全声波方程波场深度延拓倏逝波压制研究

Evanescent wave suppression in full-wave-equation wavefield depth extrapolation

与经典的单程波深度偏移方法相比,全声波方程波场深度偏移方法在成像质量和计算效率方面会面临新的难题,还有诸多理论问题值得深入研究,其中倏逝波的压制问题是全声波方程波场深度延拓及深度偏移方法面对的一项特殊挑战.为了解决该问题,本文提出了两种新的倏逝波压制策略.策略I是以经典频率-波数域方法为基础,提出广义低通滤波器方法;策略II是以能量守恒原理为基础,来解决波场延拓中倏逝波的振幅爆炸问题.对梯度速度模型的脉冲响应计算表明,本文提出的两种倏逝波压制策略在横向速度变化剧烈情况下的数值结果都是稳定的,并且能够实现与有限差分方法相同的波场延拓计算精度.在对盐丘模型成像时,在相同计算参数下,本文所提新策略解决了在使用传统低通滤波器压制倏逝波时无法对高角度构造进行准确成像的缺点.相较于压制倏逝波的谱投影方法,本文所提新策略不仅在成像质量上达到了与之相同的水平,而且具有更高的计算效率.通过一系列数值模拟实验表明:与传统倏逝波压制策略相比,本文所提新策略在实现全声波方程波场深度延拓及成像时,更好地平衡了计算效率和计算精度,具有更好的实用价值.

Evanescent wave suppression presents a special challenge to full-wave-equation wavefield depth-extrapolation and related depth migration methods.This is because,in contrast to classic one-way depth migration,more challenges in both imaging quality and computational cost are involved in full-wave-equation wavefield depth migration.To address this challenge,we propose two new methods for evanescent wave suppression.Method I is an extension of the classic one-way frequency-wavenumber domain method,named generalized low-pass filter;Method II is the energy norm method by using the energy conservation law to identify and eliminate energy explosion of evanescent waves.Impulse responses in a gradient velocity prove that our proposed two methods are stable and enable to achieve the similar wavefield extrapolation accuracy as the finite different method.When imaging the standard Salt model,with the same hardware configuration and parallelization implementation,the new methods solve the weakness of using the conventional low-pass filter in suppressing evanescent waves and achieve the similar imaging quality as that of full-wave-equation depth migration by using the spectral projector in suppressing evanescent waves,but with much less computational cost.The proposed methods provide two numerical algorithms to efficiently suppress evanescent waves in performing full-wave-equation wavefield depth extrapolation and migration.