Damage Processes of Polypropylene Fiber Reinforced Mortar in Different Fiber Content Revealed by Acoustic Emission Behavior

The performances of the cement-based materials can be improved by the incorporation of polypropylene fiber, but the damage processes become more complex with different fiber contents at the same time. The acoustic emission（AE） technology can achieve the global monitoring of internal damage in materials. The evolution process of failure mode and damage degree of polypropylene fiber reinforced mortar and concrete were analyzed by measuring the AE energy, RA value, AF value and b value. It was found that the cement matrix cracked on the initial stage, the cracks further developed on the medium stage and the fibers were pulled out on the last stage. The matrix cracked with minor injury cracks, but the fiber broke with serious damage cracks. The cumulative AE energy was proportional to the polypropylene fiber reinforced concrete and mortar＇s ductility. The damage mode and damage degree can be judged by identifying the damage stage obtained by the analysis of the AF value.

Acoustic field simulations of logging while drilling by cylindrical finite difference with variable grids

This study proposes an elastic finite difference(FD)time domain method with variable grids in three-dimensional cylindrical coordinates.The calculations will diverge and become less accurate by conventional cylindrical FD as the grid size gradually becomes more extensive with the increasing radius.To prevent grids from being too coarse in far fields,we compensate for the grid cell infl ation by refi ning the grid step in the azimuthal direction.The variable grid FD in the cylindrical coordinate systems has a higher effi ciency in solving acoustic logging while drilling(LWD)problems because the grid boundaries are consistent with those of the drill collar and the borehole.The proposed algorithm saves approximately 94%of the FD grids,80%of the computation time,and memory with a higher calculation accuracy than the FD on rectangular grids for the same models.We also calculate the acoustic LWD responses of the fl uid-fi lled borehole intersecting with fractures.Refl ections are generated at the fractures,which can be equivalent to an additional scattering source.The mode conversions between the collar and the Stoneley waves are revealed.The Stoneley spectra are more sensitive to the fracture.Finally,the logs in a heterogeneous formation with two refl ectors far from the borehole are modeled,and a means of estimating the azimuth of geological interfaces from refl ections is proposed.

Influences of clean fracturing fluid viscosity and horizontal in-situ stress difference on hydraulic fracture propagation and morphology in coal seam

The viscosity of fracturing fluid and in-situ stress difference are the two important factors that affect the hydraulic fracturing pressure and propagation morphology. In this study, raw coal was used to prepare coal samples for experiments, and clean fracturing fluid samples were prepared using CTAB surfactant. A series of hydraulic fracturing tests were conducted with an in-house developed triaxial hydraulic fracturing simulator and the fracturing process was monitored with an acoustic emission instrument to analyze the influences of fracturing fluid viscosity and horizontal in-situ stress difference on coal fracture propagation. The results show that the number of branched fractures decreased, the fracture pattern became simpler, the fractures width increased obviously, and the distribution of AE event points was concentrated with the increase of the fracturing fluid viscosity or the horizontal in-situ stress difference. The acoustic emission energy decreases with the increase of fracturing fluid viscosity and increases with the increase of horizontal in situ stress difference. The low viscosity clean fracturing fluid has strong elasticity and is easy to be compressed into the tip of fractures, resulting in complex fractures. The high viscosity clean fracturing fluids are the opposite. Our experimental results provide a reference and scientific basis for the design and optimization of field hydraulic fracturing parameters.

Determination Method of Focal Depth of Local Earthquake Using the Travel-time Difference between Pn and sPn

In this paper,we derived the relationships between the travel time difference of sPn and Pn and the local earthquake focal depth.In these equations,the travel time difference of sPn and Pn is not related to the epicentral distance,but depends only on the regional crustal mode and the focal depth.According to the equations,we provided a simple and accurate method to determine local earthquake focal depth by using the travel time difference between phase sPn and Pn.This method has been used to determine the focal depths of two earthquake of MS6.1 and MS5.6 which occurred at the junction of Panzhihua and Huili,Sichuan on August 30 and 31,2008.The results were compared to those from other sources such as the China Earthquake Networks Center,and the comparison shows that the results are accurate and reliable.

Non-contact Acoustic Contour Imaging Method Using Phase Difference in Extremely Shallow Underground

Buried object which exists at extremely shallow underground can be detected by using an acoustic vibration and a SLDV （scanning laser Doppler vibrometer）. Non-contact acoustic exploration can be realized by using air coupled sound. It was confirmed that discovery and the identification of a buried thing were possible using the OFR method （optimum frequency range method）. However, in this method, only the amplitude of the vibration speed spectrum is used and did not use the phase information. Therefore a new imaging method is proposed that used the phase information of the vibration speed spectrum. From the experimental results, the possibility of the outline extraction by using the phase difference between each scan point is confirmed. As a future task, the phase difference image will become clearer by changing the scan point density and long duration time of emitted wave.

3D numerical simulation in acoustic-elastic coupled media with staggered-grid finite-difference method

Acoustic-elastic coupled media is often encountered in most marine explorations, and accurate simulation of acoustic-elastic coupled media is of great significance. At present, the study of acoustic-elastic coupled media still assumes that the solid of the acoustic-elastic coupled media is isotropic, but this assumption is not in accordance with the actual situation. In this paper, we derive the solid media of acoustic-elastic coupled media from isotropic media to anisotropic media, and propose an acoustic-elastic coupled medium based ontransverse isotropic media with vertical symmetric axes(VTI) to improve the accuracy of forward modeling. Based on the relationship between the Thomsen parameter and the coefficient matrix of the anisotropic elastic wave equation, we transform the Thomson parameter into a velocity model with anisotropic properties. We use a staggered grid finite difference method to simulate the propagation of a wavefield in a three-dimensional acoustic-elastic coupled media. We obtain the snapshots of the wave field when the solid of the acoustic-elastic coupled media is an isotropic medium and a VTI media. When the solid of the acoustic-elastic coupled media is considered VTI media, we can observe the qP wave and qS wave that cannot be observed in the isotropic medium from the wave field snapshot. We can also find that the seismic records obtained by the method we use are more realistic. The algorithm proposed in this paper is of great significance for high-precision ocean numerical simulation.

Fracture identification based on remote detection acoustic reflection logging

Fracture identification is important for the evaluation of carbonate reservoirs. However, conventional logging equipment has small depth of investigation and cannot detect rock fractures more than three meters away from the borehole. Remote acoustic logging uses phase-controlled array-transmitting and long sound probes that increase the depth of investigation. The interpretation of logging data with respect to fractures is typically guided by practical experience rather than theory and is often ambiguous. We use remote acoustic reflection logging data and high-order finite-difference approximations in the forward modeling and prestack reverse-time migration to image fractures. First, we perform forward modeling of the fracture responses as a function of the fracture-borehole wall distance, aperture, and dip angle. Second, we extract the energy intensity within the imaging area to determine whether the fracture can be identified as the formation velocity is varied. Finally, we evaluate the effect of the fracture-borehole distance, fracture aperture, and dip angle on fracture identification.

Numerical simulation of an acoustic field generated by a phased arc array in a fluid-filled borehole

The acoustic tools widely used in borehole well logging and being developed in borehole acoustic reflection imaging do not have the function of azimuthal measurement due to a symmetric source, so they can not be used to evaluate the azimuthal character of borehole formation. In this paper, a 3D finite difference method was used to simulate the acoustic fields in a fluid-filled borehole generated by a traditional monopole source and a phased arc array. Acoustic waveforms were presented for both cases. The analysis of the simulated waveforms showed that different from the monopole source, the acoustic energy generated by the phased arc array transmitter mainly radiated to the borehole in a narrow azimuthal range, which was the key technique to implement azimuthal acoustic well logging. Similar to the monopole source, the waveforms generated by the phased arc array in the fluid-filled borehole also contain compressional （P） waves and shear （S） waves refracted along the borehole, which is the theoretical foundation of phased arc array acoustic well logging.

ACOUSTIC REFLECTION FROM A BAFFLED ELASTIC/PIEZOELECTRIC PLATE

A finite difference/boundary integral procedure to determine theacoustic reflected pressure from a fluid-loaded bi-laminated plate isdescribed. The bi-laminate is composed of a piezoelectric layer andan elastic layer in contact with the fluid, and is held by anacoustically hard baffle. In the numerical model, the fluid pressureat fluid/solid interface is replaced by a continuum of point sourcesweighted by the normal acceleration of the elastic plate, and thegoverning equation system is solved in the solid domain.

ACOUSTIC REFLECTION FROM A PERIODIC ELASTIC/PIEZOELECTRIC PLATE

The acoustic reflected pressure from a periodic elastic/piezoelectric laminated plate is studied for the purpose of acoustic reflection control.A finite difference/boundary integral procedure to determine the reflected pressure from the fluid-loaded plate is described.In the numerical model,a Green's function in the form of infinite sum is employed and a boundary integral is performed to replace the fluid pressure at fluid/solid interface by a continuum of point sources weighted by the normal acceleration of the elastic plate.The equation system is then solved only in the solid domain.It is demonstrated that an appropriate applied voltage potential across the piezoelectric layer has the effect of cancelling the fundamental propagating mode,and there,is no reflection for frequencies up to the cut-off frequency of the next propagating mode if the fundamental mode has been eliminated.

Full-waveform Velocity Inversion Based on the Acoustic Wave Equation

Full-waveform velocity inversion based on the acoustic wave equation in the time domain is investigated in this paper. The inversion is the iterative minimization of the misfit between observed data and synthetic data obtained by a numerical solution of the wave equation. Two inversion algorithms in combination with the CG method and the BFGS method are described respectively. Numerical computations for two models including the benchmark Marmousi model with complex structure are implemented. The inversion results show that the BFGS-based algorithm behaves better in inversion than the CG-based algorithm does. Moreover, the good inversion result for Marmousi model with the BFGS-based algorithm suggests the quasi-Newton methods can provide an important tool for large-scale velocity inversion. More computations demonstrate the correctness and effectives of our inversion algorithms and code.

Effect of the fluctuant acoustic channel on the gain of a linear array in the ocean waveguide

The inhomogenous ocean waveguide,which leads the amplitude and phase of the signal arriving at a hydrophone array to fluctuate,is one of the causes that make the array gain deviate from its ideal value.The relationship between the array gain and the fluctuant acoustic channel is studied theoretically.The analytical expression of the array gain is derived via an acoustic channel transfer function on the assumption that the ambient noise field is isotropic.The expression is expanded via the Euler formula to give an insight into the effect of the fluctuant acoustic channel on the array gain.The result demonstrates that the amplitude fluctuation of the acoustic channel transfer functions has a slight effect on the array gain;however,the uniformity of the phase difference between the weighting coefficient and the channel transfer function on all the hydrophones in the array is a major factor that leads the array gain to further deviate from its ideal value.The numerical verification is conducted in the downslope waveguide,in which the gain of a horizontal uniform linear array(HLA)with a wide-aperture operating in the continental slope area is considered.Numerical result is consistent with the theoretical analysis.

Explosions and seismic phenomena based on exciting of acoustic-electromagnetic waves

During earthquakes and strong underground explosions it is possible to observe two different effects. The first one is connected with the destruction of media, and this causes acoustic and later hybrid acoustic-electromagnetic waves in an epicenter in the atmosphere and in the ionosphere. Another one is connected with cracks in crystals of rocks, which seems more interesting, because it is possible to recognize the natural earthquakes and industrial explosions. In the first effects due to nonlinear elastic modules the acoustic waves move through the lithosphere and transform their spectra from VLF (very low frequencies ~ 1 - 10 kHz) at the depth of about 30 km into the lower part of ELF (extremely low frequencies, ~ 3 Hz - 1 kHz) on the Earth’s surface, then they pass the atmosphere and penetrate into the ionosphere. During the nonlinear acoustic passage through the atmosphere and the ionosphere, the spectrum transforms from ELF into ULF (ultra low frequencies, < 1 Hz) range. In this review article the classification of spectrum and analysis of two cases of the destruction of rocks in the lithosphere is presented. The rocks possess piezoelectric and piezomagnetic properties. In this case the electromagnetic emission is excited by the fracturing in plates of crystals. The difference of emission from piezoelectric and magnetic plates in cases of industrial explosions and natural seismic events including volcanic phenomena gives a possibility to analyze the method of its identification. The consideration is based on the model of the plate of a finite size with an uniformly moving crack.

Efficient solution of large-scale matrix of acoustic wave equations in 3D frequency domain

In 3D frequency domain seismic forward and inversion calculation,the huge amount of calculation and storage is one of the main factors that restrict the processing speed and calculation efficiency.The frequency domain finite-difference forward simulation algorithm based on the acoustic wave equation establishes a large bandwidth complex matrix according to the discretized acoustic wave equation,and then the frequency domain wave field value is obtained by solving the matrix equation.In this study,the predecessor's optimized five-point method is extended to a 3D seven-point finite-difference scheme,and then a perfectly matched layer absorbing boundary condition(PML)is added to establish the corresponding matrix equation.In order to solve the complex matrix,we transform it to the equivalent real number domain to expand the solvable range of the matrix,and establish two objective functions to transform the matrix solving problem into an optimization problem that can be solved using gradient methods,and then use conjugate gradient algorithm to solve the problem.Previous studies have shown that in the conjugate gradient algorithm,the product of the matrix and the vector is the main factor that affects the calculation efficiency.Therefore,this study proposes a method that transform bandwidth matrix and vector product problem into some equivalent vector and vector product algorithm,thereby reducing the amount of calculation and storage.

Numerical Simulation of Acoustic-Gravity Waves Propagation in a Heterogeneous Earth-Atmosphere Model with Wind in the Atmosphere

A numerical-analytical solution for seismic and acoustic-gravity waves propagation is applied to a heterogeneous “Earth-Atmosphere” model. Seismic wave propagation in an elastic half-space is described by a system of first order dynamic equations of elasticity theory. Propagation of acoustic-gravity waves in the atmosphere is described by the linearized Navier-Stokes equations with the wind. The algorithm proposed is based on the integral Laguerre transform with respect to time, the finite integral Fourier transform along the spatial coordinate with the finite difference solution of the reduced problem.

VTI介质修正声学近似qP波波动方程与模拟

各向异性介质中纵横波通常耦合在一起传播,纵横波解耦是地震波传播理论研究的重要内容。经典的声学近似通过设置垂向qSV波速度V_(S0)为0来解耦qP波场,但是存在退化qSV波等问题。本研究将垂向qSV波速度V_(S0)考虑成波数k_(x)、k_(z)和各向异性参数ε、δ的函数,对声学近似进行修正,推导了VTI介质修正声学近似qP波频散关系和波动方程。VTI介质修正声学近似qP波波动方程包含椭圆项和非椭圆项两部分,因此采用混合有限差分/伪谱算法进行求解,即采用有限差分法求解椭圆项、伪谱法求解非椭圆项。频散关系分析和数值示例表明,基于修正声学近似的qP波波动方程不包含退化qSV波,是纯qP波方程,与弹性波方程模拟结果吻合较好,且具有较高精度;该方程在ε≥δ和ε<δ的VTI介质中均是稳定的,并且精度高于声学近似模拟结果。

基于深度神经网络的7065铝合金厚板应力检测模型

针对工业生产中传统超声应力检测法对铝合金厚板在不同拉伸率和不同温度条件下存在的测量误差的问题,以7065铝合金厚板为实验对象,提出一种在不同拉伸率和不同温度条件下的基于树突神经网络的应力预测模型与传统超声检测法融合的应力检测模型,然后使用改进的GSA-GRNN对该应力检测模型进行温度补偿。以南南铝公司生产的7065铝合金厚板为研究对象,使用恒温槽为超声检测提供恒温环境,分别对不同拉伸率、不同温度下的7065铝合金厚板进行超声检测,将声时差、拉伸率作为输入参数,应力作为输出参数,创建一个基于树突神经网络的应力检测模型,然后将应力检测模型的输出作为输入,使用改进的GSA-GRNN建立温度补偿模型对应力检测模型进行温度补偿。研究结果表明:融合了传统超声声时差的检测模型均方根误差为0.84636,相关系数为0.99743,和其他神经网络模型对比,该模型拥有更好的精度;在对该模型进行温度补偿后,模型的应力均方根误差和相关系数分别可以达到0.78848和0.99844,模型的精度得到了进一步的提升。证明基于数据驱动的神经网络融合传统超声检测可以有效降低检测误差,同时省去传统检测方法人工计算应力的时间,提高了检测效率。研究结果可以为基于数据驱动的应力检测模型提供进一步的优化参考。

基于声学多普勒频差法的自动测流设备应用

实现流量在线监测是提高水文测报现代化水平的重要前提,目前水文自动测流技术研究中多普勒频差原理广泛应用,自动测流设备适用程度较高,能够实现自动化监测,但不同设备又都有其局限性与应用范围,具体自动测流方案的选择要充分考虑测流断面的特性,同时必须根据历史数据与资料进行充分论证和精准分析。结合石梁河水文站测流断面,从技术方面对垂直式声学多普勒流速剖面仪和侧扫雷达设备进行充分论证和精准分析。

基于交错网格有限差分算法的页岩声波各向异性校正方法研究

页岩具有层理发育的特征,这会引起强烈的声波各向异性,导致直井与水平井声波测井数据之间差异明显,因此在水平井储层参数计算中无法直接应用基于直井的岩石物理解释模型.为了解决这一问题,本文以页岩波速各向异性实验数据为基础,引入交错网格有限差分算法,首先在直井井孔模型中(VTI介质)模拟了声波的发射和接收,随后通过弹性系数矩阵的Bond变换,模拟了在井斜角不为0的情况下(TTI介质)井孔中的声场传播,以任意井斜角与井斜角为0情况下纵波慢度差值相对值为纵坐标,以相对应的井斜角为横坐标,建立起了纵波各向异性校正公式.模拟与应用结果表明:井孔中波形曲线与实轴积分法(RAI)得到的波形曲线一致,同时利用慢度相似相关算法(STC)得到的地层纵波慢度与给定的实验测量值吻合很好,在28种地层弹性参数的情况下,平均相对误差为2.3%;纵波慢度差值相对值与井斜角关系曲线显示,在井斜角小于30°的条件下,纵波慢度差值相对值变化较小,随着井斜角大于30°后,相对值变化增大,当井斜角为90°也就是水平井模式下,纵波慢度差值相对值达到最大.根据纵波各向异性校正公式,对水平井纵波曲线进行了慢度校正.利用纵波校正前后计算的水平井有效孔隙度与导眼井岩心分析有效孔隙度相对误差分析表明,纵波校正后计算的有效孔隙度计算精度有了明显的提高,证明了该方法具有非常好的应用效果,可用于页岩水平井纵波慢度校正.

基于银膜的自准直光纤法布里-珀罗腔设计及声源定位研究

提出一种基于银膜的自准直外腔式光纤法布里-珀罗(EFPI)干涉仪声传感器阵列,用于中低频声信号的检测以及二维平面声源定位。传感器阵列由三个相同结构的EFPI组成,结构简单、制作简便。银膜的应用和准直器的引入提高了传感器的声压灵敏度,扩大了声源定位范围。实验结果表明:单个传感器声压灵敏度为185 mV/Pa,最小可探测声压为52.7μPa/Hz^(1/2)@500 Hz。在声源指向性实验中,声源设置在传感器正前方时,其声压灵敏度达到185 mV/Pa,声源设置在传感器侧面(90°,270°)时,其声压灵敏度衰减仅为4.5%。传感器阵列结合到达时间差技术,成功实现高精度的声源定位,系统的理论空间分辨率为0.71 cm,最大定位误差不超过2.8 cm,定位范围为200 cm×200 cm,具有成本低、实用性高等优点。