以頻域及時頻分析輔助孔內震測走時分析自動化

Abstract

現有的孔內震測方法多採用以人工手動挑取初達波的走時分析方法，然而手動挑波結果往往因分析人員的經驗、訊號品質優劣而有差異，當資料量多繁雜時，相當費時費工；而當震波訊號的訊雜比較低時，判讀上的困難將使其難保正確性；此外，手動挑取之初達時間往往只在時間域上判釋，並無法得知其所得速度之頻率範圍。雖然陸續一直有學者進行初達波自動化選取之相關研究，但所提出之方法基本上皆是基於時間域之分析，且常常是有條件限制之適用性，對於訊號品質較差的資料易有判讀錯誤的情形。故本研究從訊號分析的角度，利用頻率域及時間－頻率域之訊號分析方法，透過有效的濾波，一方面由不同場域觀察震波訊號之特性，另一方面藉以訂定出半自動挑波法，並透過現地資料之應用探討不同孔內震測技術之適用性。 本研究在頻率域下採用頻譜分析法，透過簡化之均質均向土層假設，利用各頻率下之相位角差斜率以獲得相位速度；而在時間－頻率域上採用多重濾波法及小波轉換繪製時頻譜，並提出在時頻譜上利用振幅起伏起始點之自動挑取方法獲得頻帶波相平均初達時間，進而計算該頻帶下之初達波速。經現地資料測試，頻率域之簡化頻譜分析法較適用於震源穩定且有兩個接受器之孔內懸盪式震測法，此分析可同時提供剪力波速並可有額外之頻率訊息，可提供工程師據以進行進一步之動態分析，雖然此方法具有自動化分析之高度潛力，但其對於訊雜比低之訊號結果誤差較大，需進一步進行分析探討；而採用時頻分析法所訂定出之半自動挑法，無論採用多重濾波法或小波轉換法於現地懸盪式震測及下孔震測之資料驗證，皆有良好表現，整體波速趨勢與手動挑波結果一致，且相對穩定，並具有良好的抗雜訊能力，然而此時頻譜半自動挑波方法受限於時頻譜分析技術之時頻域間的解析度限制，針對不同震源類型的孔內震測試驗須調整分析參數達到最佳化，以便半自動挑波法能正確判釋波相初達時間，本研究根據現地試驗資料針對孔內懸盪式震測法以及下孔式震測法分別提出建議之使用參數。 綜合成果顯示，頻譜分析法較適合應用於震源穩定且高頻的懸盪式震測法，惟對訊雜比低之訊號需後續進一步探討；而時頻分析法對於各項孔內震測皆可有良好應用成效，且有良好之抗雜訊能力，可廣為推廣應用。

The data reduction of existing borehole seismic methods are currently based on travel-time analysis in which first-arrival times of the seismogram are manually picked. However, manual picking is not efficient and depends heavily on the data quality and analyst’s experience. Besides the difficulty of picking first arrivals in the time domain, no information is obtained regarding the effective frequency of the analysis. Several automatic picking methods were proposed to increase the efficiency of data processing. But most of them are based on time-domain analysis and work well only on some specific conditions. Data reduction methods based on the frequency domain and time-frequency domain were proposed in the study. The aim of the study was to observe the behavior of the borehole seismic waves from different domains (i.e. frequency and time-frequency domain) and propose a semi-automatic procedure for more objective first-arrival time picking. Field data from PS-Log testing and down-hole testing were used to evaluate the feasibility of the proposed procedure, in particular for the shear wave velocity. In the frequency domain, the phase velocity can be calculated from the phase shift of two receiving stations in each frequency. To avoid the difficulty of unwrapping phase angles in lack of low frequency components, the formation around the borehole is assumed homogeneous and isotropic such that the phase velocity can be calculated from the slope of the phase difference versus frequency at certain frequency range. A better understanding of the seismic signal can be achieved by time-frequency analysis, such as multi-filter analysis or wavelet transform. In the time-frequency domain, a semi-automatic procedure was proposed to determine the first arrival times from the envelope of the time-frequency spectrum at specific frequency range. Evaluation using field data shows that the spectral analysis method is suitable for PS-Log testing with stable source and two simultaneous recording channels. The method is easy to be automated and yields Vs and the corresponding effective frequency. However, more study should be conducted to overcome occasional failure in low S/N data. The semi-automatic time-frequency spectrum analysis, either based on multi-filter analysis or Morlet wavelet transformation, works well both in PS-Log testing and down-hole testing. The picked first arrival times from the proposed method agree well with the manually-picked results in high quality data. The procedure is stable and yields reasonable results in data with low S/N. But it should be noted that the method inherit the tradeoff between frequency and time resolution from the time-domain analysis. The parameters used in the time-frequency analysis should be optimized for different source types. The parameters for PS-Log testing and down-hole testing were suggested respectively in the study and verified by the field data.