逆时偏移在声反射成像测井中的应用方法研究

声反射成像测井中常用的基于射线理论的Kirchhoff积分偏移算法和基于单程波理论的F-K偏移算法均可实现井旁缝洞反射体的快速偏移成像,但其仅适用于地层垂向变化较弱的速度场和高陡角裂缝的偏移成像,无法实现低角度反射体的准确偏移归位,产生偏移假象误导测井解释.逆时偏移基于全波动方程,可适应强垂向变化速度场,实现近似水平反射体的偏移成像.本文详细分析了将逆时偏移应用于声反射成像测井时存在的数据准备、时间采样间隔匹配和成像条件改进等若干问题,通过设置多组理论模型来说明算法对井旁不同反射体的识别能力.模拟资料和实际资料处理结果证实,较F-K偏移算法,逆时偏移算法成像精度更高、收敛性更好,可有效实现近似水平构造偏移归位.改进的归一化互相关成像条件可解决深部地层的远井壁成像衰减问题,降低测井解释的多解性.逆时偏移将成为声反射成像测井高精度偏移技术的发展方向.

Study on application method of reverse time migration in acoustic reflection imaging logging

The Kirchhoff integral migration based on ray theory and the F-K migration based on one-way wave theory are common in acoustic reflection imaging logging. These algorithms can achieve fast migration imaging of fracture reflectors by boreholes, but they are only appropriate for the velocity field with weak variation in the vertical direction and the imaging of high angle cracks. They cannot achieve the migration of low angle fracture reflectors, which will result in the imaging artifacts and the misunderstanding of logging interpretation. Based on the full wave equation, reverse time migration (RTM) can adapt to the velocity field with strong variation in the vertical direction and achieve the imaging of the approximate horizontal reflectors. In this paper, several problems of using RTM in the data processing of acoustic reflection imaging logging are analyzed in details, the data preparation, the match of time sampling interval and the improvement of imaging condition. The recognition ability of RTM algorithm of different reflectors is illustrated by setting multiple sets of theoretical models. The processing results of synthetic data and real data demonstrate that, compared to the F-K migration algorithm, RTM has a higher imaging accuracy and a better convergence, which can effectively achieve the migration homing of approximate horizontal structures. The modified normalized cross-correlation imaging condition can solve the problem of imaging attenuation far from boreholes in deep formation, and reduce the multi-solution of logging interpretation. RTM can be described as the development direction of acoustic reflection imaging logging high precision migration technology.