AVO investigations of shallow marine sediments
AVO investigations of shallow marine sediments
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DOI：
10.1046/j.1365-2478.2001.00246.x
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发表时间：
2001-03
影响因子：
2.6
通讯作者：
M. Riedel;F. Theilen
中科院分区：
文献类型：
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作者：
M. Riedel;F. Theilen
幅度随偏移量变化 (AVO) 分析基于 Zoeppritz 方程，可以将反射和透射系数计算为偏移量或入射角的函数。本文介绍的高频（高达 700 Hz）AVO 研究已用于确定浅海环境（20 m 水深）中沉积物的物理特性。可以约束的属性包括纵波和横波速度、体积密度和声衰减。使用更高频率需要特殊分析，包括仔细的几何形状以及源和接收器方向性校正。过去，海洋沉积物被建模为弹性材料。然而，包含吸收的粘弹性模型更为真实。当入射角大于 40° 时，从粘弹性模型导出的 AVO 函数与不存在临界入射角时具有纯弹性特性的函数不同。横波速度对反射系数的影响很小（尤其是海底遇到的低横波速度）。因此，很难从 AVO 趋势中提取 S 波参数。另一方面，纵波速度和密度显示出相当强的影响。衰减（由品质因数 Q 描述）会影响反射系数，但无法从 AVO 函数中唯一确定。为了测量地震图中的反射系数，需要确定共中点 (CMP) 道集中直达波和海底反射的振幅，并针对球面发散以及源和拖缆方向性进行校正。在显示泥浆和巨石粘土不同 AVO 特征的 CMP 位置，沉积物物理性质是通过使用顺序二次规划 (SQP) 反演技术确定的。泥浆反演泥沙物性为：纵波速度α=1450±25 m/s，横波速度β=90±35 m/s，密度ρ=1220±45 kg/m3，纵波品质因数QP=15±200，横波品质因数QS=10±30。砾石粘土反演沉积物物理性质为：α=1620±45 m/s，β=360±200 m/s，ρ=1380±85 kg/m3，QP=790±660，QS=25±10。
Amplitude‐variation‐with‐offset (AVO) analysis is based on the Zoeppritz equations, which enable the computation of reflection and transmission coefficients as a function of offset or angle of incidence. High‐frequency (up to 700 Hz) AVO studies, presented here, have been used to determine the physical properties of sediments in a shallow marine environment (20 m water depth). The properties that can be constrained are P‐ and S‐wave velocities, bulk density and acoustic attenuation. The use of higher frequencies requires special analysis including careful geometry and source and receiver directivity corrections. In the past, marine sediments have been modelled as elastic materials. However, viscoelastic models which include absorption are more realistic. At angles of incidence greater than 40°, AVO functions derived from viscoelastic models differ from those with purely elastic properties in the absence of a critical angle of incidence. The influence of S‐wave velocity on the reflection coefficient is small (especially for low S‐wave velocities encountered at the sea‐floor). Thus, it is difficult to extract the S‐wave parameter from AVO trends. On the other hand, P‐wave velocity and density show a considerably stronger effect. Attenuation (described by the quality factor Q) influences the reflection coefficient but could not be determined uniquely from the AVO functions. In order to measure the reflection coefficient in a seismogram, the amplitudes of the direct wave and the sea‐floor reflection in a common‐midpoint (CMP) gather are determined and corrected for spherical divergence as well as source and streamer directivity. At CMP locations showing the different AVO characteristics of a mud and a boulder clay, the sediment physical properties are determined by using a sequential‐quadratic‐programming (SQP) inversion technique. The inverted sediment physical properties for the mud are: P‐wave velocity α=1450±25 m/s, S‐wave velocity β=90±35 m/s, density ρ=1220±45 kg/m3, quality factor for P‐wave QP=15±200, quality factor for S‐wave QS=10±30. The inverted sediment physical properties for the boulder clay are: α=1620±45 m/s,β=360±200 m/s,ρ=1380±85 kg/m3,QP=790±660,QS=25±10.