earthquakes occur and low frequency seismic waves dominate the ensuing seismic wavefield, the resonance frequencies of the meters are excited and they react according to more common features. In the latter case, the signals from the two instruments are strongly correlated to each other (up to 0.9). In this paper the behaviors of spring gravimeters in the frequency range of the disturbances produced by local and large worldwide earthquakes are presented and discussed.