用剖面模拟法反演地层品质因数

QUALITY FACTOR INVERSION USING SECTION MODELING METHOD

本文叙述了用VSP资料的下行初至波进行地层品质因数(Q)反演的方法(剖面模拟法)。我们选每个层中脉冲宽度对脉冲到达时的回归斜率作为衡量模拟剖面与实际VSP记录逼近程度的标准,把反演Q值归结为求解多元非线性最优化问题。为了消除解的非唯一性及提高反演速度,首先对预处理后的VSP资料作SVD(奇异值分解变换)低通滤波,以消除噪声及上行波的影响,然后把多元非线性最优化问题转化为一元问题。我们用谱比值法求模型参数初值,为了消除震源不一致性的影响,探讨了参考脉冲的选取问题。本反演方法的优点是不受震源能量变化的影响,对初值要求不严,适用于Q值与频率有关的情况,以及能减小误差积累等。VSP理论合成记录及实际资料的试算表明,用剖面模拟法反演Q值是行之有效的。

This paper presents a method (section modeling) that acts as quality factor inversion based upon the first arrivals of downgoing waves on VSP data. In its implementation, the regressive slope on the curve of impulse width-arrival time for each layer are considered as the extent to which a modeled section arc drawn near to real VSP data. In this way, the inversion for quality factor becomes a problem of multivariablc nonlinear optimization. In order to remove the ambiguity in solutions and to increase the operating speed, the preprocessed data arc SVD (Sigular Value Decomposition) low-pass filtered to reduce the effect from noises and upgoing waves. Then, the multivariablc nonlinear optimization problem is transformed into a problem involving only one variable. The initial value of model parameters is calculated using the spectral ratio method. In consideration of the effect from the inconsistency among seismic sources, the paper also gives attention to the selection for reference pulses. The method suggested is advantageous in that the method stands up to the variation of source energy and no strict requirement is needed on the selection of initial values. Besides, the method is applicable to cases in which Q values appear to be frequency-dependent and less errors will be accumulated during processes. Examples on both synthetic and real VSP data show that the section modeling method is effective in quality factor inversion.