三维各向异性裂缝介质正演模拟的三种交错网格适应性比较及Lebedev方法的改进

研究了三维各向异性裂缝介质的正演模拟的三种不同有限差分法,即标准交错网格(SSG)、旋转交错网格(RSG)和Lebedev方法(LS),详细对比分析这三种交错网格方法在模拟复杂各向异性介质的优势与局限.提出一种新的改进方法,简化了LS有限差分法在任意各向异性介质中的正演模拟.为了模拟三维大规模复杂各向异性介质的地震响应,提出一个优化的正演模拟计算流程:将模型参数分为模型介质参数和模型构造参数.该计算流程适用于三种有限差分法中的任何一种.使用LS方法实现任意三维各向异性裂缝介质地震响应的三维全波场模拟,通过使用三种不同的有限差分模拟方法进行二维和三维模型数值模拟试验,验证了所提出方法有效.

大地电磁三维矢量有限元正演误差分析及其优化

利用基于六面体的矢量有限元法进行大地电磁正演模拟时,在电性变化剧烈的区域有可能因为网格剖分的不够细致,导致六面体单元中的场值无法通过十二条棱边的插值准确表达,从而形成误差.另外,由于有限元剖分网格不可能无限延伸,而大地电磁场的边界条件在无穷远处才能得到满足;所以不恰当的网格剖分策略可能造成数值模拟计算结果误差过大.本文研究了不同频率下误差产生的规律及原因,并提出了减小误差的方法.最后根据误差分析对正演过程进行了优化,即保证了计算精度又大大提高了正演速度.

地下厂房洞室群岩体参数的增量位移反分析

考虑大岗山水电站地下厂房洞室群围岩参数获取的复杂性和不确定性,提出针对大型地下洞室群分层开挖的岩体参数增量位移反演分析法,采用粘弹性(H-K)本构模型,基于地下厂房分层开挖实测位移,通过合理选择待反演参数,以每层开挖引起的增量位移计算值与实测值残差平方和为目标函数,利用遗传模拟退火算法优化目标函数,实现各向同性粘弹性参数的增量位移反分析。对反演得到的等效岩体参数进行正演分析,再对后续厂房开挖引起的围岩增量位移进行预测,可进一步判断围岩稳定性。

Three-dimensional acoustic wave equation modeling based on the optimal finite-difference scheme

Generally, FD coefficients can be obtained by using Taylor series expansion （TE） or optimization methods to minimize the dispersion error. However, the TE-based FD method only achieves high modeling precision over a limited range of wavenumbers, and produces large numerical dispersion beyond this range. The optimal FD scheme based on least squares （LS） can guarantee high precision over a larger range of wavenumbers and obtain the best optimization solution at small computational cost. We extend the LS-based optimal FD scheme from two-dimensional （2D） forward modeling to three-dimensional （3D） and develop a 3D acoustic optimal FD method with high efficiency, wide range of high accuracy and adaptability to parallel computing. Dispersion analysis and forward modeling demonstrate that the developed FD method suppresses numerical dispersion. Finally, we use the developed FD method to source wavefield extrapolation and receiver wavefield extrapolation in 3D RTM. To decrease the computation time and storage requirements, the 3D RTM is implemented by combining the efficient boundary storage with checkpointing strategies on GPU. 3D RTM imaging results suggest that the 3D optimal FD method has higher precision than conventional methods.

Designing optimal number of receiving traces based on simulation model

Currently, the selection of receiving traces in geometry design is mostly based on the horizontal layered medium hypothesis, which is unable to meet survey requirements in a complex area. This paper estimates the optimal number of receiving traces in field geometry using a numerical simulation based on a field test conducted in previous research （Zhu et al., 2011）. A mathematical model is established for total energy and average efficiency energy using fixed trace spacing and optimal receiving traces are estimated. Seismic data acquired in a complex work area are used to verify the correctness of the proposed method. Results of model data calculations and actual data processing show that results are in agreement. This indicates that the proposed method is reasonable, correct, sufficiently scientific, and can be regarded as a novel method for use in seismic geometry design in complex geological regions.

Multiobjective particle swarm inversion algorithm for two-dimensional magnetic data

Regularization inversion uses constraints and a regularization factor to solve ill- posed inversion problems in geophysics. The choice of the regularization factor and of the initial model is critical in regularization inversion. To deal with these problems, we propose a multiobjective particle swarm inversion （MOPSOI） algorithm to simultaneously minimize the data misfit and model constraints, and obtain a multiobjective inversion solution set without the gradient information of the objective function and the regularization factor. We then choose the optimum solution from the solution set based on the trade-off between data misfit and constraints that substitute for the regularization factor. The inversion of synthetic two-dimensional magnetic data suggests that the MOPSOI algorithm can obtain as many feasible solutions as possible; thus, deeper insights of the inversion process can be gained and more reasonable solutions can be obtained by balancing the data misfit and constraints. The proposed MOPSOI algorithm can deal with the problems of choosing the right regularization factor and the initial model.

Damage evolution analysis of cast steel GS-20Mn5V based on modified GTN model

A modified Gurson-Tvergaard-Needleman （GTN） model that accounts for the mixed （isotropic and kinematic） hardening of cast steel GS-20Mn5V was developed and implemented in the finite dement program ABAQUS/Standard via a user-defined material subroutine UMAT. This model couples the stress state and damage evolution （pore volume fraction increase） by a classic method that assumes that the total void volume fraction is divided into a nucleation and a growth part. A parametric study was conducted to assess the effect of modified GTN model parameters on mechanical properties such as the nucleation, growth and coalescence of voids and to obtain the optimal parameter combination by the orthogonal test method. The predicted load-displacement curves of notched specimens with the optimal parameters are favorably compared to the experimental curves. Therefore, the modified GTN model can be used to predict the damage evaluation and fracture behavior of GS-20Mn5V.