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由近地表结构、衰减和散射引起的振幅随炮检距变化(AVO)观测值的趋势和变化,可以用拟频谱黏弹性2-D模拟进行数值合成。近地表结构产生振幅聚焦和散焦,极大地畸变了在炮检距时窗内的AVO观测值,畸变的尺度相当于构造的横向变化。衰减和散射减弱了所有炮检距上的绝对振幅。散射和波的干涉加大了与AVO测量值有关的变化。根据内在衰减、与散射有关的视衰减和几何聚焦的相对影响,一个归一化的AVO响应可以随炮检距增大或减小(相对于有关的弹性的、非散射的1-D解而言),从而,与预测为密度、速度、孔隙度或泊松比的反差函数的特性相似。如果只有相对(归一化)振幅可用的话,就很难区分对绝对振幅有主要贡献的那些参数的影响。例如,固有衰减产生的递减振幅的趋势(相对于一个弹性平层模型而言)可以被大排列长度范围内的聚焦/散射或各向异性的影响所抵消。当噪声水平足够高时,有关散射和衰减对AVO影响的诊断信息就丢失了。因此,基于均匀层状弹性模型的AVO观测值解释必须小心使用,因为一般来讲它不是唯一的。AVO参数中的横向变化是烃类检测的关键,这样由上覆岩层特征的横向变化产生的AVO横向变化有可能被错误地解释,尤其当记录排列长度比较小时。
The quasi-spectral viscoelastic 2-D simulation can be used for the numerical synthesis of the amplitudes caused by near-surface structure, attenuation and scattering as a function of offsetting variation (AVO) observations. The near-surface structure produces amplitude focusing and defocusing, greatly distorting the AVO observations at the offset time window, and the scale of distortion corresponds to the lateral variation of the structure. Attenuation and scattering attenuate the absolute amplitude at all offsets. Scattering and wave interference increase the variation associated with AVO measurements. A normalized AVO response may increase or decrease with offset based on the relative effects of intrinsic attenuation, scatter-related apparent attenuation, and geometric focus (relative to the associated elastic, unscattered 1-D solution Thus, the characteristics of the inverse function predicted as density, velocity, porosity or Poisson’s ratio are similar. If only relative (normalized) amplitudes are available, it is difficult to distinguish between those parameters that have a major contribution to the absolute amplitude. For example, the tendency of diminishing amplitudes produced by inherent attenuation (relative to a flat elastic model) can be offset by the effects of focusing / scattering or anisotropy over a large array length. When the noise level is high enough, diagnostic information on the effects of scattering and attenuation on AVO is lost. Therefore, interpretations of AVO observations based on a uniform layered elastic model must be used with caution because, in general, it is not unique. Lateral variations in the AVO parameters are the key to hydrocarbon detection so lateral variations in AVO due to lateral variations in overburden character may be misinterpreted, especially if the recording arrangement length is relatively small.