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本文针对地球天然脉冲电磁场(ENPEMF)信号的非平稳性特点,通过采用归一化STFT-WVD变换(NSTFT-WVD)提取芦山MS7.0地震前地表天然脉冲电磁场信号的时频特性,来研究大地震发生前地表天然脉冲磁场的时频特点。通过对线性调频信号分别进行Short-Time Fourier Transform(STFT)和WignerVille Distribution(WVD),对比分析两者时频聚集的优缺点,提出采用NSTFT-WVD变换方法,可得到时频聚集特性更优且可抑制交叉项的时频分析方法。结果表明,对于ENPEMF非平稳信号,NSTFT-WVD变换能较为真实地反映震前ENPEMF信号的时-频-能量谱的联合分布特性,即震前1~2天呈现较为明显的脉冲全频率段静默状态,通道2和通道3数据的震前时频表现基本一致,具有较好的临震前兆的特点。
In this paper, according to the non-stationary characteristics of ENPEMF signal, the time-frequency characteristics of natural pulsed electromagnetic field before Lushan MS7.0 earthquake are extracted by using normalized STFT-WVD transform (NSTFT-WVD) Time - frequency characteristics of natural pulsed magnetic field before earthquakes. By comparing short-time Fourier Transform (STFT) and WignerVille Distribution (WVD) with chirp signal, the advantages and disadvantages of time-frequency aggregation are analyzed and compared. The NSTFT-WVD transform method is proposed to obtain better time- Time-frequency analysis method that can suppress cross terms. The results show that for ENPEMF non-stationary signals, the NSTFT-WVD transform can more truly reflect the joint distribution of the time-frequency-energy spectrum of the pre-earthquake ENPEMF signal, that is, the full pulsed full frequency segment is silenced 1 to 2 days before the earthquake The state, channel 2 and channel 3 data are basically the same before and after the pre-earthquake time-frequency data, and have the characteristics of good precursory precursors.