5:15 PM - 6:30 PM
[SCG43-P02] Visual observation using a small ROV around the lake-bottom seepage area of the Lake Biwa
Keywords:Lake Biwa, Lake-bottom seepage, ROV
There is a flat depression at the depth of 90-100 meters around the northwestern area of the Biwa lake off Takashima city. Many lake bottom seepage points on the depression area have been pointed out, and the distribution of these points is lined in a north-south direction. The contribution of ground water seepage from the lake bottom is estimated to be about 10 to 20 %, however, their actual condition is not clear at the lake-bottom.
To confirm the relationships between the sedimentary structure and lake bottom seepage, we conducted the sedimentary structure survey using sub-bottom profiler loaded on the R/V Hassaka of The University of Shiga Prefecture. Three SBP survey lines were carried out in the east - west direction in the depression era. The penetration depth of this survey was about 15 m. Some strong reflectors were detected in all survey lines. The deepest strongest reflector is presented at the depth of 12-15 m, and this corresponds to the Kikai-Akahoya (K-Ah) tephra (Iuchi et al., 1987). Next, fish finder was also used for detection of acoustic anomaly of water column related to the seepage on the lake bottom. We traced the seepage points from north to south to detect the water column anomaly using a fish finder. As a result, we found a clear water column anomaly around the 50 m height mound at the southern area of the depression.
Therefore, we installed the BlueROV2 manufactured by Blue Robotics Inc. To detect the existence of lake-bottom seepage by the visual observation around the water column acoustic anomaly area, and to correct the seepage water sampling for chemical analysis at the seepage point. In the default specification, the pressure case is made of acrylic, however, we changed it to an aluminum pressure case for durability and also adopted detachable connector for maintainability. This ROV is characterized by its high maintainability and ease of user function expansion. Therefore, we are considering adding the function to close the sampling bottom by the command from the ship. We will report the visual observation results using this ROV, which is scheduled to be conducted in March 2021.
To confirm the relationships between the sedimentary structure and lake bottom seepage, we conducted the sedimentary structure survey using sub-bottom profiler loaded on the R/V Hassaka of The University of Shiga Prefecture. Three SBP survey lines were carried out in the east - west direction in the depression era. The penetration depth of this survey was about 15 m. Some strong reflectors were detected in all survey lines. The deepest strongest reflector is presented at the depth of 12-15 m, and this corresponds to the Kikai-Akahoya (K-Ah) tephra (Iuchi et al., 1987). Next, fish finder was also used for detection of acoustic anomaly of water column related to the seepage on the lake bottom. We traced the seepage points from north to south to detect the water column anomaly using a fish finder. As a result, we found a clear water column anomaly around the 50 m height mound at the southern area of the depression.
Therefore, we installed the BlueROV2 manufactured by Blue Robotics Inc. To detect the existence of lake-bottom seepage by the visual observation around the water column acoustic anomaly area, and to correct the seepage water sampling for chemical analysis at the seepage point. In the default specification, the pressure case is made of acrylic, however, we changed it to an aluminum pressure case for durability and also adopted detachable connector for maintainability. This ROV is characterized by its high maintainability and ease of user function expansion. Therefore, we are considering adding the function to close the sampling bottom by the command from the ship. We will report the visual observation results using this ROV, which is scheduled to be conducted in March 2021.