本研究主要目的為分別從實驗、理論及現場觀測探討土石流產生地表振動（或稱地聲）之特性，包括土石流地聲之頻率範圍、傳遞速度及衰減係數。本研究可分成下列五個主要部份: 1、以實驗方法探討石頭摩擦及撞擊土石材料時其地聲頻率之特性，首先了解土石基本運動型態之地聲特性後，再進行人工土石流實驗，以了解土石流地聲特性及其與上述土石基本運動型態地聲之關係。2、利用地聲探測器（geophone）探討不同類型土石流地聲之特性；土石流類型包括礫石型、一般型及泥流型。3、於土石流觀測站現場量測土石撞擊河床或於河床上滾動所產生地聲之頻率、傳遞速度及衰減係數。4、利用彈性波在孔隙介質中傳遞之理論計算土石流地聲傳遞速度，並與實驗值比較。5、分析自動化土石流觀測系統所蒐集之土石流現場之地聲資料，並探討其特性。
　　上述第一個部份的研究（第四章），主要是利用水中麥克風（hydrophone）探討石頭摩擦及撞擊土石材料時地聲頻率及強度。在了解土石基本運動之地聲特性後，再於人工渠道中模擬土石流，並探討其地聲頻率及強度之特性。實驗量測所得地聲時域訊號經快速傅立葉轉換（Fast Fourier Transform，FFT）及Gabor Transform（第三章）分析，以求得地聲之頻域訊號及其時間–頻率訊號，並比較兩種訊號分析方法之適用性。實驗結果顯示石頭摩擦土石材料產生之地聲頻率屬於低頻範圍，約在10到300 Hz，主要分佈在20～80 Hz之間。而石頭撞擊土石材料產生之地聲頻率在10到500 Hz之間，且低頻訊號依然明顯。此外，石頭落下的高度對地聲頻率沒有明顯的影響，但會改變地聲訊號之強度。礫石型土石流地聲頻率的範圍分佈在20-300 Hz之間，坡度愈陡或土石材料粒徑愈大，地聲強度愈強。
第二個部份的研究主要是以地聲探測器量測土石碰撞時所產生之地表振動，實驗所得地表振動之時域訊號利用快速傅立葉轉換（FFT）及Gabor Transform轉換為頻域訊號及時間–頻率訊號，以探討土石地聲之頻率、傳遞速率及隨距離衰減特性。實驗可分為兩部份，第一部份是於實驗室內量測石頭以自由落體方式撞擊水槽內之土石材料所產生之地聲。此部份之實驗主要是探討地聲探測器傾斜不同角度時所測得地聲訊號之差異（第五章）。這些資訊可提供土石流觀測示範站中地聲探測器安裝及接收訊號之參考。第二部份的實驗則是利用地聲探測器探討不同類型土石流地聲之特性（第六章）；土石流類型包括礫石型、一般型及泥流型。實驗結果顯示礫石型土石流所造成地表振動的頻率主要在10到300Hz之間；而泥流型土石流地聲的頻率主要介於5到20Hz之間。一般型土石流地聲的頻率則介於兩者之間，當其土石材料中之礫石成份增加時，10到300Hz之地聲振幅會明顯增加。
　　第三個部份的研究主要是於土石流觀測站現場量測土石撞擊河床、或於河床上滾動所產生地聲之頻率、傳遞速度及衰減係數（第七章）。實驗地點有兩處，一處是南投縣信義鄉豐丘土石流觀測站，另一處為南投縣信義鄉神木村愛玉子溪土石流觀測站。實驗結果顯示石頭以自由落體方式撞擊豐丘野溪所得地聲頻率介於20到150Hz之間，且隨著石頭重量增加地聲之顯著頻率（superior frequency）也會降低；石頭於豐丘野溪上滾動所得地聲頻率頻率範圍與單一石頭撞擊河床所得地聲頻率頻範圍一致。土石地聲傳遞速率約為833 – 1000 m/s。神木村愛玉子溪土石地聲實驗顯示石頭撞擊河床所得地聲頻率亦介於20到150Hz之間，而土石地聲傳遞速率約為333 – 400 m/s。此外，在假設地聲為圓柱波（cylindrical wave）的情形下，本研究亦分析出地聲之衰減係數。
　　第四個部份的研究主要是利用彈性波在孔隙介質中傳遞的理論計算土石流地聲傳遞速度（第二章），並與實驗值比較（第七章）。本研究分別利用Gassmann理論、Biot理論及Duffy and Mindlin（1957）的球狀顆粒組合理論（sphere-pack model of granular rocks）求出彈性波在孔隙介質及飽和孔隙介質中傳遞的速度。
　　第五個部份的研究主要是介紹行政院農業委員會水土保持局所建置之自動化土石流觀測系統之儀器及訊號傳輸設備（第八章），並分析土石流觀測站在現場所蒐集之土石流造成地表振動訊號之特性（第九章）。文中詳細探討了2004年7月2日敏督利颱風期間在南投縣信義鄉神木村愛玉子溪量測到的台灣地區第一筆現場土石流觀測資料。由資料中得知土石流發生前河道流量明顯變小，土石流波湧前端呈現波浪狀並有巨礫集中的現象，而土石流波前通過後其流深迅速變小，前端觀測流速介於10—13 m/sec，前端流深約5.5—6 m，最大觀測粒徑約4—5m，平均流深約2m，持續時間達5分鐘之久。本文另以Gabor Transform分析土石流流動造成的地表振動訊號（亦稱土石流地聲），發現當土石流波湧抵達地聲探測器時，由於前端巨礫集中，因此地聲頻率較低，約在10到30Hz之間；而當波湧主峰通過時，頻率範圍較寬，介於10到250Hz之間；而在土石流尾端，由於石頭較小，地聲頻率主要在60-80 Hz之間。由神木村愛玉子溪現場所埋設之序列式地聲探測器的訊號求得土石流波湧的平均速度為 。此外，不同的量測地點之地質也會影響土石流地聲訊號的強度。

　This work investigates the main characteristics of the ground vibrations, or sometimes called underground sounds, generated by debris flows. The ground vibrations were discussed in terms of the frequency range, the propagation speed and the decay rate. Experimental facilities were set up in the laboratory as well as in the field to measure the frequency, the phase speed and the decay rate of the ground vibrations. The source of tremor was produced by a rock that falls freely and hits a gravel bed in the laboratory experiments or a riverbed in the field experiments. Effects of different types of debris flows on the ground vibrations were also tested in a flume. These debris flows include the bouldery, the cobble-gravelly and the muddy debris flows. The field experiments were performed at the Fongciou and Shenmu (Aiyuzih creek) debris flow monitoring stations, Sinyi Township, Nantou County, Taiwan.
　The ground vibration signals were detected at the early stage of this research by a hydrophone and later by geophones. The vibration signals in the time domain were analyzed using both the fast Fourier transform and the Gabor Transform to represent the signals in both the frequency and time-frequency domains. Different approaches for determining the phase speed of the elastic waves in the porous media were applied to determine the propagation speed of the ground vibrations and compare with the field test data. The ground vibration data associated with real debris flows obtained from the debris flow monitoring system were examined to reveal the properties of the ground vibrations. These data were also compared with the field test data to clarify the main source of the ground vibration and to understand the mechanism of these vibrations.
　The measurement results reveal that the frequency of the underground sound generated by the rock-gravel bed friction is relatively low, being mostly between 20 and 80 Hz. In contrast, the frequency range of the collision sound is relatively higher, between 10 and 500 Hz. The frequency of the underground sound caused by the debris flows in the flume is in the range of 20-300 Hz. The field experimental data at the Fongciou debris flow monitoring station show that the frequency of the ground vibration is in the range of 20-150 Hz and the propagation speed is between 833-1000 m/s. While the experiments in Shenmu station show that the ground vibration is in the same range of 20-150 Hz and the propagation speed is about 333-400 m/s.
　The frequency of ground vibration resulting from a real debris flow occurred at July 2, 2004 in Aiyuzih creek, Shenmu village, Sinyi Township, ranges within 250 Hz and mainly between 5 to 100 Hz. The superior frequency appeared at 60 Hz. The amplitude of ground vibration depends on the location where the instrument is installed.

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