基于广义S变换的透射槽波埃里相识别
Research on identifying the airy phase of transmitted channel waves based on generalized S-transform
责任编辑: 叶佩
收稿日期: 2020-06-1 修回日期: 2021-05-12
基金资助: |
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Received: 2020-06-1 Revised: 2021-05-12
作者简介 About authors
陈波(1984-),男,在读博士研究生,主要从事地震信号分析及层析成像方面的研究工作。Email:
透射槽波勘探是查明煤矿工作面地质情况的重要物探方法之一,其频散曲线埃里相识别的合理性直接关系到后续工作面层析成像的准确性。目前,S变换已被广泛应用到频散分析中,然而由于S变换窗函数固定,其应用效果受到限制,为进一步提高频散曲线埃里相的识别精度,本文将窗函数可调的广义S变换引入到频散曲线的分析中。对广义S变换的窗函数采用时间半高宽进行分析,窗函数时间半高宽在给定频率范围内越宽,其时间分辨率低;频率分辨率增高,窗函数宽度越窄,则时间分辨高、频率分辨低。给出了时间半高宽与广义S变换参数的关系式,可根据实际情况定量调节时频分辨率。计算表明,在合理选择广义S变换参数的前提下,广义S变换能有效提高时频分辨率,改善频散曲线埃里相特征,利于解释人员准确拾取透射槽波埃里相。
关键词:
Transmission seismic exploration in the coal mining face is one of the important geophysical surveys to figure out the geological hazards of coal seam.The rationality of identifying the airy phase from the transmitted channel waves is closely related to the accuracy of the tomography.Nowadays,S-transform is widely used in the analysis of dispersion curve,however,its application is limited with the fixed window.In order to improve the identification accuracy of the airy phase of the dispersion curve,this paper introduces the generalized S-transform with adjustable window into the analysis of dispersion curve.The window of generalized S-transform is analyzed by temporal full width at half maximum (temporal FWHM).For a frequency range,a wider window indicates a lower temporal resolution and a higher frequency resolution,and a narrower window indicates a higher temporal resolution and a lower frequency resolution.The time-frequency resolution can be adjusted quantitatively according to the relationship between the temporal FWHM and the parameters of generalized S-transform in application.The synthetic and real data result show that the generalized S-transform can effectively improve the time-frequency resolution and the airy phase characteristics of the dispersion curve.It is helpful for the interpreter to pick up the airy phase of the transmitted channel waves accurately.
Keywords:
本文引用格式
陈波, 朱国维, 武延辉, 杨振强, 周俊杰.
CHEN Bo, ZHU Guo-Wei, WU Yan-Hui, YANG Zhen-Qiang, ZHOU Jun-Jie.
0 引言
我国煤炭资源丰富,分布广[1],但煤层赋存地质条件差,致灾因素较多,而地面地震勘探精度低,无法满足煤矿企业对安全生产的需求,因此在煤矿工作面内开展透射地震勘探为精细查明工作面内的地质情况提供了可能[2,3]。相对于围岩,煤层速度低,在煤层内激发地震波会产生沿煤层传播的槽波[4,5]。由于在煤层内传播的槽波携带了煤层的地质信息,因此通过对透射槽波的分析可以查明工作面内的地质异常。频散是透射槽波的重要特征,埃里相分布在频散曲线上速度最低能量强的位置,其特征明显,通过对频散曲线埃里相的分析可以获得工作面内煤层的速度及厚度[6,7,8],从而查明工作面内的地质情况。因此,准确地从透射槽波频散曲线中识别埃里相显得尤为关键。
1 广义S变换
Stockwell等[11]提出了S变换,其定义式如下:
其逆变换为:
式中:t为时间,h(t)为待变换的时间序列信号;f为频率,
式中:
图1
图1
广义S变换窗函数时间半高宽随频率变化
Fig.1
Temporal FWHM of generalized S-transform window versus frequency
式中:f1、f2为频率,tFWHM1、tFWHM2为窗函数时间半高宽。当频率为f1时,窗函数的时间半高宽为tFWHM1;当频率为f2时,窗函数的时间半高宽为tFWHM2。附录A给出了详细推导,可以通过给定两个频率点的时间半高宽快速确定A和B。
附录A:
广义S变换窗函数:g(t)=
设频率为f1时,窗函数的时间半高宽为tFWHM1,频率为f2时,窗函数的时间半高宽为tFWHM2。则有方程组:
化简得:
解得A、B为:
图2
图2
频率阶梯变化的非平稳信号的S变换和广义S变换(A=0.4239,B=0.471)对比分析
a—频率阶梯变化的非平稳信号;b—
Fig.2
Comparison of S-transform and generalized S-transform (A=0.4239,B=0.471) for non-stationary signal with frequency step change
a—non-stationary signal with frequency step change;b—S-transform results of
图2b、2c分别为图2a所示信号的S变换和广义S变换结果,从变换的结果可以看到,广义S变换信号的频率分辨率显著提高,在时频域中,阶梯状信号的能量聚集性更好,即频率分辨率提高,由海森堡不确定性原理,其时间分辨率降低,图2c中持续时间较短的高频信号能量相对发散,表明了其时间分辨率降低。图2d展示了图2b和图2c虚线处的线谱,相比S变换,广义S变换信号能量更集中,频率分辨率得到大幅提高。图2e为50 Hz时的S变换和广义S变换窗函数比较,广义S变换窗函数呈现出“矮”、“宽”特征,图2f为窗函数时间半高宽对比图,广义S变换窗函数整体比S变换宽,因此,其频率分辨率高于S变换。任取两点,由式(4)和式(5)即可确定A、B值,本例中A=0.4239、B=0.471。
2 合成数据应用
表1 合成Love型槽波物性参数
Table 1
岩性 | 厚度/m | 横波速度/(m·s-1) | 密度/(kg·m-3) |
---|---|---|---|
顶板砂岩 | ∞ | 1800 | 2600 |
煤层 | 2.4 | 1000 | 1300 |
底板砂岩 | ∞ | 1800 | 2600 |
合成的Love型槽波如图3a所示,道间距为5 m,共41道。取第30道进行群速度频散曲线提取,提取过程简述如下:
图3
图3
合成Love型槽波数据S变换和广义S变换(A=0.4239,B=0.471)提取频散曲线对比分析
a—合成Love型槽波记录;b—
Fig.3
Comparison of the dispersion curve extracted by S-transform and generalized S-transform (A=0.4239,B=0.471) for synthetic Love channel waves
a—synthetic Love channel waves;b—the 30th channel wave in
首先将地震道变换到时频域S(t,f),然后用炮检距x除以t,即得v=x/t,最后对S(v,f)重新排序并插值得到群速度频散曲线图。
为进一步说明广义S变换A、B取值对频散曲线埃里相特征的影响,与图3相反,我们取一组值A=2.1195、B=2.355,使广义S变换窗函数时间半高宽整体比S变换窄,广义S变换窗函数呈现出“高”、“窄”特征,如图4a所示,图4b为窗函数时间半高宽对比,广义S变换窗函数整体比S变换窄,其时间分辨率高于S变换,由海森堡不确定性原理,频率分辨率低于S变换。利用此广义S变换对图3b数据进行处理,提取的群速度频散曲线如图4c所示(埃里相所在区域由黑色圆圈所示),可以看到,相比S变换提取的群速度频散曲线图3c,图4c中埃里相所在区域黑色圆圈所示完全无法识别,因此,通过图3、图4的分析可知,利用广义S变换适度提高频率分辨率更有利于埃里相的识别。
图4
图4
合成Love槽波数据广义S变换(A=2.1195,B=2.355)提取频散曲线
a—50 Hz时的S变换和广义S变换窗函数振幅随时间变化;b—S变换和广义S变换窗函数时间半高宽对比;c—
Fig.4
Dispersion curve extracted by generalized S-transform (A=2.1195,B=2.355) for synthetic Love channel waves
a—S-transform and generalized S-transform window amplitude versus time at 50 Hz;b—comparison of temporal FWHM between S-transform and generalized S-transform;c—dispersion curve of
图5
图5
合成Love槽波数据中加入随机噪声后S变换和广义S变换(A=0.4239,B=0.471)提取频散曲线对比分析
a—
Fig.5
Comparison of the dispersion curve extracted by S-transform and generalized S-transform (A=0.4239,B=0.471) for synthetic Love channel waves with adding random noise
a—synthetic Love channel waves with adding random noise to
3 实际资料应用
图6
图6
实际槽波数据的S变换和广义S变换(A=0.4239,B=0.471)提取频散曲线对比分析
a—某工作面第12炮第10道透射槽波记录;b—
Fig.6
Comparison of the dispersion curve extracted by S-transform and generalized S-transform(A=0.4239,B=0.471) for real data
a—the 10th transmitted channel wave of the 12th shot in a coal mining face;b—dispersion curve of
4 结论
1)讨论了广义S变换中参数的取值与时频分辨率的关系,并推导了其参数与窗函数时间半高宽的具体表达式,为定量表征时频分辨率奠定基础。
2)广义S变换的数值试验表明,通过合理选择参数,广义S变换能够有效提高时间或频率分辨率,根据海森堡不确定性原理,在提高频率分辨率的同时,时间分辨率会有所下降,反之亦然,因此,需根据实际情况合理选择参数,提高时频分辨率。
3)将广义S变换引入到透射槽波频散曲线提取中,研究认为广义S变换能有效改善频散曲线的时频分辨率,通过选取合适的参数能进一步有效突出频散曲线中埃里相特征,使解释人员能准确快捷地识别透射槽波埃里相。
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