南海北坡被動大陸邊緣海底沉積層之震波速度特性

Abstract

傳統上要了解海洋沉積物的各項物理性質，是透過岩心取樣或鑽井等直接測量的方式，現今我們可以透過分析地震波的反射訊號來觀察沉積物的性質。本研究利用多頻道反射震測系統蒐集四條橫跨南海北坡被動大陸邊緣的測線，使用速度頻譜法分析海洋沉積物的震波速度，最後將層間速度與單程走時迴歸，探討不同沉積環境的震波速度特徵。利用地形特徵將研究區域分成大陸斜坡、下部斜坡與海盆三種環境，研究結果顯示，水平方向上的速度變化較平緩，大致隨著地質構造的分佈而改變其速度構造；垂直方向上的速度則隨著深度增加而變快。將速度構造之數值取出內插後，以層間速度為Y軸、海床下沉積物單程走時為X軸做速度曲線圖，使用線性迴歸求取速度函數，發現在三種區域的速度曲線不盡相同，大陸斜坡的速度分佈範圍較廣，解釋是由於沉積速率快，且有觀察到許多崩塌現象，導致沉積物結構較鬆散；在下部斜坡與海盆之速度曲線集中，說明在地層中沉積物未受到太大的擾動。透過迴歸方式，可將這些速度特徵以二次式表示，分別為大陸斜坡：V=1511+2003t-20.342t^2；下部斜坡：V=1511+2373.8t-406.18t^2；海盆：V=1511+2111.2t-146.77t^2。速度函數可以幫助我們了解該區域的走時與速度之間的關係也可以提供相同區域的震測測線做時-深轉換的依據，由函數的係數也可以觀察到在大陸斜坡速度有較慢的上升趨勢，而下部斜坡上升的最劇烈。此外為評估速度分析的準確度，本研究利用ODP site1148鑽井資料比對速度分析的結果，發現其層間速度隨地層上升的趨勢相當接近，說明使用速度頻譜法分析海底沉積物之震波速度是可行的方式。

P-wave velocity is an important physical property of marine sediments, it also plays an important role in seismic data processing. Traditionally, we obtain physical properties of marine sediments by direct measurements of cored samples, but most of such samples come from very shallow sediments beneath seafloor, except where samples from deep sea drilling investigation are available. In order to obtain P-wave velocity information of sedimentary strata at depth, seismic velocity analysis techniques are often used. This study presents a summary of P-wave velocities of the sediments in the Northern South China Sea (NSCS) continental margin. Data from 2-D multichannel seismic reflection surveys are analyzed, using velocity spectrum analysis (VSPEC) technique to derive interval velocities in marine sediments. Through careful VSPEC, we can build a velocity model along the seismic profile which reveals velocity variations in the study area. We establish the characters of P-wave velocities for 3 geological provinces in NSCS: continental slope, lower slope, and ocean basin. The characters of depth relationship are usually presented in the form of equations relating interval velocity V with one way travel time t. The following relations are established for each of the 3 geological provinces: in the lower slope of NSCS-: V=1511+2373.8×t-406.18×t^2; in the Northwest Subbasin of NSCS: V=1511+2111.2×t-146.77×t^2; in the continental slop area of NSCS: V=1511+2003×t-20.342×t^2. In addition, we compared our velocity analysis results with those derives from logging data of the ODP site 1148 located in the Northwest Subbasin of NSCS. At site 1148, four seismic horizons from Pleistocene to Oligocene can be recognized, and so are our results from VSPEC. The velocity information from core data are in consistent with the velocity information derived from seismic data in this study.