Stochastic characterization and seismic response of upper and middle crustal rocks based on the Lewisian gneiss complex, Scotland
Stochastic characterization and seismic response of upper and middle crustal rocks based on the Lewisian gneiss complex, Scotland
复制标题
基于苏格兰刘易斯片麻岩杂岩的上中地壳岩石的随机表征和地震响应
DOI：
10.1111/j.1365-246x.1994.tb00925.x
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发表时间：
1994
影响因子：
2.8
通讯作者：
K. Holliger
中科院分区：
文献类型：
--
作者：
A. Levander;R. England;S. Smith;R. Hobbs;J. Goff;K. Holliger
总结
根据苏格兰刘易斯片麻岩杂岩的地质图和岩石物理数据，我们建立了中地壳和上地壳地震速度不均匀性的统计模型。我们所测量的不均匀程度与使用常规反射和折射技术的结晶地壳的地震勘探有关。我们在26.8米的网格上数字化了两张1：10560的拉克斯福德（太古代）时代刘易斯岩石地质图。这两张地图都被认为是苏格兰西北部上地壳和中地壳的代表，并且都被认为提供了地壳部分的横截面视图。数字地图的特点是其岩性人口的统计数据，并通过其二维空间自相关函数。利用岩石物理学数据，为不同的岩性指定适合中地壳的地震速度。主要有三种岩性：长片麻岩（Vp = 6.2 km s−1）、镁铁质斜长角闪岩（Vp = 6.75 km s −1）和中等成分片岩（Vp = 6.5 km s −1）。这两个地图都有自仿射空间结构。
第一张地图覆盖了向斜的核心。它的自相关函数定义了一个分形维数为2.78，水平特征长度约为244米，垂直相关约为133米（纵横比为2：1）的介质。它具有基本上三峰速度（岩性）人口组成的37%硅质片麻岩，43%镁铁质斜长角闪岩，和20%的片岩。这张地图代表了30%-40%的拉克斯福阶岩石。第二幅图是平面图，可绕垂直于走向的轴旋转90°以给出横截面。该图的分维值为2.55，水平相关长度约为111 m，垂直相关长度约为38 m（纵横比3：1）。它有一个几乎双峰人口组成的77%的片麻岩和22%的镁铁质角闪岩。它代表了60- 70%的拉克斯福德岩石。
最后，我们研究的地震响应的上部到中部地壳的基础上，我们的统计模型，使用声学和弹性的2-D有限差分合成地震记录。短炮检距记录表明，刘易斯上地壳产生的散射波，显着干扰信号反射从更深层次。透射散射Q和尾波衰减率的测量证实，在刘易斯型地壳的地震散射是强大的。刘易斯地壳模型的偏移CMP响应显示了在上地壳中经常观察到的特征性“盐和胡椒”图案，并且被错误地描述为“不均匀”。我们认为“半透明”是一个更合适的描述。
SUMMARY
We have developed statistical models of upper and middle crustal seismic velocity heterogeneity based on geologic maps and petrophysical data from the Lewisian gneiss complex, Scotland. the level of heterogeneity we have measured is relevant to seismic exploration of the crystalline crust using conventional reflection and refraction techniques. We digitized two 1:10560 geologic maps of Laxfordian (Archean) age Lewisian rocks on a 26.8m grid. Both maps are believed to be representative of the upper and middle crust in north-western Scotland, and both are believed to provide cross-sectional views of parts of the crust. the digital maps were characterized by the statistics of their lithologic populations and by their 2-D spatial autocorrelation functions. Different lithologies were assigned seismic velocities appropriate for the mid-crust using petrophysical data. Three lithologies are dominant: silicic gneisses (Vp∼6.2 km s−1), mafic amphibolites (Vp— 6.75 kms−1), and intermediate composition schists (Vp—6.5kms−1). Both maps have self-affine spatial fabrics.
The first map covers the core of a syncline. Its autocorrelation function defines a medium with a fractal dimension of 2.78, a horizontal characteristic length of about 244m and a vertical correlation of about 133m (aspect ratio is 2:1). It has an essentially trimodal velocity (lithology) population consisting of 37 per cent silicicgneiss, 43 per cent mafic amphibolites, and 20 per cent schists. This map is representative of 30-40 per cent of Laxfordian rocks. the second map is a plan view which can be rotated 90° about an axis perpendicular to strike to give a cross-section. This map is characterized by a fractal dimension of 2.55, with a horizontal correlation length of about 111m and a vertical correlation of about 38m (aspect ratio 3:1). It has a nearly bimodal population consisting of 77 per cent silicic gneisses and 22 per cent mafic amphibotites. It is representative of 60-70 per cent of Laxfordian rocks.
Lastly we examine the seismic response of an upper to middle crust based on our statistical models using acoustic and elastic 2-D finite-difference synthetic seismograms. Short-offset shot records demonstrate that a Lewisian upper crust produces scattered waves which significantly disrupt signals reflecting from deeper levels. Measurements of transmission scattering Q, and coda decay rates confirm that seismic scattering in Lewisian-type crust is strong. the migrated CMP response of a Lewisian crustal model shows the characteristic ‘salt and pepper’ pattern often observed in the upper crust, and described, incorrectly, as ‘transparent'. We suggest that ‘translucent’ is a more appropriate descriptor.