复杂地区地震资料低信噪比的原因及对策(一)——表层散射波是导致地震资料低信噪比的根本原因
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摘要
复杂地区的低信噪比问题,是长期困扰油气地震勘探工作者的重要难题。针对大量干扰记录,结合现场实际,采用由简单到复杂、由个别到一般,逐步深入的分析方法,得出了若干重要认识与结论:①波在传播过程中遇到不均匀体——散射源都要产生波的散射;②表层介质的不均匀性最强、结构最复杂,是散射源的主要分布区域;③直达波、反射波等传到不均匀体都有可能产生散射,但散射波的主要能源是面波;④在复杂地区,散射干扰是形成资料低信噪比的关键因素。同时还发现表层散射源主要有:两种不同物性介质的结合部;凹凸不平的低降速带底界;高地势,特别是高速岩层的高陡山区;复杂地形;厚度较大的第四系分布区;溶洞、溶缝分布区等。分析结果认为:散射源可按其大小、散射次数和散射波形态分类;散射强度与不均匀体和围岩阻抗差大小、个体大小、离激发点与测线距离、地势高低、低降速带厚度、地表光滑性等因素有关。这些认识与结论是在理论与实践结合上对散射波的形成机制认识的重大进展,对散射波的压制具有重要指导意义。
A low signal-to-noise ratio of seismic data in complex areas has long been a major problem challenging seismic survey for hydrocarbon exploration.Based on a large number of interference seismic records and field practices,some important conclusions are drawn by progressive analytical methods from simple to complex and from individual to general.Seismic waves scatter when they meet an inhomogeneous object,i.e.a scattering source.The surface medium is the strongest in heterogeneity and the most complex in structure,thus is the major place where inhomogeneous objects occur.Seismic waves such as direct waves and reflected waves may scatter when they meet inhomogeneous objects,but the major energy source of scattered waves is surface waves.Scattering interference is the major cause of a low signal-to-noise ratio in complex areas.In addition,surface scattering sources mainly include interface between two different media,the lower boundary of uneven and low subweathered zones,high surface relief areas especially highly steep mountain areas with high-velocity rocks,complex topography,Quaternary cover areas with great thickness,and areas with solution cavity and dissolved fractures.The scattering sources can be classified according to their volume,scattering number,and geometry of scattered waves.The intensity of scattering is related to the impedance contrast between the inhomogeneous object and surrounding rocks,the volume of inhomogeneous object,distances to shot point and seismic line,slope and relief,the thickness of low weathered zone,smoothness of surface,etc.These understandings are of great significance to the elimination of scattered waves.
A low signal-to-noise ratio of seismic data in complex areas has long been a major problem challenging seismic survey for hydrocarbon exploration.Based on a large number of interference seismic records and field practices,some important conclusions are drawn by progressive analytical methods from simple to complex and from individual to general.Seismic waves scatter when they meet an inhomogeneous object,i.e.a scattering source.The surface medium is the strongest in heterogeneity and the most complex in structure,thus is the major place where inhomogeneous objects occur.Seismic waves such as direct waves and reflected waves may scatter when they meet inhomogeneous objects,but the major energy source of scattered waves is surface waves.Scattering interference is the major cause of a low signal-to-noise ratio in complex areas.In addition,surface scattering sources mainly include interface between two different media,the lower boundary of uneven and low subweathered zones,high surface relief areas especially highly steep mountain areas with high-velocity rocks,complex topography,Quaternary cover areas with great thickness,and areas with solution cavity and dissolved fractures.The scattering sources can be classified according to their volume,scattering number,and geometry of scattered waves.The intensity of scattering is related to the impedance contrast between the inhomogeneous object and surrounding rocks,the volume of inhomogeneous object,distances to shot point and seismic line,slope and relief,the thickness of low weathered zone,smoothness of surface,etc.These understandings are of great significance to the elimination of scattered waves.
引文
[1]勾丽敏,刘学伟,雷鹏,等.金属矿地震勘探技术方法研究综述——散射波地震勘探方法[J].勘探地球物理进展,2007,30(2):85-91.
[2]李灿苹,刘学伟,王祥春,等.地震波的散射理论和散射特征及其应用[J].勘探地球物理进展,2005,28(2):81-89.
[2]李灿苹,刘学伟,王祥春,等.地震波的散射理论和散射特征及其应用[J].勘探地球物理进展,2005,28(2):81-89.
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