AVO理论在海底弹性参数反演中的应用研究

发布时间：2018-03-27 05:10

本文选题：AVO效应　切入点：AVO反演　出处：《中国地质大学(北京)》2017年博士论文

【摘要】：海底沉积物弹性参数的获取对于天然气水合物勘探、海底冷泉探测等有着重要意义。在海洋地震勘探研究中,研究目标往往集中在海底以下目标体的探测。AVO (Amplitude Variation with Offset)技术作为一种从叠前地震数据中提取地下介质物性参数的有效手段,主要关注了海底以下地层物性参数的反演,对于与海水交界的海底沉积物弹性参数的反演往往被忽略。本文研究目的在于利用海底反射的AV0效应进行海底沉积物弹性参数的反演。海底反射界面可视为一个流体-固体界面,该界面处满足法向应力连续、法向位移连续、切向应力为零三个边界条件。本文基于该界面处的边界条件推导了描述平面纵波入射海底时透反射效应的Zoeppritz方程。海底反射界面是一个强弹性差异界面,常规的弱弹性差异假设下的近似方程不适用于海底反射。本文利用级数展开的方法在不对界面两侧弹性参数做任何假设的情况下推导了反射系数随入射角变化关系的近似方程。描述海底反射系数随入射角变化关系的精确方程和近似方程构成了本文的理论基础。基于海底反射系数随入射角变化关系的精确方程和近似方程,给出了一种海底沉积物三弹性参数的两步线性反演方法。该方法在第一步反演中利用近角度区间内反射系数随入射角正弦平方之间的一次函数关系,求取反射系数的梯度和截距信息;在第二步反演中利用大入射角区间内的反射系数求取海底横波速度与纵波速度之比;最后由梯度、截距和横纵波速度之比求取海底沉积物的纵波速度、横波速度和密度。在该方法的基础上进一步讨论了子波准确性对反演结果的影响,并给出了一种基于岩石物理约束的直接利用反射振幅进行海底沉积物弹性沉积物参数反演的策略。最后通过模型例子分析了两步线性反演方法中近角度区间估计准确性对反演结果的影响。针对两步线性反演方法的不足,本文基于无约束优化算法,直接利用精确的反射系数方程对海底沉积物弹性参数进行反演。从常步长梯度算法入手,详细研究了包括最速下降法、牛顿法、共轭梯度法在内的各种无约束优化算法在海底沉积物弹性参数反演中的表现。这类方法直接利用全角度范围内的反射系数信息进行反演,不需要事先对近角度区间进行估计,这大大提高了反演结果的稳定性。最后对南海实际地震资料进行了反演与初步分析。
[Abstract]:The acquisition of elastic parameters of seabed sediment is of great significance for gas hydrate exploration and submarine cold spring exploration. As an effective method to extract the physical parameters of underground media from pre-stack seismic data, the research object is usually focused on the detection of the subsea target. The technique of AVO amplitude Variation with Offsetts, as an effective means to extract the physical parameters of underground media, mainly pays attention to the inversion of the physical parameters of the strata below the sea floor. The inversion of the elastic parameters of seafloor sediments at the interface with sea water is often ignored. The purpose of this paper is to use the AV0 effect of sea floor reflection to retrieve the elastic parameters of seabed sediments. The seabed reflection interface can be regarded as a. Fluid-solid interface, The interface satisfies the continuity of normal stress and normal displacement. The tangential stress is zero three boundary conditions. Based on the boundary conditions at this interface, the Zoeppritz equation is derived to describe the permeable reflection effect when the plane longitudinal wave is incident to the seafloor. The submarine reflection interface is a strongly elastic differential interface. The conventional approximation equation under the assumption of weak elastic difference is not suitable for submarine reflection. In this paper, the relation between reflection coefficient and incidence angle is derived without any assumption of elastic parameters on both sides of the interface by using series expansion method. The exact equation and approximate equation describing the relation of submarine reflection coefficient with incident angle form the theoretical basis of this paper. Based on the exact equation and approximate equation of submarine reflection coefficient varying with incident angle, In this paper, a two-step linear inversion method for three elastic parameters of seafloor sediment is presented. In the first step, the first step inversion is based on the first order function between the reflection coefficient and the sinusoidal square of the incident angle in the near angle interval. The gradient and intercept information of reflection coefficient are obtained. In the second step, the ratio of sea bottom shear wave velocity to longitudinal wave velocity is obtained by using the reflection coefficient in the range of large incidence angle. The ratio of intercept to shear wave velocity is used to calculate the velocity, shear wave velocity and density of seafloor sediment. Based on this method, the influence of wavelet accuracy on inversion results is discussed. A direct inversion strategy of elastic sediment parameters using reflection amplitude based on the physical constraints of rock is presented. Finally, the near angle interval estimation in the two-step linear inversion method is analyzed by a model example. The effect of accuracy on inversion results. Based on the unconstrained optimization algorithm, the elastic parameters of seafloor sediment are inversed directly by using the exact reflection coefficient equation. Starting with the constant step size gradient algorithm, the most rapid descent method and Newton method are studied in detail. The performance of various unconstrained optimization algorithms, including conjugate gradient method, in the inversion of elastic parameters of seafloor sediments. This kind of method directly uses the reflection coefficient information in the whole angle range for inversion, and does not need to estimate the near-angle interval in advance. This greatly improves the stability of the inversion results. Finally, the inversion and preliminary analysis of the actual seismic data in the South China Sea are carried out.
【学位授予单位】：中国地质大学(北京)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：P631.4

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