摘要
在石油地震勘探的地面多分量记录中，垂直分量上往往只有纵波（P波），径向水平分量上往往只有垂直偏振横波（SV波），而极少见到如地震学中常出现的P波与SV波共存于同一分量记录中的情形。本文从理论上对此作出了解释。其具体过程是，首先根据弹性半空间模型，导出了P波、SV波分别到达自由面的位移响应和Wichert关系式，然后针对不同根波与纵波的速度比（vs／vP）绘制其相应的曲线。通过分析发现，在致密岩石（vs／vs比值高）出露地区，因受自由面效应的影响使得到达P波或SV波在铅垂分量记录和径向水平分量记录上都有显示；而在疏松层（不仅vs／vp比值低，而且vs和vP本身数值也不大，为低速带）覆盖区，由于受自由面效应的影响以及低速带的折射作用，使得到达P波和到达SV波分别反映到错垂分量和径向水平分量上去了。由此可以预见，在高波速比（vs／vp）的表层地区，诸如永久冻土带、石灰岩或其它坚硬基岩出露地带，地面弹性波记录表现为矢量波场；在低波迭比（vs／vp）地区，则近似表现为两个标量波场，垂直和径向水平分量分别以P波和SV波为主，便于资料的采集与处理。须指出的是，目前的各种弹性波正演模拟法以及超声波物理模型试验都应该提供具有小波速比（vs／vp）表层的结果。
In the multicomponent seismograms of surface hydrocarbon seismic explo-ration, compressional wave usually occurs in vertical component, Vertically-polar-ized shear wave (SV-wave) usually appears in radial horizontal component, and werarely see the theoretical phenomenon in seismology that P-wave and SV-wave co-exist in one component. This actual phenomenon is explained theoretically. Ex-planatory procedure consists of two steps:- Free-surface displacement responses and Wichert relations, which are relat-ing to P-wave and SV-wave, are respectively derived from an elastic half spacemodel.. The corresponding curves are plotted according to ratio of S-wave velocityto P-wave velocity (vs/vp).We can see from analysis that the influence of free-surface effect makes P-waveand SV-wave coexist in both vertical component and horizontal component in com-pacted-rock cropping area where vs/vp is high ; but in loose ground area (low veloci-ty layer where vs/vp is low, vs and vp are low), the influence of free-surface effectand the refraction due to low velocity layer make P-wave and SV-wave occur sepa-rately in vertical component and horizontal component.Hence one can foresee that surface elastic wave data behave as vector wavefield in high vs/vp surface area (such as permafrost, limestone crop and other com-pacted-rocd crop); but in low vs/vp area, they nearly behave as two scalar wavefields (P-wave mainly occurs in vertical component, and SV-wave in horizontalcomponent, favouring data acquisition and processing). In addition, author makesfollowing suggestions:. Various forward modelings of elastic waves and the physical modeling of ul-trasonic waves ought to offer the result of low vs/vp surface layer so as to cope withreal conditions in hydrocarbon exploration area.. All methods (such as AVO and inversion) in which prestack amplitude in-formation is used and those methods for extracting elastic polarization parametermust consider free-surface effect so as to improve accuracy-
出处
《石油地球物理勘探》
EI
CSCD
北大核心
1998年第2期139-150,共12页
Oil Geophysical Prospecting
关键词
自由面效应
位移
偏振
地震勘探
油气勘探
free-surface effect, displacement response, polarization charac-teristic, Wichert relation
