南沖繩海槽仿三維震測資料處理與應用

Abstract

本研究以海研一號所配備的二維震測資料收集系統，在強烈海流造成的浮纜偏移效應下，搭配震測浮纜尾端浮標的定位系統與密集的二維震測線，首次嘗試以真實三維震測資料處理技術，成功建立位於南沖繩海槽品質相當不錯的三維震測影像。 在經濟部中央地調所原定2017年9月的資料收集，原本打算在南沖繩海槽進行大範圍的二維震測資料收集，希望對南沖繩海槽的地質能夠有更進一步瞭解，但是在進行震測資料收集時，卻發現具有強烈的黑潮影響著震測浮纜，使得震測浮纜接收器造成偏移(seismic cable feathering)，如果按照原定計畫進行二維震測資料收集，勢必因為描線落點位置的不夠正確，造成回收的資料品質與剖面的真實性產生誤差，因此當時在航次進行途中，決定改變資料收集方式，改為收集測線間距約340公尺的密集二維震測線，搭配震測浮纜尾端的定位系統，藉由幾何計算找出描線真正的位置，在許多條密集二維震測線所建立空間之中，建立三維震測影像。 沖繩海槽為琉球弧溝隱沒系統後方的張裂性弧後盆地，本論文的研究區域位於此弧溝隱沒系統最南端的南沖繩海槽中，最新一期的盆地擴張中心區。南沖繩海槽是相當年輕且正在活躍的弧後盆地，於本研究中的震測剖面可看到許多疑似火成入侵的強振幅震測訊號，同時具有許多對稱於盆地擴張中心區的正斷層發育。 依照本研究的資料處理方式，在以僅有的二維震測資料收集設備的情況下，建立了品質不錯的三維震測影像。於處理出的震測影像結果顯示，在南沖繩海槽中，具有許多的正斷層存在。並於本研究結果的震測影像中發現，許多明顯的火成侵入特徵與切穿海床的構造存在，推測南沖繩海槽中的許多斷層，可能是氣柱噴發的良好管道之一。

According 2D seismic acquisitive equipment to produce 3D seismic image by streamer feathering, dense 2D seismic survey and precise GPS system which setting on the tail buoy at the end of seismic cable. The Southern Okinawa Trough (SOT) is the back arc basin behind the Ryukyu subduction zone, which is the youngest rifting region with highly geothermal activities. The highly geothermal value may suggest the strong hydrothermal circulation in this area. Hydrothermal mineral deposits consist of rare earth elements are formed in situ through hydrothermal alternation. For further understanding, the 3D seismic image play an important role in geophysical investigation, which could picture a reality of geological structures distributed underground. The 3D seismic data used in this research was collected unpredictably during the cruise where there was a strong Kuroshio Current passing through the east site of Taiwan. The strong current affected the seismic cable, causing seismic cable feathering. The seismic cable feathering induce incorrect seismic data. To fix this flaw, data acquisition pattern during the cruise needed to be changed from original survey. Under the premise that the original seismic data convert into dense 2D seismic lines with the interval around 340 meter, GPS data of seismic traces should be calculated. With such kind of improvement, the flaw in 2D seismic data processing become an advantage in 3D seismic data processing. 3D seismic image processed with the steps in this research, could show a distinct seismic image to reveal igneous intrusion and normal faults development in the rifting region of SOT. The normal faults which cut through the igneous intrusion may be a pathway of fluid migration. After the processing steps that applied in my research, high quality 3D seismic images could be processed by only 2D seismic acquisitive equipment.