三参量速度分析中的构造形态约束

在三参量速度分析中,需要根据三维地震勘探资料充求取地下界面的倾向和倾角,然后确定均方根速度.由于进行速度分析所用的地震数据不可能很多,如果地震资料的信噪比又不高,就很难求准地层的倾向和倾角,进而使均方根速度有较大误差,降低了三参量速度分析的整体效果.只要原始资料的信噪比不是特别差,处理参数也没有太大的问题,在叠加剖面上总是能看到地下构造的形态.由于叠加剖面在时间和空间两个方向上都有较大的尺度,所以在叠加剖面上确定同相轴的倾角比较准确.叠加剖面上同相轴的倾角实际上是反射层的时间视倾角.如果知道纵测线和横测线两个方向上的时间视倾角,又知道地层的平均速度,就能求得反射层的真倾向和真倾角.这样求出的倾向和倾角能表示较大范围的反射层结构的总体情况,虽然它与速度分析点上的倾向和倾角有一定的差异,但作为后者的参考值和约束还是可以接受的.采用大范围反射层的倾向和倾角对三参量速度分析进行约束之后,避免了计算过程中倾向和倾角出现较大的误差,改善了低信噪比资料的三参量速度分析的效果.理论模型数据和实际资料处理结果表明,该方法是切实可行的,可以提高三维地震资料的处理水平.

Structural shape restriction in three-parameter velocity analysis

In the three-parameter velocity analysis,we first derive dip direction and dip angle of reflection interface from 3-D seismic data,then obtain mean square-root ve-locity. Only limited seismic data are used in velocity analysis,so that if the seismic data have low signal/noise ratio,we can hardly estlmate satisfactory dip direction,dip angle and mean square-root velocity,thus reducing the effect of three-parameter velocity analysis.Generally speaking,structural shape can be seen in stacked seismic section solong as the signal/noise ratio is not too low and the processing parameters are rea-sonalble on the whole. Stacked seismic section has big vertical and lateral sizes,sothe dip angles of seismic reflection legs in the stacked seismic section can be deter-mined quite correctly.The dip angle of reflection leg in stacked seismic section is actually the time ap-parent dip angle of reflection interface. Having known the time dip angles in inlineand crossline directions,and the corresponding average velocity,we can compute thetrue dip direction and true dip angle of reflection interface. The dip direction and dipangles we obtained in this way can show the general structural shape in quite largeextent. Although the dip directions and dip angles slightly deviate from those atpoints where velocity is analsyed,they can be the reference and restriction of thelatters.After the three-parameter velocity analysis is restricted by using the dip direc-tions and dip angles of wide reflection interfaces,big errors of dip direction and dipangle can be avoided desirably,thus improving the velocity analysis of the seismicdata whose signal/noise ratio is low. It is practically and theroretically shown thatthe method is feasible and improves the processing of 3-D seismic data.