Detecting artifacts in seismic profiles
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摘要: 假象识别是正确解释地震剖面的前提，也是描述地震成像保真度（精准度）的主要标准之一. 本文通过分析反射地震剖面实例，对几种常见的、易被误解的地震成像假象的特征进行了分类阐述，提出基于成因识别地震成像假象的思路. 从地震剖面成像过程来看，假象的成因包括信号误判、成像畸变和波速模型误差这三个主要因素. 许多不易被识别的假象起源于地震数据处理成像过程中对信号的误判，犹如“张冠李戴”；成像畸变假象的根源包括成像照明度、资料频宽和方法假设等方面的缺陷，造成抹痕等成像结果畸变假象；当地震波速度模型误差较大时，地震成像结果不仅会出现成像位置错误，还可能在波阻抗不连续点出现散射拖尾假象. 因此，衡量地震成像精准度的标准应包括成像结果的分辨率、位置准确度和假象识别三个要素. 这些要素相互影响，因此应该根据研究目标和实际情况，综合评判地震成像中假象的影响.Abstract: Detecting imaging artifacts is a precondition for interpreting seismic profiles, as well as one of the fidelity (accuracy) measures for all seismic images. Based on field seismic reflection profiles, this paper analyzes some common and often miss-interpreted seismic imaging artifacts, and proposes to detect the artifacts through analyzing their origins. From the perspective of seismic imaging process, the origins of seismic imaging artifacts include three main factors: mistaking the signals, imaging distortions and errors in the reference velocity model. Many deceiving artifacts are originated by mistaking the signals in the processing and imaging processes. The imaging distortions are due to insufficiencies in seismic illumination, data frequency bandwidth and assumptions of the imaging methods, leading to smearing and other imaging distortion artifacts; With an erroneous reference velocity model, seismic images may show not only position errors, but also diffraction artifacts at discontinuous positions of seismic impedance. Hence, the measurable elements of the fidelity of seismic images should include resolution, position accuracy and artifact detection. These elements are connected with each other, thus should be evaluated jointly in detecting artifacts, according to the given study objectives and resources available.
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Key words:
- seismic profiles /
- seismic imaging /
- artifact detection /
- artifact origins /
- fidelity
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图 1 对深海地震剖面一个较连续同相轴的两种解释. （a）反射波时间偏移剖面，显示洋底沉积层与大洋基底的分界面，和基底内一个同相轴（箭头所示）. （b）该同相轴或被解释为断层，如红色虚线所示. （c）经分析，该同相轴是由洋底沉积层内多次波造成的假象，如图中沉积层的镜像投影所示
Figure 1. Two interpretations of a continuous reflector on a deep-sea seismic profile. (a) Time-migrated reflection profile shows the interface between ocean bottom sediments and basement, and a linear anomaly (denoted by arrows). (b) A plausible fault interpretation of the anomaly, denoted by a dash line. (c) Detailed analysis indicates the anomaly is an artifact from mistaking internal multiples in the sediments as primary reflections, as shown by the mirror projection of the sediments
图 2 对比两个水平地震时间切片（Biondi, 2006）. （a）三维地震成像切片；（b）二维地震成像切片. 在箭头所指处，三维成像剖面只显示一条河道；二维成像剖面则显示多条河道，因其误把二维测线侧向反射能量当成测线之下信号成像，造成假象
Figure 2. Two seismic time slices based on (Biondi, 2006): (a) 3D imaging; (b) 2D imaging. At the location marked by arrows, the 3D slice shows a single channel, while the 2D slice shows multiple channels, as they contain artifacts from miss-focused energy from laterally outside the 2D survey lines
图 3 在美国加州圣安德烈斯断层（SAF，图中距离为0处垂直黑线）附近，两个VSP剖面上的抹痕假象. 资料由测井（粉红色虚线）中布设的32个检波器采集天然微震和地表人工震源而成. （a）根据4个微震和3个地表震源资料制作的偏移剖面. （b）根据7个微震和2个地表震源资料制作的偏移剖面。图中颜色表示纵波反射振幅，字母和黑线表示原作者解释的几个小断层。图中圆弧状彩色条带皆沿着以震源和检波器为焦点的双程等时线分布，因此我们认定这些断层反射图像含有大量抹痕假象（Chavarria et al., 2003）
Figure 3. Along-isochron smearing artifacts on migrated P-wave profiles using VSP data acquired near the San Andres fault (SAF), at 0 km in distance on each profile. The data of microearthquakes and surface shots were acquired by 32 receivers placed in a vertical bore (pink dash line). (a) Migrated image of the data from 4 microearthquakes and 3 shots nearest to the well. (b) Migrated image of data from 7 microearthquakes and 2 shots that are near the well. Letters in these sections and dashed lines in (b) denote interpreted faults. Both cross sections show many colored arcs and stripes following the two-way traveltime isochrons with the sources and receivers as foci, leading to our conclusion that these fault images contain lots of smearing artifacts (Adapted © Chavarria et al. (2003), some rights reserved exclusive licensee AAAS. Distributed under CC BY-NC)
图 4 空间混叠假象示意. （a）一个理论褶皱剖面（Zhou, 2014）. （b）当垂向采样率不足时，在点圈所示剖面（a）高倾角薄层部位出现空间混叠假象，呈现与真实倾角方向反向排列的点状互层. （c）一条地震剖面实例，时间采样间隔1 ms. （d）当时间采样间隔粗化到6 ms时，剖面（c）中许多高倾角薄层部位出现空间混叠假象
Figure 4. Illustration of spatial aliasing. (a) A sketch profile of folded strata（Zhou, 2014）. (b) After an insufficient vertical sampling of profile (a), spatial aliasing artifacts appear in three dashed ellipses, where the steeply dipping thin strata become spotty dipping lines of opposite dipping angles. (c) A field reflection profile of 1 ms in sample time interval. (d) After coarsening the sample time interval of profile (c) to 6 ms, spatial aliasing artifacts appear at many places of steeply dipping thin strata
图 5 波速模型变化对一个模拟盐丘地震偏移剖面的影响（Lazarevic, 2004）. （a）正确波速模型产生的剖面. （b）把正确波速模型降慢10%（乘以0.9）产生的剖面. （c）把正确波速模型提快10%（乘以1.1）产生的剖面
Figure 5. The impacts of velocity variation on migrated profiles of a synthetic salt model（Lazarevic, 2004）. (a) Profile of the correct velocity model. (b) Profile of a slower velocity model (multiplied the model of (a) by 0.9). (c) Profile of a faster velocity model (multiplied the model of (a) by 1.1)
图 6 在美国南加州一条南北地震剖面上对比三个针对莫霍面的研究。（a）平面图显示剖面位置、区内地震（粉色小点）、固定地震台（蓝色实三角）、LARSE反射地震剖面的检波器（黑三角形）和炮点（红圈）以及主要断层（浅蓝线）。（b）剖面图对比三个莫霍面研究结果：（1）层析成像莫霍面（黄虚线）基于彩色层所示天然地震走时层析P波速度模型（修改自Zhou et al., 2010）；（2）反射莫霍面（浅蓝线）根据人工地震资料获得的反射地震剖面（修改自Fuis et al., 2007）；（3）6个地点的接收函数莫霍面深度范围（修改自Zhu, 2002），由红蓝工字形符号表示。剖面上方5个蓝色框显示剖面所穿过主要断层的名称缩写：SMF为圣塔莫尼卡断层，SSF为圣塔苏珊娜断层，SGF为圣盖博瑞断层，SAF为圣安德烈斯断层，GF为咖劳克断层
Figure 6. Comparison between three independent studies of the Moho discontinuity on a south-to-north seismic profile in Southern California. (a) Map along the profile showing regional earthquakes (pink dots), permanent seismic stations (blue filled triangles), geophones (black open triangles) and shots (red open circles) of a LARSE reflection line, and major faults (light blue lines). (b) Profile view of the Moho discontinuity: (1) Tomography Moho (yellow dashed line) based on P-wave velocities using earthquake traveltime data（modified from Zhou et al., 2010）; (2) PmP Moho (solid and dotted line in light blue color) based on reflection data（modified from Fuis et al., 2007）; (3) Moho depth ranges at six locations (red bars with blue circles) based on receiver functions（modified from Zhu, 2002）. Five box labels over the profile show acronyms of faults: Santa Monica (SMF), Santa Susana (SSF), San Gabriel (SGF), San Andreas (SAF), Garlock (GF)
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