L-Va曲线监控偏移时间剖面构造畸变现象及其校正

Monitoring and correction of structural distortion in time-migration sections based on the velocity pitfall curve L-Va

如何提前预测时间偏移剖面构造形态是否发生畸变现象及其畸变类型,进而判别深度剖面构造形态是否合理,是复杂构造地震资料处理和解释的难点和重点。为此,提出一种监控时间偏移剖面上构造畸变现象的方法,以构造+层速度模型为基础,沿层提取加权平均速度,得到沿层速度陷阱L-Va曲线(L代表层位;Va代表加权平均速度)。L-Va曲线形态主要有3种类型:①水平直线,揭示沿层没有发生速度陷阱,时间偏移剖面构造形态未发生畸变;②下凹曲线,揭示沿层产生“速度下拉效应”的速度陷阱,时间偏移剖面构造形态发生“向斜型”畸变;③上凸曲线,揭示沿层产生“速度上拉效应”的速度陷阱,时间偏移剖面构造形态发生“背斜型”畸变。观测L-Va曲线变化趋势和几何形态,可判断时间偏移剖面目标层构造形态畸变程度、背斜高点漂离方向和距离,有利于指导变速时深转换和叠前深度偏移时克服速度陷阱,对时间偏移剖面构造畸变现象进行有效校正,判定深度剖面构造形态的合理性,提高构造圈闭落实精度。一般地,同一个勘探区域和同一个构造带的地层、构造特征和层速度结构基本相同,因此,有井约束构造+层速度模型实例分析得到的认识,可以推广到无井约束的构造+层速度模型,从而监控无井约束偏移时间剖面构造畸变现象。该方法操作简单、实用,可以满足山地复杂构造油气勘探精度需求。

While processing and interpreting seismic data from complex structures,it is important(yet difficult)to correctly predict structural distortions in time-migration sections and,at the same time,determine the reasonability of the interpretation of structures in the depth domain.In this paper,a method for monitoring structural distortions in time-migration sections was proposed.The weighted average velocity along the target stratum was calculated through a model that integrates structural information and interval velocity so as to obtain the velocity pitfall curve L-Va.This curve can have three distinct shapes,namely:①a horizontal straight line,indicating that no velocity pitfall effects occur along the stratum,and no structure is distorted in time-migration sections;②a concave curve,indicating a“velocity pull-down effect”along the stratum and that synclinal deformation occurred in time-migration sections;③a convex curve,indicating a“velocity pull-up effect”along the stratum and that anticlinal deformation occurred in time-migration profiles.The extent of distortion,the anticline crest deviation,and the deviation distance can be determined by observing the patterns of the L-Va curves.This can help eliminate the velocity pitfall effect during viable velocity time-depth conversion or prestack depth migration,thereby effectively correcting structural distortions in time-migration sections and improving the accuracy of structural traps.Generally,in a given exploration area or tectonic zone,there is a correspondence between the strata,the structural characteristics,and the interval velocity.Therefore,knowledge gained from well-constrained structures using the interval velocity model can be applied to the structural model directly,without using well constraints to monitor the structural distortions in time-migration sections.The method is practical and easily implementable,and thus can meet the demands of oil and gas drilling and exploration in complex mountainous structures.