5:15 PM - 6:45 PM
[ACC26-P09] Detail lake-bed topography mapping of Lago Grey in Patagonia by Multi-beam echosounder survey
Keywords:glacier, MBES, Lake-bed geometry
Understanding mechanisms controlling rapid retreat of lake-/ocean-terminating calving glaciers is urgently required because the calving glaciers contributes the global sea level rise. The complex variation of calving glaciers is affected by the presence of lakes or the ocean. Recently, the lake-/ocean-bed topography plays an important role for controlling the stress balance of at the termini, thereby the rapid retreat. For ocean-terminating glaciers, detail ocean bathymetry, which acquires using multi-beam echosounder (MBES), was utilized to analyze the glacier change. On the other hand, the number of detail lake-bathymetry is limited due to the rarity of ship mounted MBES system operated for glacial lakes, which limits our understanding of the mechanism for the rapid retreat of the lake-terminating glaciers. Here, we report the lake-bed geometry which we operated MBES system on a ship for a glacier lake in Southern Patagonia.
Glaciar Grey, a lake-terminating glacier, has three separated termini, all of which flowing into the Lago Grey. The central terminus retreated by 2 km in 1999, and the eastern terminus retreated by 1.5 km in 2017–2021. Bathymetry by single-beam sonar surveys in 2017–2018 revealed the presence of 400-m-deep lake-bed geometry in front of the central terminus.
In this study, we measured the geometry of glacier-ice submerged in lake and lake-bed geometry by using a portable MBES system (EM2040P, KONGSBERG Ltd.). The system was consisted with a transducer, an inertial motion sensor, and two GNSS antennas. We fixed the system onto a small boat (length: ~5 m, width: ~2 m), by using metal pipes and rushing belts. The number of sonar-beam was 512, and the measurement range was 550 m in maximum. The pulse frequency was selected from 200, 300, and 400 kHz. We operated the system with changing the direction, angle, and frequency of the beam depending on the situation of data acquisition (depth and resolution). The MBES surveys were carried out on four days (4, 5, 11, 14 May 2023) considering the lake and the glacier situation (weather, surface wave, iceberg, and calving activity). Position accuracy of point clouds acquired by MBES was improved by using several corrections. Coordinates of the boat was corrected by kinematic GNSS with a static station on the bedrock at lakeshore. Distance between the transducer and measured points was corrected by using vertical sound-velocity profiles measured 10 points during the MBES survey. Sonar tilt of each survey was corrected by using a patch test with a single characteristic lake-bed geometry. We manually removed outlier points by checking all of the swath measured. Finally, 5-m resolution lake-bed terrain model was constructed from lake-bed point clouds.
Lake-bed geometry of Lago Grey was constructed over 12 km2 from four-days surveys. The bathymetry data revealed the presence of complex geometry, which includes important information for interpreting past glacier dynamics and showing past glacier activity as below. Moraine-like terrains were found at the 2-km down-glacier side of the Central terminus, and at the 1-km down-glacier side of the Eastern terminus from each 1979 terminus position. The height of moraine-like terrain was 100 m on the glacier-side and 20 m on the other side from the Central terminus. These features show that ice-front position beyond the maximum advance from the record of land-/aerial-photographs, which showing the ice-front position on the little ice age maximum. The deepest part of Lago Grey in front of the Central terminus was flat, over an area of 0.5 km2. Bathymetric map shows bed-rock like bumpy features alongside the flat terrain and some of the parts were covered by the sediments. These features imply that the flat area is covered by the glacier delivered sediments. A 300-m-deep and 1.5-km-long narrow valley was found from the lake-bed geometry in front of the Eastern terminus. The rapid retreat of Eastern terminus since 2017 was due to flotation of terminus that caused by the ice-front retreat into this valley.
MBES survey revealed a detail lake-bed geometry of Lago Grey. The lake-bed geometry shows possibilities of analyzing past-glacier activities back beyond the era that aerial photographs and satellite images were captured.
Glaciar Grey, a lake-terminating glacier, has three separated termini, all of which flowing into the Lago Grey. The central terminus retreated by 2 km in 1999, and the eastern terminus retreated by 1.5 km in 2017–2021. Bathymetry by single-beam sonar surveys in 2017–2018 revealed the presence of 400-m-deep lake-bed geometry in front of the central terminus.
In this study, we measured the geometry of glacier-ice submerged in lake and lake-bed geometry by using a portable MBES system (EM2040P, KONGSBERG Ltd.). The system was consisted with a transducer, an inertial motion sensor, and two GNSS antennas. We fixed the system onto a small boat (length: ~5 m, width: ~2 m), by using metal pipes and rushing belts. The number of sonar-beam was 512, and the measurement range was 550 m in maximum. The pulse frequency was selected from 200, 300, and 400 kHz. We operated the system with changing the direction, angle, and frequency of the beam depending on the situation of data acquisition (depth and resolution). The MBES surveys were carried out on four days (4, 5, 11, 14 May 2023) considering the lake and the glacier situation (weather, surface wave, iceberg, and calving activity). Position accuracy of point clouds acquired by MBES was improved by using several corrections. Coordinates of the boat was corrected by kinematic GNSS with a static station on the bedrock at lakeshore. Distance between the transducer and measured points was corrected by using vertical sound-velocity profiles measured 10 points during the MBES survey. Sonar tilt of each survey was corrected by using a patch test with a single characteristic lake-bed geometry. We manually removed outlier points by checking all of the swath measured. Finally, 5-m resolution lake-bed terrain model was constructed from lake-bed point clouds.
Lake-bed geometry of Lago Grey was constructed over 12 km2 from four-days surveys. The bathymetry data revealed the presence of complex geometry, which includes important information for interpreting past glacier dynamics and showing past glacier activity as below. Moraine-like terrains were found at the 2-km down-glacier side of the Central terminus, and at the 1-km down-glacier side of the Eastern terminus from each 1979 terminus position. The height of moraine-like terrain was 100 m on the glacier-side and 20 m on the other side from the Central terminus. These features show that ice-front position beyond the maximum advance from the record of land-/aerial-photographs, which showing the ice-front position on the little ice age maximum. The deepest part of Lago Grey in front of the Central terminus was flat, over an area of 0.5 km2. Bathymetric map shows bed-rock like bumpy features alongside the flat terrain and some of the parts were covered by the sediments. These features imply that the flat area is covered by the glacier delivered sediments. A 300-m-deep and 1.5-km-long narrow valley was found from the lake-bed geometry in front of the Eastern terminus. The rapid retreat of Eastern terminus since 2017 was due to flotation of terminus that caused by the ice-front retreat into this valley.
MBES survey revealed a detail lake-bed geometry of Lago Grey. The lake-bed geometry shows possibilities of analyzing past-glacier activities back beyond the era that aerial photographs and satellite images were captured.