台灣大屯火山群聲音訊號之定位研究

Abstract

大屯火山群坐落在台灣的北端，近年來各項研究均指出，大屯火山群應屬於活火山(Zellmer et al., 2015; Lin, 2016)。在全球標準時間2014年3月16日，大屯火山群的小油坑噴氣孔區域有一起小型的蒸氣噴發事件被次聲波測站及地震測站接收到 (Lin, 2017)。在火山活動監測的過程中，由大屯火山寬頻地震網所記錄到的訊號，除了典型的幾種火山型地震之外，也常觀察到頻率位於15-45Hz的高頻訊號，但僅能確定此訊號非地震訊號，近年增設次聲波測站也能觀測到此記錄。 本研究針對地震儀所記錄到，主要頻率落在15-45Hz的聲音訊號，觀察比對每日的地震站及次聲波測站訊號之時頻圖，找出高頻聲波訊號並截取，對波形進行15-45Hz之帶通濾波，並利用AIC演算法自動判別訊號到時(Akaike, 1974; Hendriyana et al., 2018)，假設聲音在空氣中以340m/s的速度傳遞，最後透過格點搜尋法加以定位。 本系統目前已成功定位出17個聲波事件之聲源的位置，將所有測站波形依照定位結果，由近而遠排列，其結果亦可驗證此系統之定位結果。期望透過聲音訊號的定位系統開發，提供一個快速且有效火山活動監測方法。

The Tatun Volcano Group (TVG) is located at the northern tip of Taiwan, various of studies indicate that TVG might be considered as an active volcano. We believe that the monitoring of future volcanic activity in this area is important, because a tiny scale phreatic eruption in TVG was recorded by seismic station and infrasonic station on March 16 2014(UTC) (Lin, 2017). In addition to typical types of volcanic earthquake, an unusual seismic tremor with high-frequency (15-45 Hz) was recorded by the Tatun volcano broadband seismic network, at the same time we can also see the acoustic signal which was recorded by infrasound stations. In this study, we develop a system to find the precise source location for acoustic signals which was recorded by seismogram. Comparing the daily seismic and infrasound signals by plotting the spectrogram to find the high-frequency acoustic signals. Then, we applied a band-pass filter (15-45 Hz) and calculate the envelope of the signals, and find the first arrival times automatically by using the AIC algorithm (Akaike, 1974; Hendriyana et al., 2018). Finally, performing a basic grid search to find the source location assume that the propagation speed (340 m/s). This study has successfully positioned the acoustic signals location, which is consistent with the results by using Signal Stacking Subprocess (Lin, 2017).