振幅隨支距變化(AVO)分析可提供一個準確辨識甲烷水合物及可能生成機制的方法。其目的最終為透過岩石物理學進一步確定岩石的類型以及包括流體含量、孔隙度、密度和震波速度等相關的石油物理參數。經由AVO屬性分析可以輔助標定水合物的分布。透過針對平行海床地形且與海床反射極性相反的海底仿擬反射(BSR)來辨識甲烷水合物與游離氣的存在。經過詳細的AVO特徵分析可瞭解沉積物中有水合物層的存在及可造成P波震波速度的增加，並且在甲烷水合物穩定區的頂部產生強反射。穩定區底部的震波與AVO屬性分析為確認BSR存在的重要分析步驟。其下的游離氣層可形成”亮點”，亦為證實甲烷水合物存在的重要特徵。 本論文專注於台灣西南海域下枋寮盆地的長支距MGL0908-TST和短支距MCS937-10震測資料的資料處理與分析工作。依據Ak-Richard的兩項近似值(two term approximations)進行AVO與屬性分析。並進一步透過彈性波模擬進行合成震波記錄模擬，同時透過合成資料比較AVO效應分析的結果，以確認與實際資料間的緊密相關性。實際震波記錄資料需要透過保持震幅的幾何擴散損耗補償資料處理，以避免人為影響波形。同時，針對水合物存在區域的合成記錄與實際資料兩者間AVO效應的比較進行探討。合成AVO模擬的研究可確認天然氣沉積物的反射係數會隨角度的增加而減少。針對下枋寮盆地標定的目標區域，透過重合前同中點集合進行AVO分析可確實的辨識出甲烷水合物的存在證據。AVO效應的發生，主要是因應被水合物地層所覆蓋且圈合著的游離氣層，隨著距離與角度的改變所造成的彈性波阻抗變化。AVO分析中，可觀測到雙程走時2250毫秒附近的反射訊號與其下方有不同程度的AVO異常。進一步透過AVO模擬可以明確地指出其下方天然氣的存在。AVO分析主要以AVO屬性中的截距(intercept, A)、梯度(gradient, B)、sign(A)*B及泊松比(Poisson’s ration)等參數來偵測其存在的可能性。梯度分析(gradient analysis)和綜合圖表(crossplot)皆顯示為第三類型AVO反應，顯示游離氣的確存在於高速層(甲烷水合物)的下方。經由AVO分析後，可以確定下枋寮盆地中有甲烷水合物實質存在的量化證據。進一步更實際的針對該區進行量化工作則需仰賴未來三維資料分析的工作。;Amplitude versus offset (AVO) analysis provides an accurate method to identify gas hydrate and possible occurrence mechanism. The ultimate goal is to determine type of rock and associate petro-physical parameters including fluid content, porosity, density and seismic velocity. Areal distribution of hydrate can be easily identified through AVO attributes. A bottom simulating reflector (BSR) which sub-parallels to the seafloor topography with reverse polarity compare to sea-floor reflection is a helpful hint to identify the presence of gas hydrate. The preliminary features indicate the presence of hydrate layer within the sediments is that the velocity will increase and also generating a strong reflector on top of gas hydrate stability zone. This study focus on MGL0908-TST and MCS937-10 seismic data in lower Fangliao Basin, Taiwan In this study, AVO synthetic modelling for AVO analysis were performed based on Aki-Richard’s two term approximations and elastic modeling. Amplitude preserved seismic data processing with compensation of geometrical spreading loss were required and avoid artificial influence on the waveform. Synthetic compared with the real data gathers at hydrate concentration zones for AVO effects are investigated. Synthetic AVO study confirms the AVO theory that the gas-saturated sediment exhibit reduction of reflection coefficient with increasing incidence angle. For real data application, the AVO analysis under CMP gather at specific location acquired in the lower fangliao basin were investigated in order to confirm the presence of gas hydrate. The AVO effects occur mainly due to the presence of free gas-bearing zone usually trapped by the overlying hydrate formations. AVO analysis is performed on reflection event around TWT 2250 mili-second can be observed with indication of gas supply from below show distinct AVO anomaly. The AVO anomalies were identified from AVO attributes such as intercept (A), gradient (B), product A*B and Poisson’s ratio attributes. The gradient analysis and crossplot show Class 3 AVO response indicating the free gas beneath a high velocity layer, in this case, gas hydrate. The existence of gas hydrate in lower Fangliao Basin can be confirmed by AVO analysis.
