基于伴随状态法二维时间域勒夫波全波形反演研究

Research on two-dimensional Love-wave full waveform inversion in time domainbased on adjoint state method

高频面波技术作为一种先进的地球物理勘探技术可被用来估计近地表横波速度.然而,基于频散曲线反演的高频面波技术对于非层状复杂的地质模型是不适用的.为此,本文对勒夫波进行时间域全波形反演以估计近地表横波速度.有别于经典的伴随状态法,本文基于一阶勒夫波伴随状态法求取模型参数梯度,该伴随状态法可以求取精确的伴随源.为削弱震源与近自由地表的耦合作用,本文对梯度进行适当的预调节.采取共轭梯度算法优化目标函数,并通过三点抛物法确定最优步长.分别对两个模型的无噪声和噪声勒夫波数据,应用本文方法测试.结果表明:经过50次迭代后,真实模型的主要异常已经被精确重建;噪声会加强震源与近自由地表的耦合作用,在近自由地表附近产生假异常;基于伴随状态法的全波形反演对于异常体的边界刻画能力较弱.

High-frequency surface wave techniques as advanced geophysical exploration techniques are used to estimate the near-surface shear wave velocity. However, high frequency surface-wave techniques based on inversion of dispersion curves are not suitable to non-layered complex earth models. In this paper, Love-wave full waveform inversion in the time domain is carried out to estimate the near-surface shear wave velocity. we calculate the gradient of objective function with respect to model parameters by first-order Love-wave adjoint state method. Unlike the classical adjoint state method, the adjoint state method can get the exact adjoint source. In order to weaken the coupling between source and near free surface, the gradient is preconditioned in this paper. The conjugate gradient algorithm is used to optimize the objective function, and the optimal step size is determined by the three-point parabolic method. We apply our method to noise-free and noise-contaminated synthetic data of two models. The results show that after 50 iterations, the main anomalies of the real model have been reconstructed accurately, and the noise will strengthen the coupling between the source and the near free surface, and produce false anomalies near the free surface, and the full waveform inversion based on the concomitant state method is weak in the boundary drawing of the abnormal body.