三維底質回聲影像技術建立：以永安海脊與下枋寮盆地為例

Abstract

本研究將過往2維的底質回聲剖面轉化為空間上的影像，成功發展了3維的底質回聲影像技術。3維回聲影像能幫助我們清楚分析海床下複雜構造的幾何形貌，正確定義出沉積物分佈範圍或地層之走向。 近年來利用反射震測於臺灣西南海域已發現大量的海底仿擬反射(BSR)存在，此一訊號被認為是天然氣水合物存在的重要指標。過往的調查通常為大範圍的普查，但針對滲漏型天然氣水合物儲集層仍缺乏較明確的描述，無法合理評估開採效益。本研究利用仿3維震測在具有探勘潛力的地區作業時，伴隨收集而來的回聲剖面資料做3維影像處理。首先以下枋寮盆地區的資料來發展此項技術，比對前人的研究成果做為驗證，然後將發展出來的技術應用在永安海脊區，再加入岩心和地化數據來探討海床上流體之滲漏現象。 在下枋寮盆地區是由八座泥貫入體包圍所組成，此處泥貫入體的抬升影響了沉積物的傳輸，使盆地呈階段性的發育，由於枋寮海底峽谷上游提供有限的沉積物量，加上盆地目前受泥貫入體阻擋，減少了沉積物填滿與溢出(fill and spill)的下坡充填過程，使得此區域以塊體運動堆積(MTDs)為主要的盆地填充沉積。本研究利用3維影像在此區發現三處塊體崩落堆積物，一處位於盆地中部東側斜坡腳下，水深約960公尺，從中心向西南方向約1.450平方公里外伸沉積體，厚度約20公尺；另一處位於盆地北部西側斜坡腳下，向東延伸面積約1平方公里、厚7公尺的沉積物；第三處的沉積物在研究區盆地北部，邊界範圍可再往北延伸超出研究區的範圍。由這些海床上沉積物的堆疊情形及分佈，能幫助我們了解近代盆地內的發育趨勢。 永安海脊西側斜坡盆地的發育受到古河道的多次切割與填充，承接了峽谷源頭帶來的砂質沉積物，良好的滲透率可以做為天然氣水合物移棲的空間。本研究發現在東側與西側斜坡盆地的岩心資料薄粉砂層顯示有高甲烷濃度的岩心深度，所對應到的底質回聲剖面為強振幅的反射層，且部分薄粉砂層面在被斜坡盆地截切的地帶，發現有麻坑現象的分布。利用3維回聲影像描繪出粉砂層面的分佈範圍，約從永安海脊西側斜坡盆地北部往南延伸3400公尺到盆地之中部，往西則延伸到海底峽谷的位置，深度約在1437至1470公尺之間，在永安海脊東側粉砂層走向約為北偏東30度，向東南傾斜1.5度，深度878至1047公尺。這些層面可能為淺部的甲烷滲漏管道，利用這些資訊期望對未來天然氣水合物探勘井位提供有利的評估資訊。

This study utilizes sub-bottom profile images to identify seafloor features, such as mud volcanoes, fluid seepage features and mass transport deposits, for gas hydrate and geohazard investigations. 3D chirp image blocks have been successfully constructed by compiling the 2D chirp sonar profile data collected through dense (50 or 100 m) line-spacing surveys. 3D chirp images can help us to recognize the true geometry of complex structural features on and just beneath the seafloor, also can better define the distribution of seafloor sediments and the directions of shallow strata. Gas hydrate as a potential new energy resource has been noticed worldwide in recent years. Offshore southwestern Taiwan, clear bottom simulating reflectors (BSRs) have been widely observed on seismic profiles. In this study, we try to get details of subsurface seepage structures from 3D chirp images. First, we build a 3D chirp sonar image block of the Lower Fangliao Basin. Then we compare this 3D chirp sonar block to previous research for technical verifications. Finally, we apply this technique to Yung-An Ridge area, linking chirp sonar images with core sample analyses and geochemical data to explore the subsurface fluid migration path. The mud diapiric intrusions have resulted in a series of structural highs in and around the Lower Fangliao Basin. In this area, the major influence of sedimentation is mass transport deposits (MTDs) as the Fangliao submarine canyon provides a limited amount of sediments, and the basin is currently blocked by mud diapirs. By analyzing 3D chirp sonar images, MTDs are recognized in three blocks. The first one is located in the middle of the basin with depth about 960 meters, thickness about 20 meters, and covers an area of 1.450 square kilometers. Another MTD is located west of the northern part of the basin. It is 7 m thick sediments.with an area of about one square kilometers eastward. The 3rd MTD is in the northern part of the basin in the study area, which can be extended further northward beyond the study area. The slope basin west of the Yung-An Ridge (YAR) receives sandy sediments from submarine canyons. Highly permeabe sand bodies can be acted as fluid conduits. In the west slope basin, MD core data show that the thin silt layers which contain high concentration of methane would correspond to the strong amplitude reflections on chirp sonar profiles. Similar situation can also be seen on the intra-slope basineast of the YAR. The use of 3D chirp images can depict the distribution of these silt layers. These silt layers may become the seepage structures for active gas transporting upward to the seafloor. In the slope basin west of the YAR, the silt layer extends to 3400 meters from north to the middle basin. In the eastern part of intra-slope basin, this layer is about north by east 30 degrees and tilted 1.5 degrees to the southeast.