彰濱外海高解析震測地層及流體特徵分析

彰濱外海離岸風場區地質環境可能易產生沉積物液化與流體化作用，若能有效掌握地層特性及流體特徵，將有助於離岸工程的建設。本研究透過海洋地質及地球物理方法，發展及應用高解析火花式放電反射震測技術，結合海、陸域地質岩心資料，針對彰濱外海淺部地層特性及流體特徵進行探討，並評估可能的液化潛勢。針對單頻道高解析震測資料，除運用常規處理方法，亦針對資料中的複反射、湧浪效應、振幅變化不均等問題，應用進階處理技術進行處理，與一般彰濱外海的已發表資料相比，資料品質獲得大幅度改善，可提升對地層分析及流體特徵的辨識效率與可信度。利用震測相分析，根據外部形貌差異將流體特徵分為垂直移棲型與水平富集型，再以內部特徵將垂直移棲型細分為P1(反極性)、P2(極性正常)、P3(連結麻坑)型；而水平富集型又可分為H1(楔型狀)、H2(透鏡狀)、H3(平板狀)型。P1、P2型密集分布於古、今濁水溪出海口，推測其形成機制與濁水溪快速沉積作用有關，P3型數量少，零星分布於彰濱外海，三者皆移棲至近海床地層，故形成年代近今。水平富集型皆呈東北-西南向分布，其中H1、H2型位於古濁水溪口外，其分布地層推測為古濁水溪快速沉積事件下，分別由砂所形成的楔型體與遠端細粒沉積物形成的沉積體，H3型離岸較遠，其位於東彰雲沙脊發育區。透過海水面變化及岩心定年資料對比，H2、H3型應在6~4.5 ka之遠濱沉積環境中所形成，而H1型則在4.5 ka至現今之三角洲潮盆環境中發育。進一步將所辨識出的不同類型流體分布與地工分析方法所建立的液化潛勢結果比對。結果顯示，垂直移棲型多位於低液化潛勢區；水平富集型則多位於高液化潛勢區，指出兩種主要流體特徵，分別反應液化作用發生前後的結果。本研究認為地物技術所辨認出的流體特徵結合地工分析之液化潛勢結果有其一致性，若能結合地物方式的大範圍調查優勢，先建立出地質模型，再以少量精準的地工分析加以驗證，對於離岸風電場址的評估效率及成本應有正面之幫助。

關鍵字

流體特徵；液化潛勢；沉積環境；離岸風電；火花式放電震測；彰濱外海

並列摘要

The area of offshore Changhua of Central Taiwan is high potential of soil liquefaction and fluidization in the seabed. To get better understanding of strata characteristics, especially the fluid characteristics will be helpful for offshore wind farm construction. This study integrates geology and geophysics data including high-resolution single channel sparker seismic profiles, offshore and onshore wells data to recognize fluid characteristics and evaluate liquefaction potential in strata offshore Changhua. We develop and apply advanced single channel seismic data processes to suppress noises caused by multiple reflection, swell effect and unbalance amplitudes. Comparing to the other high-resolution seismic data published in the area, our results present much higher qualities. By taking advantages of good seismic images, we can provide more convinced results of seismic stratigraphy and fluid characteristics analyses. According to seismic facies analyzing results, the fluid characteristics in the study area can be classified as “vertical” and “horizontal” types. Furthermore, the vertical type can be classified as three sub-types including P1 (reversed polarity), P2 (normal polarity) and P3 (connect with pockmark); and the horizontal type includes H1 (clinoform), H2 (lenticular) and H3 (flat) sub-types. Most of P1 and P2 fluid features locate at the area nearby the paleo and modern Choshui River mouth. We speculate the origin of this two sub-types could be results of high sediment discharge of Choshui River. P3 fluid features are rarer than P1 and P2 and sporadically distributes in the study area. All vertical type of fluid feature are observed in the shallow strata and most of them can be traced below seafloor. Thus, we suggest that their formation was in the recent ages. The distribution of H1 and H2 fluid features extend southward from the paleo-Choshui River mouth. H1 features are identified in sandy and clinoform strata caused by terrigenous sediments discharged from paleo- Choshui River, and H2 features are developed in fine sediment bodies. H3 sub-type occurs in the area near by East Chang-Yuen Ridge far away from the Changhua coast. Based on the sea level change records and core dating data, we suggest that H2 and H3 features could develop within offshore sedimentary facies from 6 to 4.5 ka, and H1features should develop in submarine delta since 4.5 ka. We integrate geophysical analyses results and published geotechnical researches to discuss relations between recognized fluid characteristics and liquefaction potential in the study area. The results show that most of the vertical occurred in low liquefaction potential area; however, most of the horizontal fluid features developed in high liquefaction potential area. We suggest that the occurrences of the vertical type can be consequences of liquefaction and fluidization process in the area In contrast, the occurrences of horizontal type could be an indicator of high liquefaction potential in the strata. This study successfully presents our geophysical analyzing results of fluid characteristics lead to similar conclusions of published geotechnical analyzing results for liquefaction potential evaluation in the area. Combining advantages of geological, geophysical and geotechnical methods, we can build a geological model first, and then validate that model by limited geotechnical analysis results. Moreover, a better geotechnical model can be provided for further uses of offshore wind farm construction. Based on this study, we believe our suggestion can offer a better approach making higher economic benefits for site investigations of offshore wind farms offshore Changhua of Central Taiwan.

並列關鍵字

fluid characteristic ； liquefaction potential ； depositional environment ； offshore wind farm ； sparker seismic ； offshore Changhua

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英文文獻

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