叠前逆时偏移影响因素分析

The influence factors analyses of imaging precision in pre-stack reverse time migration

反射地震勘探中的偏移成像技术是获取地下介质构造形态最有效的手段之一.在叠前深度域偏移方法中,目前工业界采用的方法包括基于射线理论的波动方程积分解法和基于波动理论的微分波动方程单程波解法,这两类方法难以处理地震波横向速度变化剧烈的高陡倾角构造成像问题.近年来勘探地震学研究领域发展起来的叠前逆时偏移采用了双程波求解微分波动方程的算法,这种方法具有相位准确、不受介质横向速度变化和高陡倾角构造的影响、成像精度高、可以利用回转波正确成像等优点,从理论上弥补了当前工业界常规地震偏移所面临的成像缺陷.然而,叠前逆时偏移成像方法从理论走向实用尚需解决如下问题:计算速度和数据存储空间的节省、初始速度模型的建立、震源子波的选择、数值模型边界条件的定义和假像的消除等等.对于计算速度和存储量大的问题,随着计算机硬件的快速发展,将会不断得到改善,同时可以采取一些计算技术和存储策略来加以缓解.本文主要针对初始速度模型的建立、震源子波的选择、数值模型边界条件的定义和假像的消除这些因素,利用简单模型进行了分析.对于反射波造成的传播路径上的假像,给出了一种振幅补偿滤波方法.对勘探地球物理学界给出的SEG/EAGE二维盐丘模型、Marmousi模型和本研究设计的崎岖海底模型进行了叠前逆时偏移成像,均取得了较好的成像效果.

Seismic migration is one of the most effective means to acquire subsurface structures in reflection seismic exploration. Among the migration methods in pre stack depth domain at present, wave equation integral solutions based on ray theory and wave equation differential solutions based on one-way wave theory are widely applied in industry, which, however, both are difficult to deal with the seismic wave structure imaging problems of large lateral velocity variation and high-steep dip. In recent years, pre-stack reverse time migration developed in seismic exploration adopts the algorithm of using two-way wave method to find the differential wave motion equation solutions, which has some merits such as accurate phase, avoiding lateral variable speed and high-steep dip limitations, high imaging precision, and imaging overturned waves correctly. It makes up the imaging deficiency of the conventional seismic migration methods in industry at present. However, the imaging methods of pre-stack reverse time migration need to overcome some problems for realizing it on 3-D field data, such as raising the computational efficiency, saving data storage space, building the initial velocity model, selection of source wavelet, defining the numerical model boundary condition, and removal of artifacts et al. The problems of computational efficiency and large amount of storage will be overcome with the fast development of computer hardware and adopting some computational technologies and storage strategies. In this paper, we use a simple model to analyze the influences of the initial velocity model building, the source wavelet selection, the numerical model boundary condition definition and the artifacts removal. Especially, we propose the method of amplitude compensation filtering to remove the artifacts in propagation path caused by reflected waves. We implement the pre- stack reverse time migration imaging and achieve better imaging effects for the SEG/EAGE 2-D salt model and Marmousi model, and a rugged water-bottom model designed by us in this paper.