地球物理学进展 ›› 2021, Vol. 36 ›› Issue (5): 2102-2108.doi: 10.6038/pg2021FF0231

• 应用地球物理学Ⅰ(油气及金属矿产地球物理勘探) • 上一篇    下一篇

逆时偏移震源子波影响分析

杨仁虎1,2()   

  1. 1.防灾科技学院,三河 065201
    2.河北省地震动力学重点实验室,三河 065201
  • 收稿日期:2021-05-15 修回日期:2021-06-29 出版日期:2021-10-20 发布日期:2021-11-11
  • 作者简介:杨仁虎,男,1979年生,博士,副教授,主要从事地震波传播与成像等方面的研究. E-mail: yangrenhu@cidp.edu.cn
  • 基金资助:
    河北省地震动力学重点实验室开放基金(FZ202101);河北省地震局地震科技星火计划项目(DZ20200827053)

Analysis of effect of source wavelet on reverse time migration

YANG RenHu1,2()   

  1. 1. China Institute of Disaster Prevention, Sanhe 065201, China
    2. Hebei Key Laboratory of Earthquake Dynamics, Sanhe 065201, China
  • Received:2021-05-15 Revised:2021-06-29 Online:2021-10-20 Published:2021-11-11

摘要:

逆时偏移作为一种先进的地震偏移成像方法,其成像结果的好坏取决于很多因素,其中成像条件是关键的一个因素.成像条件一般采用互相关成像条件,它是将源波场沿时间进行正推,接收波场沿时间进行逆推,然后将源波场和接收波场进行互相关,从而得到偏移成像结果.源波场正推和接收波场逆推都是采用同一个波动方程,这里采用波动方程一阶应力-速度形式,数值计算方法采用交错网格有限差分方法.采用振幅补偿拉普拉斯滤波方法压制逆时偏移成像中的低频噪声.采用三次样条插值方法解决逆时偏移成像中的波形不光滑问题.在接收波场逆推过程中,是将地震记录作为边界条件.而在源波场正推过程中,需要给定震源子波作为初值条件.理论上来讲,震源子波为脉冲时是最佳的,但在实际当中难以实现.在源波场正推时,一般采用子波函数.子波函数形式非常多,最常用的是雷克子波.不同主频震源子波产生的源波场频率也不同,从而导致逆时偏移互相关成像效果也不同.那么震源子波主频对逆时偏移成像影响到底有多大呢?为了简便,本文基于不同主频雷克子波,初步分析了相应的逆时偏移成像效果,同时对速度模型存在不同误差时的情形也进行了试验分析.结果表明:在不同速度误差情况下,震源子波主频越低,成像同相轴连续性越好,散射点聚焦能量越强;震源子波主频越高,虽然分辨率提高,但成像同相轴连续性变差,散射点聚焦能量变弱,同时背景噪声增强.本文研究结果对逆时偏移成像技术在实践中的应用具有一定的参考价值.

关键词: 逆时偏移, 互相关成像条件, 震源子波, 波动方程, 速度误差

Abstract:

As an advanced seismic migration imaging method, the imaging results of Reverse Time Migration (RTM) depends on many factors, among which the imaging condition is the key one. The imaging condition generally adopts the cross-correlation imaging condition, which is that the source wavefield is propagating forward along time, and the receiver wavefield is propagating backward along time, and then the source wavefield and the receiver wavefield are cross-correlated to obtain the migration imaging results. The forward and backward propagation of the source and receiver wavefield adopt the same wave equation. Here, the first-order stress-velocity form of the wave equation is adopted, and the staggered-grid finite difference method is adopted for numerical calculation. The amplitude-compensated Laplacian filtering method is used to suppress the low frequency noise in reverse time migration. Cubic spline interpolation method is used to solve the problem of unsmooth waveform in reverse time migration. In the backward propagation process of receiver wavefield, seismic records are used as boundary conditions. In the forward propagation process of source wavefield, the source wavelet should be given as the initial value condition. Theoretically, it is optimal when the source wavelet is pulse, but it is difficult to achieve in practice. When the source wavefield is propagating forward, the wavelet function is generally adopted. There are many forms of wavelet function, the most commonly used is Ricker wavelet. The frequency of source wavefield generated by source wavelet with different dominant frequency is also different, which leads to different cross-correlation imaging effect of reverse time migration. So how much does the dominant frequency of the source wavelet affect the reverse time migration imaging? For simplification, this paper preliminarily analyzed the imaging effect of reverse time migration based on Ricker wavelet with different dominant frequency, and experimentally analyzed the case of velocity model with different errors. The results show that the lower the dominant frequency of the source wavelet is, the better the continuity of the seismic events, and the stronger the focusing energy of the scattering points. The higher the dominant frequency of the source wavelet is, the higher the resolution, the worse the continuity of the seismic events axis, the weaker the focusing energy of the scattering points, and the stronger the background noise. The results of this paper have a certain reference value for the application of reverse time migration imaging technology in practice.

Key words: Reverse time migration, Cross-correlation imaging condition, Source wavelet, Wave equation, Velocity error

中图分类号: