利用水下無人載具描繪臺灣西南海域斷層帶海床孔洞與線形槽溝特徵

Abstract

臺灣西南海域蘊藏豐富的天然氣水合物，而永安海脊與好景海脊為一探勘目標區。本研究利用美國蒙特利灣水族館研究所的自主式水下無人載具(Autonomous Underwater Vehicle, 簡稱AUV)以及搭配國家實驗研究院臺灣海洋科技研究中心的無人水下遙控載具(Remotely Operated Vehicle, 簡稱ROV)，首次於臺灣西南海域進行超高解析度水深探勘與即時海床觀測調查，希望藉由水下載具所得之超高解析度資料來觀察位於好景海脊西側逆斷層上方的海床是否有逸氣構造及自生性碳酸鹽礁之形成。 本調查範圍為好景海脊西側一條南北走向的逆斷層帶，長度約10.8公里、寬度約2.3公里、水深約975公尺至1450公尺，斷層東側為逆衝褶皺抬升的海脊高區，向西漸緩至沉積盆地。每1公尺網格點的超高解析度多音束水深資料顯示此區中有兩種非常特殊的海床形貌：海床不對稱孔洞以及線形槽溝。此外，高解析度側掃聲納影像中也觀察到一處異常高振幅反射區。 本研究區域北段海床上有許多形似冒氣孔洞的凹陷，數量超過200個，大致分布於海脊西南側斜坡底部，形狀似彗星的凹陷(comet-shaped scar)深1至8公尺，頭部深凹而尾部狹長，也有些較圓的凹陷狀似麻坑(pockmark)。高解析度底質剖面顯示海床不對稱孔洞均出現於一傾斜層面被斜坡截切的地帶，推測這些狹長形的孔洞可能是由於地層中富含天然氣的流體沿著向東北傾斜的層面移棲至海床後噴出海床，所形成崩塌形貌的海床孔洞。而於孔洞區附近觀察到的異常高振幅區，則可能是自生性碳酸鹽在海床上形成底質堅硬、表面較粗糙的區域。本研究認為海床不對稱孔洞與異常高振幅區可能均與逸氣構造有關。調查區域南段的海床上則存在一系列的南北走向線形槽溝，每個線形槽溝寬約5至10公尺、深約1至4公尺，平均長度約為1.6公里，從高解析度海床底質聲納剖面顯示線形槽溝並無向下延伸至深部，而震測資料顯示線形槽溝位於褶皺逆衝斷層上盤，本研究認為線形槽溝之形成可能與此區逆斷層活動所造成的塊體運動有關。 貼近海床的超高解析度海床調查可以觀測到船載式儀器無法解析出的細微海床形貌，對瞭解調查區域受海底下方流體或其他構造作用造成的影響提供更多的資訊，並指出海床上尚有許多先前未揭露之形貌特徵值得去探討。

In April 2013, a Taiwan-US collaborative recent cruise was conducted in the area offshore southwest Taiwan. Numerous (over 200) asymmetric seafloor depressions and a series of linear troughs were observed along a west-vergent fault zone on the west side of the Good Weather Ridge utilizing the mapping Autonomous Underwater Vehicle (AUV) of the Monterey Bay Aquarium Research Institute (MBARI). These asymmetric seafloor depressions are distributed over an area of about 1.03 km2 in the eastern part of the surveyed area between 1100 and 1200 m water depths. The zone of depressions occurs near the boundary between the smooth seafloor of sediment basin to the west and the outcrops of a series dipping exposed on a strata sloping seafloor to the east. The unprecedented high-resolution multibeam bathymetry (1 m lateral resolution) and chirp sub-bottom profiles (11 cm vertical resolution) reveal that the depressions are 1 to 8 m deeper than the surrounding seafloor and form comet-shaped scars of ~10 to 200 m length that widen downslope. Some circular depressions also occur, which would usually be described as pockmarks. Some asymmetric depressions have knickpoints within their scarps, which are horizontally aligned and suggested the existing of some weak planes. Chirp sub-bottom profiles also show horizontal reflectors, which are parallel with the exposed beds seen in the outcrops above, indicating that these depressions occur where the sediment cover over these beds pinches out. This study implies these asymmetric depressions relate to gas seepage, because the fluid possibly migrate along the lightly dipping horizontal strata and then blowout where seafloor cute the strata that generate these depressions. Furthermore, the side-scan sonar images found a small high amplitude anomalies area around the depressions, showing a strong reflection caused by authigenic carbonates. Seafloor where has authigenic carbonates become firm and roughness causing strong backscatter energy. On the southern partner of the surveyed area, a series of north-south tending troughs occur within a 4.5 km long and 1.5 km wide zone in water depths 975 and 1450 m. Seismic reflection profiles running across the fault zone show that these troughs are developed on top of a small young sediment wedge at the toe of the hanging wall above the west-vergent fault. The sediment of this wedge is uplifted and folded due to the fault activities, and the observed young troughs could be the seafloor expression of the deformation. This study implies these troughs are related to the fault and block movement. However, few areas have been surveyed at this resolution and thus we speculate seafloor asymmetric depressions and seafloor troughs like the descriptions above are not unique in this area.