﻿ 快速回转波近地表速度建模方法
 石油地球物理勘探  2019, Vol. 54 Issue (2): 261-267  DOI: 10.13810/j.cnki.issn.1000-7210.2019.02.003 0
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引用本文

GUO Zhenbo, SUN Pengyuan, QIAN Zhongping, LI Peiming, TANG Bowen, XIONG Dingyu. Fast near-surface model building with turning wave. Oil Geophysical Prospecting, 2019, 54(2): 261-267. DOI: 10.13810/j.cnki.issn.1000-7210.2019.02.003.

文章历史

Fast near-surface model building with turning wave
GUO Zhenbo , SUN Pengyuan , QIAN Zhongping , LI Peiming , TANG Bowen , XIONG Dingyu
Research & Development Center, BGP inc., CNPC, Zhuozhou, Hebei 072751, China
Abstract: Conventional first-arrival travel time tomography based on iterative inversion is computationally intensive and time consuming, especially in a large-scale data processing. In order to improve the efficiency of near-surface modeling, a fast near-surface model building with turning wave is developed by assuming that the near-surface velocity is transversely constant within the maximum offset range and varies linearly with the depth. In order to improve the accuracy of inversion, a multi-datum correction method is adopted to reduce the influence of surface undulation on the inversion results. In order to enhance the stability of inversion, a local weighted ray parameter estimation is developed to increase the robustness with respect to noise. Synthetic and real data tests verify the validity and efficiency of the proposed method.
Keywords: near-surface    tomography    velocity model building    inversion    statics    turning wave
0 引言

1 方法原理 1.1 基本原理

 $v\left( z \right) = {v_0} + gz$ (1)

 $\left\{ {\begin{array}{*{20}{l}} {{x_{\rm{c}}} = {x_0} + \frac{{\sqrt {1 - v_0^2{p^2}} }}{{pg}}}\\ {{z_{\rm{c}}} = {z_0} - \frac{{{v_0}}}{g}} \end{array}} \right.$ (2)
 图 1 地震波在速度随深度线性变换介质中的传播路径

 $r = \frac{1}{{pg}}$ (3)

 $\left\{ {\begin{array}{*{20}{l}} {H = \frac{2}{{pg}}\sqrt {1 - {p^2}v_0^2} }\\ {t = \frac{2}{g}{\rm{ln}}\left( {\frac{{1 + \sqrt {1 - {p^2}v_0^2} }}{{p{v_0}}}} \right)} \end{array}} \right.$ (4)

 ${v_{\rm{T}}} = \frac{1}{p}$ (5)

 $h = \frac{1}{{pg}} - \frac{{{v_0}}}{g}$ (6)

 $\begin{array}{l} O\left( g \right) = {\left( {Hg - \frac{2}{p}\sqrt {1 - {p^2}v_0^2} } \right)^2} + \\ \;\;\;\;\;\;\;\;\;\;\;{W^2}\left[ {tg - 2{\rm{ln}}{{\left( {\frac{{1 + \sqrt {1 - {p^2}v_0^2} }}{{p{v_0}}}} \right)}^2}} \right] \end{array}$ (7)

1.2 多基准面初至时间校正

 图 2 多基准面校正示意图
1.3 基于局部加权的稳定射线参数估计

 图 3 射线参数估计示意图 (a)射线参数整体估计方法示意图；(b)基于局部加权的拟合方法(线2)与常规拟合方法(线1)的对比

 ${O_{{\rm{lin}}}}({t_0}, p) = \sum\limits_{k = 1}^N {{{\left| {{w_k}({t_k} - {t_0} - p{H_k})} \right|}_2}}$ (8)

 ${w_k} = \left\{ {\begin{array}{*{20}{c}} 0&{\left| {{t_k} - {t_0} - p{x_k}} \right| > \Delta {t_{{\rm{max}}}}}\\ 1&{其他} \end{array}} \right.$ (9)

2 理论数据测试

 图 4 Amoco静校正基准测试模型

 图 5 原始数据回转波层析结果

 图 6 将图 5所示结果作为初始模型的常规射线层析结果

 图 7 地表以下不同深度处速度曲线对比 (a)地表以下50m；(b)地表以下100m

 图 8 原始初至与加入随机噪声后的初至对比

 图 9 利用加入噪声初至进行回转波层析的结果
3 实际资料测试

 图 10 实际数据本文方法(a)和常规射线层析(b)反演结果

 图 11 本文方法层析静校正后分炮检距叠加结果

4 讨论

(1) 关于速度局部横向均匀假设。地质构造的尺度通常要比地震观测最大炮检距的尺度大得多，在最大炮检距范围内将其假设为横向均匀是可接受的，也是地震资料处理过程中常用的一种假设条件。本文方法与常规射线类层析方法的关系可类比于叠前时间偏移与叠前深度偏移的关系。

(2) 关于速度纵向线性递增假设。由于压实的作用，地层的速度通常是随深度逐渐增加的，特别是对于距地表几百米以内的速度分布(近地表建模主要关注的区域)这种现象更为明显[22]。近地表速度分布的线性递增假设符合大部分地区地质规律。

(3) 本文方法基于射线路径圆弧状假设，换一个角度来说，等价于利用圆弧状射线路径去拟合实际观测旅行时。由于射线类层析分辨率的限制(不小于第一菲涅耳带)[23]，这种假设与常规射线类层析反演方法同样是非常接近的。图 12为利用图 6所示的反演结果对不同位置的三炮进行射线追踪的结果，由射线路径分布可见，除了速度结构特别复杂的区域外，大部分射线路径与圆弧较为接近。

 图 12 利用图 6所示反演模型计算的三炮的射线路径

5 结束语

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