子波提取
褶积模型是所有反演的基础:
地震道=地震子波*反射系数+噪声
频率域内, 褶积则为乘积的关系.
反演相当于地震道除以地震子波, 得到反射系数:
反射系数=地震道/地震子波
频域内窄频段的子波限制了信息的获取范围.The narrow band wavelet restricts the available range of information in the frequency domain.
地震子波完全由它的振幅谱和相位谱来定义:
The Wavelet is defined completely by its amplitude
spectrum and its phase spectrum:
在有限频率范围内, 相位谱通常可近似为一条直线. 直线的截距是子波的常数相位旋转, 它是子波的最佳表征. 直线的斜率标示着子波的时移.
The intercept of the line is the constant phase rotation which best characterizes this wavelet.
The slope of the line measures the time-shift of the wavelet.
极性的约定:
极性约定是一个特殊的子波相位问题. 默认的约定便是: 声阻抗的增加在零相位的地震数据上代表一个波峰.
A special wavelet phase issue is the Polarity convention.
The default convention is that an increase in acoustic impedance is represented as a peak on zero-phase seismic data:
另一个默认的约定便是: 声阻抗的增加在零相位的地震数据上代表一个波谷.
The alternate convention is that an increase in acoustic impedance is represented as a trough on zero-phase seismic data:
使用ì极性约定菜单î可以设置极性约定:
The polarity convention is set using the Synthetic
Polarity Convention menu:
地震子波在时间和空间上都存在着变化, 即具有时变性和空变性, 这是基于以下几个原因:
Wavelets in the earth vary both laterally (spatially) and temporally for a variety of reasons:
近地表效应(空变)
Near surface effects (space variant)
频率吸收(时变和空变)
Frequency-dependent absorption (space and time variant)
层间多次波(时变和空变)
Inter-bed multiples (space and time variant)
NMO 拉伸
处理过程中的人为因素
Processing artifacts
STRATA 假定子波是常数, 不随时间和空间变化: 时间不变性: 这意味着反演就是在有限的时窗内求最优化的波阻抗
Time invariant: This means that the inversion is optimized for a limited time window.
空间不变性: 这意味着去除子波的空变后被最优化处理. 通常, 许多方法有可以用来提取子波. STRATA中用了以下几种:
In general, a variety of methods can be used for wavelet extraction. Some are available in STRATA. (1) 仅用地震数据估计地震子波的振幅谱. 假设相位谱已经从别的渠道得知.
子相关autocorrelation
最大熵谱分析maximum entropy spectral analysis
交互谱分析cross spectral analysis
STRATA中统计子波的提取用自相关: Statistical wavelet extraction uses the autocorrelation
(2) 单独使用地震数据估算振幅谱和相位谱
Estimate both amplitude and phase spectra from the seismic data alone.
最小熵子波估计
高阶力矩法higher order moments
STRATA 不用这种方法, 因为STRATA认为该方法不可靠.
(3) 使用给定的测量数据估计振幅谱和相位谱
Estimate both amplitude and phase spectra using deterministic measurements.
海洋信号marine signatures
VSP 分析
STRATA中, 以ASCII文件形式读入外部子波
(4) 用地震和测井资料估算振幅谱和相位谱
Estimate both amplitude and phase spectra using both seismic and well log measurements.
STRATA中用测井资料提取全子波.
(5) 用地震资料和测井资料估算振幅谱和常数相位谱STRATA中用测井资料提取常数相位子波.
STRATA中提取子波的方法:
第一步, 是否用测井资料来估算子波的相位. 关键是看测井资料与地震资料的相关性是否好. 通常情况下, 必须首先进行手动校正测井曲线. The critical issue for this decision is how well the logs tie the seismic data. Usually, manual correlation must be done to align the logs first.
1 提取统计子波(不用井资料):
这个过程只是通对地震道进行自相关计算子波的振幅谱, 并假设已知子波的相位.
主要参数:
ï道范围(通常设置为较大值以增加统计所用的道数) Trace range (usually set this large to increase statistics)
ï时窗(至少应该为子波长度的两倍)
ï子波长度(取决于层厚和分辨率, 层厚一般取200ms, 薄层取50~100ms).
2 用测井资料提取子波:
用测井资料提取子波:
此方法用测井资料估算子波的振幅谱和相位谱. 效果取决于测井曲线和地震道的相关程度.
主要参数:
选择要用的井(只用标定效果好的井)
道范围(距井的距离)
时窗
子波长度
3 用测井资料计算单一常数相位值
该方法使用地震道的自相关计算子波的振幅谱, 与统计子波提取方法中一样, 用测井资料计算子波的相位谱, 并且相位谱被近似为一个单一的常数谱.
This procedure calculates the amplitude spectrum of the wavelet using the autocorrelation of the seismic traces,exactly as in the statistical procedure.
The phase spectrum is approximated as a single constant value, using the well logs.
这种方法比较稳定, 特别是测井资料与地震数据的相关性较差时.
This procedure is more robust than the full phase spectrum calculation, especially when the tie between logs and seismic is poor.
计算相位的步骤:
(1) 用统计子波提取方法计算子波(不用井资料).
(2) 对所提取的子波进行一系列的常相位旋转
(3) 用每一次旋转后的子波计算合成道, 并且与地震道进行相关.
(4) 选出与地震道产生最大相关值的相位旋转
子波提取中的问题:
用井提取子波时, 必须首先求出测井曲线之间的最优化相关
To extract a wavelet using logs, an optimum correlation must be done first.
正确地相关必须以子波已知为前提
To perform correlation properly, the wavelet must already be known.
实际子波提取的流程:
(1) 用统计子波提取来确定一个初步的子波, 假设子波的近似相位已知.
(2) 拉伸或压缩测井曲线来标定地震道.
(3) 使用新的测井曲线来提取新的子波.
(4) 重复第(2)、（3）步，直到提取的子波达到要求为止.。