Establishing 3D Chirp Imaging Technique : Examples from the Yung-An Ridge and Lower FangLiao Basin
Date Issued
2015
Date
2015
Author(s)
Shen, Jyun-Yan
Abstract
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.
Subjects
3D chirp images
Yung-An Ridge
Lower Fangliao Basin
seepage structure
fluid migration
mass movements
SDGs
Type
thesis
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