L1范数约束的随掘地震噪声衰减

Noise attenuation based on L1-norm constraint inversion in seismic while drilling

  • 摘要: 煤矿随掘地震勘探将掘进机截割煤岩的振动能量作为震源,规避了炸药震源具有危险性隐患及防碍正常施工的缺陷,具备在煤矿开展正常掘进作业的同时进行井下地震勘探的优势。但与炸药震源相比,掘进机截割煤岩的能量较低,通过互相关得到的类脉冲源的信噪比低;并且,由于煤矿井下狭小的掘进空间里布设了各类设备,导致环境噪声频率成分复杂且能量通常较大,进一步降低了采集到的随掘信号的信噪比,影响了后续偏移成像等数据处理质量。将煤矿随掘地震信号的噪声衰减视作求解一个反问题,并基于互相关的随掘信号在时频域的系数分布相对于环境噪声呈现稀疏性的特征,提出了在反问题的求解过程中嵌入L1范数的约束,然后通过快速迭代算法求解该反问题以达到噪声衰减的目的。与传统阈值类小波去噪方法相比,由于本方法降低了时频系数扩散的影响,提高了时频系数的分辨率,当信号频率成分复杂时,噪声衰减后有效信号保真度更高。分别通过模拟和实际数据验证了方法的有效性。其中,模拟数据的频率分布由2种形式组成:主频(40,60 Hz)独立分布以及(20,60 Hz)重叠分布,并在其基础上合成了3类不同信噪比的含噪信号(信噪比分别为10.8,6.8和1.2)。传统方法和本方法的去噪结果证实:当信噪比较高以及主频独立分布时,传统方法和本方法的噪声衰减效果相当,而对于低信噪比并且主频有重叠的信号,本方法的去噪信噪比可以达到传统方法的4倍左右。而由于实际地震勘探数据的频率成分更复杂,通过2种方法用于实际信号的去噪效果对比,可以看出本方法在保持信号变化趋势及保护信号细节方面更具优势。

     

    Abstract: The vibration energy of the roadheader during it excavates coal and rock is treated as the source in seismic while heading in coal mines,which avoids the hazards and hindering normal heading operation caused by the explosive source.Therefore,it has the advantage of carrying out underground seismic exploration while coal mine is in normal heading operation.However,due to the low energy of coal-rock excavated by the roadheader,the SNR (signal-to-noise ratio) of the pulse-like source obtained by cross-correlation is low compared with employing explosive source.In addition,because various equipment are installed in the narrow heading space of coal mine,the frequency components of environmental noise are complex and the energy is usually large,which further reduces the SNR of the signals along the excavation and affects the quality of the data processing such as the migration and imaging.The noise attenuation in seismic while heading in coal mines is seen as solving an inverse problem.Based on that the coefficient distribution of the signal in the time-frequency domain is sparse relative to the ambient noise,it was proposed to embed the constraint of L1-norm in the inverse problem.Subsequently,the inverse problem was solved through a fast iterative algorithm to achieve the purpose of noise attenuation.Compared with the traditional wavelet threshold denoising method,the proposed method reduced the influence of diffusion of time-frequency coefficient and improved the resolution of time-frequency coefficient.Therefore when the frequency component of signal was complex,the fidelity of effective signal after noise attenuation was higher.The method proposed was verified by analog and actual data respectively.The frequency distribution form of the analog data was comprised of two situations:the independent distribution of dominate frequencies (40 Hz and 60 Hz) and the overlapping distribution of dominate frequencies (20 Hz and 60 Hz),then three types of noisy signals with different SNRs where synthesized (the SNR were 10.8,6.8 and 1.2 respectively).The denoising results of the traditional method and proposed method confirmed that when the SNR was high and the dominate frequencies were independently distributed,the denoising results of the two methods where comparable,and for signals with low SNR and overlapping dominate frequencies,the SNR of the denoising signals of the proposed method could reach about 4 times than that of the traditional method.However,since the frequency components of the actual seismic exploration data were more complicated,the comparison of the two denoising results of the actual signals showed that the proposed method had more advantages in maintaining trends and details of the signal.

     

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