臺灣西南海域增積岩體之P波速度分析

P-wave Velocity Analysis for the Accretionary Wedge Offshore SW Taiwan

柯佾萍(Yi-Ping Ko)

指導教授 ： 林玉儂

共同指導教授 ： 許鶴瀚(Ho-Han Hsu)

國立臺灣大學/理學院/海洋研究所/碩士(2022年)

https://doi.org/10.6342/NTU202201831

摘要

流體對於海床穩定是重要的控制因素之一。增積岩體中，沉積速率較高，以致於含水沉積物大量堆積，加上側向擠壓應力與壓密作用形成脫水之作用條件，均會導致岩層中流體增加。這些深部生成流體若沿斷層或其他通道移棲向上，易聚積於低滲透率蓋層之下，而岩層中流體聚集會提升孔隙壓力且降低沉積物顆粒所承受之應力。當孔隙壓力大於靜水壓力時，便發生超額孔隙水壓現象，此一現象導致岩層中有效應力降低，容易引發岩層破裂或海底滑坡等地質災害，對海底纜線或是海下工程施作皆可能造成危害。因此，了解岩層中流體分布情形或是孔隙壓力等資訊是非常重要的。本研究利用流體於速度剖面中的低速特性，透過分析臺灣西南海域增積岩體之速度構造來了解流體分布的狀況。本研究方法為速度建模與重合前深度移位(PreSDM)速度分析法，使用的資料為2009年TAIGER計畫於臺灣西南海域收集之長支距多頻道反射震測資料MGL0908-1A測線。在前處理資料流程中，為取得高品質之速度模型，本研究應用進階前處理步驟，包含去除受波器鬼波、曲波域去除複反射、資料正規化與殘餘複反射消除。在速度建模流程中，我們建立了初始速度模型與兩次疊代速度模型及其深度移位，在不同次的疊代中，我們透過加入速度模型之各向異性，來增加對於地層內速度描述之精確度，以得到更好的成像效果。最後，再由可信度測試方法來驗證剖面中兩處低速異常帶之真實性。經上述處理後，我們最終取得長支距多頻道測線MGL0908-1A最新深度成像結果，並與重合後時間移位影像、不同疊代速度模型深度移位影像、以及先前已發表成像結果進行比較。上述比較顯示本研究重合前深度移位後之成像品質有顯著提升。而最終速度模型亦提供更多速度細節與資訊，通過可信度測試確認剖面中兩處低速異常帶存在，由於其分布位置與BSR特徵分布相符，因此我們推論此兩處之低速異常為BSR之下游離氣體聚集所導致。總體而言，本研究成果不僅有助於西南海域構造之了解，速度構造可以解析流體分布之範圍，在未來並可進一步由此推算孔隙壓力等資訊，將可降低日後水下工程施作的各種風險。

關鍵字

震測處理 ； 重合前深度移位 ； 速度構造 ； 流體 ； 臺灣西南海域 ； P波速度各向異性

並列摘要

Fluid is a critical element when assessing slope stability. In the submarine regime, subsurface fluid may facilitate submarine landslides which pose great threats to telecommunication through ocean bottom cables. It is therefore critical to assess the distribution and pressure level of subsurface fluids. Fluid naturally results from physical and chemical compaction of saturated sediments during the burial process. Once the fluid is expelled from the sediments, it tends to migrate upwards through faults or other conduits. In places where seal layers exist, fluid accumulation may lead to the rise of pore pressure over the hydrostatic level. These overpressure patches usually carry a signature of reduced P-wave velocity, so a detailed velocity analysis can reveal such critical spots in the subsurface. In this study, we attempt to obtain the fluid information by applying velocity model building and pre-stack depth migration (PreSDM) on a large-offset marine seismic survey acquired in offshore SW Taiwan during the TAIGER project. We first adopt state-of-art preprocessing steps, including deghosting, curvelet-domain demultiple and data regularization to achieve higher signal-to-noise ratio. We then adopt a velocity model building procedure which consists of two iterations of tomography followed by depth migration. In addition, to consider the directional dependence of P-wave velocities, we gradually include anisotropy information at throughout the iterations. Finally, we carried out a reliability test in two sites to verify the low-velocity anomalies. The final model yields both improved imaging quality in the depth-migrated stack and great details about the velocity anomalies possibly associated with fluids in the accretionary wedge. In conclusion, our results not only contribute to understanding the structures offshore of SW Taiwan, but also the fluid distribution and pore pressure which can be deduced from velocity structure to decrease the risk of seabed facilities.

並列關鍵字

seismic processing ； pre-stack depth migration ； velocity structure ； fluid ； Southwestern Taiwan ； P-wave velocity anisotropy

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