| 摘要: | 中國西南陸坡天然氣水合物沈積環境特徵的AVO和淺部構造量化分析
摘要
震測勘探測線MGL0905-8RR1的科研議題是TAIGER計畫項目的一部分。該項目透過與Lamont-Doherty地球觀測站（美國）和MOST（台灣）進行海、陸聯合炸測資料的收集與分析工作，在NEP-MOST基金支持下進行天然氣水合物（GH）的勘探相關項目研究。該震測探勘測線收集了從Formosa 海脊到Yuan-An 海脊，並到達台灣西南外海岸的Good Weather海脊幾個目標區的震測數據。本研究透過垂直入射波線追跡（NIRT）、地震屬性、AVO / AVA分析、時深轉換和成像界面構造特徵的界定進行了天然氣水合物的勘探科學研究。 BSR（海底仿擬反射）是指示GH存在的震測反應。沿著該震測探勘測線本研究提出了十個潛在的天然氣水合物位置。地震屬性和AVO分析是進一步分析與確定主要GH目標很重要的科研步驟。首先透過研究地震屬性，然後進行AVO分析，以識別沿著該震測勘探測線的BSR分布。這些屬性包括截距（A），梯度（B），乘積（A * B），泊松比，流體因子，曲率，（A + B）/ 2和S波反射率。截距和梯度屬性是確認GH存在的主要依據。根據AVO / AVA屬性研究，所有目標區域都具有IV級的AVO分類。十分之七的地點可確定為天然氣水合物存在的高潛力位置。跟據交叉圖(cross plot)，針對Formosa 海脊、Yuan-An 海脊到Good Weather海脊等地區探討AVO效應和進行岩石物理特性的詳細研究。雖然在台灣西南部可能可發現具有AVO III級的潛在天然氣水合物（GH）礦區，但結果表明該測線的所有GH目標都與AVO IV級別類可相對應。AVO IV級別類通常具有負截距和正梯度的特性。這種AVO特徵可能是目標區域的沉積過程和環境變化引起的，這些在主動和被動邊緣區域產生沉積物的岩性變化，可在交叉圖中造成不同的AVO與AVA效應行為表現。沉積物含量的改變和沉積環境變化會影響截距和梯度的響應。
NIRT方法用於針對主要沉積界面進行成像並定義每個界面的平均速度，執行時間到深度的轉換。具有相同或相似平均速度的地層可以被解釋和假設為主要沉積過程發生在相同的地質時間間隔。沿著該震測勘探測線主要可分為兩個區域。基於所定義的模型界面和合成的地震記錄與震測剖面相比對，可以通過定量分析進行解釋。進行反覆的疊代從事時間 - 深度轉換，透過調整速度及對應的到時和伴隨深度的改進，從時間和深度殘差值來評估收斂。經四次疊代之後，RMS行程時間殘差和STD值都減小到定值。 NIRT技術提供了能夠量化結構界面並確定地層速度的結果。在福爾摩沙海脊的西北部，本研究定量地定義了三個主要界面，相應的速度值分別為1484米/秒，1528米/秒和1644米/秒。包括永安海脊在內的福爾摩沙海脊地區東南部有五個界面，相應的速度值分別為1484 m / s，1528 m / s，1614 m / s，1644 m / s和1684 m / s。確定的主界面在時間偏移剖面中清楚地顯示出高聲抗對比度和強反射振幅。連續沉積界面分層的分佈特徵表明本區具穩定的沉積過程並均一的覆蓋於深海和淺海沉積環境。同時水下的風化，海底坍塌產生的濁流和研究區域下可能的泥貫入體也對沉積特徵的橫向變化起著重要作用。在這項研究中，沿MGL0905-8RR1震測剖面觀測到的地質特徵包括大量沉積物質傳輸（MTD），BSR，斷層，流體傳輸管道，河道填充或地槽填充沉積特徵。
關鍵詞：天然氣水合物，AVO分析，AVO屬性，垂直入射波線追跡，地質特徵。
;Quantitative AVO and Structure Analysis of Gas Hydrate and Sedimentation Features Cross China Continental Slope, SW Taiwan
Abstract
A seismic survey line, MGL0905-8RR1, which is part of TAIGER project that joint with Lamont-Doherty Earth Observatory (USA) and MOST (Taiwan) for seismic surveys associate with gas hydrate (GH) exploration project supported under NEP-MOST fund. The seismic survey line which collected the data across several targets from Formosa Ridge to Yuan-An Ridge and reach Good Weather Ridges in offshore southwest of Taiwan. Seismic attributes, AVO/AVA analysis, time-to-depth conversion and imaging interface structure features through Normal Incidence Ray Tracing (NIRT) were performed for gas hydrate expeditions. BSR (Bottom Simulating Reflector) is a seismic response to indicate the GH presence. Ten potential gas hydrates locations were proposed along the studied seismic line. Seismic attribute and AVO analysis serve as the step to identify the major GH targets that could be important for further analysis. Seismic attributes were studied first followed by AVO analysis to identify BSRs along the seismic line. These attributes include Intercept (A), Gradient (B), Product (A*B), Poisson’s ratio, Fluid Factor, Curvature, (A+B)/2 and S-wave reflectivity. Intercept and Gradient are the main attributes to confirm the GH existence. From AVO/AVA attributes studies, all the target zones have AVO classification of class IV. Seven out of ten locations are identifying as high potential locations for gas hydrates existence. Detailed studies on both Formosa Ridge and Yuan-An ridge areas are discussed on AVO effects and rock physics properties based on cross plots. Although AVO class III could be potentially and commonly found in southwest Taiwan, however, the results show that all the GH targets along the seismic line correspond to the AVO class IV. Class IV usually has properties of negative intercept and positive gradient. Such AVO feature could be caused by depositional processes and environment changes between identified target regions that produce lithology changes across active and passive margins. The sediments properties changes may produce different AVO behavior in the cross plot. The sediments contents and environment changes can affect the response of intercept and gradient.
NIRT approach serves the purpose for imaging the main interfaces and to define average velocity for each interface in order to perform time-to-depth conversion. The layer with same or similar average velocity can be interpreted and assumed as the major sedimentation process occurred at the same geologic time interval. Seismic line is mainly divided into two areas. Based on the defined model interfaces and synthetic seismograms compared with stacked section, interpretation can be performed through quantitative analysis. Through iterative time-to-depth conversion by adjusting velocity and therefore travel-time and follow-up with depth improvement, the convergence is evaluated from time and depth residuals. Both RMS travel-time residual and STD values decreases to a constant value after four iterations. NIRT technique provides the results which able to digitize structure interfaces and determine formation velocity. At northwest of Formosa Ridge, three main interfaces with the corresponding velocities values of 1484 m/s, 1528 m/s and 1644 m/s respectively were quantitatively defined. At southeast of Formosa Ridge area including Yuan-An Ridge has five interfaces with the corresponding velocities values of 1484 m/s, 1528 m/s, 1614 m/s, 1644 m/s and 1684 m/s respectively. The determined main interfaces clearly show high contrast with strong reflection amplitude in the time migration section. The distribution of continuous layering feature indicates stable sedimentation process cover both deep and shallow marine depositional environment. Meanwhile weathering, submarine slump creating turbidity current and mud diapirism (?) beneath the study area also play important roles for lateral variation of sedimentary features. In this study, the proposed geological features observed in the seismic section along MGL0905-8RR1 include Mass Transport Deposit (MTD), BSRs, faults, fluid conduct, channel fill or trough fill sedimentation features.
Keywords: gas hydrates, AVO analysis, AVO attributes, Normal Incidence Ray Tracing, geological features. |
