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    請使用永久網址來引用或連結此文件: http://ir.lib.ncu.edu.tw/handle/987654321/77003


    題名: 台灣南部的體波與表面波波場逆推:應用於TAIGER T4b寬角度折射/反射資料;Body- and Surface-waves Wavefield Inversion in Southern Taiwan: Application to TAIGER T4b Wide Angle Refraction/Reflection Data
    作者: 維蒂利;Widiyansari, Roma
    貢獻者: 地球科學學系
    關鍵詞: Tau-p轉換;Tau-p逆推;MASW;表面波;地滾波;TAIGER;Tau-p Transform;Tau-p Inversion;MASW;Surface Waves;Ground-roll;TAIGER Project
    日期: 2018-05-18
    上傳時間: 2018-08-31 13:10:07 (UTC+8)
    出版者: 國立中央大學
    摘要: 本論文以近地表炸測記錄的表面波與體波進行南台灣地下速度構造成像。表面波與體波傳為表面波只沿著地球表面,而體波與此同時向地球內部深處傳遞。針對地殼尺度的速度構造調查,體波與表面波記錄的波場處理與逆推常適用於無走時挑定成像地球地表下構造。
    震測資料集來自台灣2008年的台灣大地動力學國際合作研究計畫(TAIGER)進行地殼尺度寬角度折射/反射(WARR)探勘。南線陣列由西向東跨越台灣南部內含四個炸點(S1, S2, S3, S4),共配置有609個間距為200公尺的受波器。
    我們使用兩種方法分別來求取沿著T4b 線的Vp 和Vs速度模型。第一種方法是透過表面波的多頻道分析(MASW)技術的執行來取得近地表速度構造。剪力波速度是在工程相關應用介質剛性最好的指針。近地表土壤或組成的剪力波速度特性是可以透過非均質介質的基本模態頻散曲線分析來回復。四個炸點中,炸點(S1) 1與炸點2(S1)具相當數量的地滾波記錄能應用MASW方法。第二種為基於tau-p波場逆推的方法,透過波場轉換與向下連續於長支距(或區域等支距)震測資料經由全域(或區域)搜尋演算法達成求取壓縮波(Vp)構造。透過波場處理策略可獨自獲得Vp 和Vs速度模型。
    由逆推的剪力波與體波速度構造中,我們可明顯看到含岩盤邊界的台灣海岸平原西南部有略向西傾的速度梯度存在。假設上新世至中新世的剪力波為1.5公里/秒,西部海岸平原沖積層的厚度在西端約1.8公里且向東變薄至0.9公里。Vs30場址分類中,炸點S1為Class D1,炸點S2 為Class D3,兩者皆為 “stiff soil”。 體波波場逆推結果顯示地表下構造可成像至深度逼近20公里。在炸點S1和S2資料顯示,西部海岸平原的厚地層(~ 3公里)速度較低(Vp為1.5-4 公里/秒,Vs為250-1500 公里/秒)且具高梯度變化造成顯著的地震混響(折射的自由表面複反射)。強烈側向速度變化的證據標識出海岸平原與接近炸點S2東側的西部麓山帶之間的過渡帶。強烈速度過渡帶也可在中央山脈與海岸山脈的炸點S4看到。5.0至6.5公里/秒的Vp速度等值線穾顯地殼地層的一般特徵,西邊偏厚(各自對應深度為8和16公里),向東變薄(各自對應深度為4和9公里)。沿著T4b線檢查6.0和6.5公里/秒的等值線,顯示一個相對平坦的構造特徵從海岸平原延伸至西部麓山帶,其在距離40-78公里處增厚並朝著中央山脈和海岸山脈方向而變薄的現象,提供地殼增厚與變薄的地質構造與地殼運動特徵的生動看法。6.0公里/秒的等值線顯示在海岸平原處深度13公里至西部麓山帶深度約7公里,突顯可能有潛在上部與下部地殼邊界(康拉德不連續面)。最薄(薄片化)的大陸地殼會在大陸邊緣的典型裂谷帶發生。當6.5公里/秒和更高速的下部地殼地層範圍從6.9(或7.0)至7.5公里/秒可能顯示相似的特徵,並突顯下部地殼與上部地函邊界(莫荷不連續面)的存在,並可經由波場處理與速度逆推研究進一步標定。針對深度2.5-5公里進行更小心的近地表速度分佈調查,地殼厚度會比層析成像研究的結果更薄些。以試錯過程來挑選初達波到時有些令人乏味、無效能、耗時,且導致速度模型過度平滑。本論文的優點是並不需手動進行走時挑選,自動化且穩定的更新速度模型。量化獲得的模型可有效地檢查合成與實際走時記錄的一致性。

    關鍵字: Tau-p轉換, Tau-p逆推, MASW, 表面波, 地滾波, TAIGER
    ;Surface and body waves generated by near-surface explosions and recorded along T4b line is investigated for imaging subsurface velocity structure in Southern Taiwan. Surface waves propagate only at the Earth’s surface meanwhile body waves propagate deep through the body/interior of the Earth. For crustal-scale velocity structure investigation, wave-field processing and inversion of both body and surface waves records can be very useful to image Earth’s subsurface structures without picking travel-time.
    Seismic dataset used in this research is a crustal-scale Wide Angle Refraction/Reflection (WARR) survey under TAiwan Integrated GEodynamics Research (TAIGER) project conducted in 2008 in Taiwan. The south main array consist of four shot points (S1, S2, S3, S4) across southern Taiwan from west to east. A total of 609 geophones was deployed with receiver interval of about 200m.
    We use two approaches to estimate separate Vp and Vs velocity models along T4b line. The first method is through implementation of Multichannel Analysis of Surface Wave (MASW) technique to determine near-surface velocity structure. Shear wave velocity is the best indicator of material stiffness for engineering related applications. The near-surface soil or formation shear wave velocity (Vs) characteristics can be retrieved by analyzing the dispersion curve associated with the fundamental mode of surface waves in heterogeneous media. MASW method is only applicable for shot number 1 (S1) and shot number 2(S1) that possess significant amount of ground-rolls records. The second method is based on tau-p wave-field inversion. Through wave-field transformation and downward continuation approach, application of long-offset (or local short-offset) seismic data to estimate compressional wave velocity (Vp) structures can be achieved via direct global (or localized) search algorithm. Through wave-field processing strategy, independent Vp and Vs velocity models can be obtained.
    From inverted shear and body wave velocity structure, we can see obvious velocity gradient exist in southwestern Taiwan Coastal Plain with the bedrock boundary is slightly west-dipping. Assume the shear wave velocity of Pliocene to Miocene bedrock is 1.5 km/s, the thickness of alluvial sediments at the west-end of western Coastal Plain is ~1.8 km and become thinner toward the east with depth ~0.9 km. The Vs30 site classification for shot S1 is Class D1 and for shot S2 is Class D3 which all corresponds to “stiff soil”. From body wave wave-field inversion, subsurface structure can be imaged up to depth of approximately 20 km. In the Western Coastal Plain, a thick sediment layer (~ 3 km) with fairly low velocity (Vp= 1.5-4 km/s and Vs=250-1500 km/s) and high gradient changes produce significant seismic reverberations (refracted free-surface multiples) shown in data S1 and S2. Evidence of strong lateral velocity changes marked the transition between Coastal Plain and Western Foothills near east side of shot point S2. Strong lateral velocity transition also can be observed between Central Range and Coastal Range at shot point S4. The contour lines for Vp values between 5.0 to 6.5 km/s highlight the general feature of crust layer which thicken in the west (depth=8 and 16 km respectively) and thinned toward the east (depth =4 and 9 km respectively). By examining the contour value of 6.0 and 6.5 km/s, a relatively flat structure feature extend from Coastal Plain to Western Foothills; thickening at the distance between 40-78 km and thinning toward Central Range and Coastal Range provide a vivid view of crustal thickening and thinning feature associate with tectonic structure along the T4b line. The 6 km/s contour shown at the depth of 13 km from Coastal Plain and reaches the depth of ~7 km at Coastal Range may highlight the potential boundary between upper and lower crust (Conrad discontinuity?). The extreme thinning (necking) of the continental crust can be occurred by a typical rift zone at continental margin. The 6.5 km/s and a high velocity lower crust layer with velocity ranging from 6.9 (or 7.0) to 7.5 km/s may show similar features and may highlight the existing boundary between lower crust and upper mantle (Moho discontinuity?) which can be identified from wavefield processing and velocity inversion studies. With more careful investigate near-surface velocity distribution within 2.5-5 km, crust thickness can be thinner than the result from tomography studies. Picking the first arrival time with trial-and-error processes are somehow tedious, not efficient and time-consuming may lead to overly smoothed model. The advantages of the proposed method are no need to manually pick travel time and automatically and stable update of velocity model. The quality of the derived model is effectively checked by the consistency between computed travel-time with seismic records.

    Keywords: Tau-p Transform, Tau-p Inversion, MASW, Surface Waves, Ground-roll, TAIGER Project
    顯示於類別:[地球物理研究所] 博碩士論文

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