5:15 PM - 6:45 PM
[SCG48-P09] The Challenger Deep bathymetry sounded during R/V Hakuho-maru KH-23-9 cruise
Keywords:Multibeam bathymetry, Challenger Deep, R/V Hakuho-maru
Exploration of the deepest part of the Earth's surface has been a long standing challenge spanning centuries. The Challenger Deep in the Mariana Trench (~11.3°N/142.2°E) stands as the deepest ocean floor on Earth, consisting of three individual small basins or depressions. The history of acoustic bathymetry and pressure measurement here is extensive, commencing with the British ship HMS Challenger II, followed by the Soviet ship Vityaz, and the American manned bathyscaph Trieste in 1960. Subsequent to these early efforts, modern multi-beam acoustic bathymetry has been conducted by the Japanese ships, including S/V Takuyo in 1984, R/V Hakuho-maru in 1992, and R/V Kairei in 1998, 1999, and 2002. Additionally, American ships R/V Kilo Moana in 2008 and USNS Sumner in 2010 as well as the German ship R/V Sonne in 2016, have contributed to the exploration of the Challenger Deep.
The most recent observations were carried out by the DSSV Pressure Drop, conducting Kongsberg EM124 multibeam echo-sounding at 8kt and vertical oceanographic observations for sound velocity correction. These observations, including full-depth CTD probe data installed on the DSV Limiting Factor, determined the deepest seafloor depth value to be 10,924 ± 15 m in the eastern basin of the Challenger Deep (Bongiovanni et al., 2021). Concurrently, Greenaway et al. (2021) reported a revised depth of 10,935 m ± 6 m from the submersible transects, incorporating acoustic altimeter profiles referenced to in-situ pressure and corrected for observed oceanographic properties, atmospheric pressure, gravity and gravity-gradient anomalies, and water-level effects.
Thus, the Challenger Deep is the deepest part where many studies have evaluated bathymetric value; in other words, it is an area that contributes to the performance evaluation of multibeam bathymetry. In this study, we introduce new sounding data in this area using a multibeam echo-sounder of Kongsberg EM124 newly installed on the R/V Hakuho-maru and show the results of the bathymetric feature of mapped three basins of the Challenger Deep. This exploration was conducted during the KH-23-9 cruise, and the Challenger Deep bathymetric data was obtained mainly on 30 November 2023.
East-west and north-south survey lines were established in three basins, namely, east, central, and west, with observations conducted in electric thrust mode at a ship speed of 4 knots. Between survey lines, the ship navigated at 8 knots, with data acquisition partly occurring at 15 knots in direct mechanical thrust mode. A swath angle of 40/40 degrees was set during the survey, and the "Very deep mode" was operationally tested near the Challenger Deep. An expendable conductivity, temperature, and depth probe 4 (XCTD4) observed the ocean structure down to 1,914 m, calculating seawater sound velocity profiles combined with ocean model values (global dataset of LEVITUS) in the deeper layer.
On the east-west survey line, data was acquired well and densely with only one survey line. Minor data loss was confirmed on the other side of the small ridge structure. On the north-south survey line, the basin bottom was smoothly captured at the beam directly under the ship (at a distance of ~1km). On the other hand, the data is noisier on the outer swath, probably due to the wobble of the bow and the roll motion caused by the ocean current. As a result, the east-west survey line shows better quality, but there is still a tendency for the depth values in the outer swath to be deeper around a specific point in the swath (probably the seam of the beam sector), and this was observed about once every few pings. There was no difference in ping quality between different thrust modes or vessel speeds.
Preliminary results of obtained bathymetry distinctly indicate that the central basin is the shallowest in the Challenger Deep. The shape of the three basins is consistent with previous studies. Although preliminary, the maximum depth of eastern and western basins indicates a range of 10,915–10,930 m and 10,910–10,920 m, respectively. These values are generally consistent with bathymetric values most recently examined by Greenaway et al. (2021), but are thought to be a few meters to 20 meters shallower. This verification requires at least a close examination of the seawater sound velocity structure.
Finally, it should be emphasized that the present observations have shown that even with a single survey line at cruising ship speeds up to 15 knots and seawater sound velocity profiles with shallow XCTDs and deep ocean models, fairly realistic bathymetric values can be obtained even at the deepest ocean floor on Earth.
The most recent observations were carried out by the DSSV Pressure Drop, conducting Kongsberg EM124 multibeam echo-sounding at 8kt and vertical oceanographic observations for sound velocity correction. These observations, including full-depth CTD probe data installed on the DSV Limiting Factor, determined the deepest seafloor depth value to be 10,924 ± 15 m in the eastern basin of the Challenger Deep (Bongiovanni et al., 2021). Concurrently, Greenaway et al. (2021) reported a revised depth of 10,935 m ± 6 m from the submersible transects, incorporating acoustic altimeter profiles referenced to in-situ pressure and corrected for observed oceanographic properties, atmospheric pressure, gravity and gravity-gradient anomalies, and water-level effects.
Thus, the Challenger Deep is the deepest part where many studies have evaluated bathymetric value; in other words, it is an area that contributes to the performance evaluation of multibeam bathymetry. In this study, we introduce new sounding data in this area using a multibeam echo-sounder of Kongsberg EM124 newly installed on the R/V Hakuho-maru and show the results of the bathymetric feature of mapped three basins of the Challenger Deep. This exploration was conducted during the KH-23-9 cruise, and the Challenger Deep bathymetric data was obtained mainly on 30 November 2023.
East-west and north-south survey lines were established in three basins, namely, east, central, and west, with observations conducted in electric thrust mode at a ship speed of 4 knots. Between survey lines, the ship navigated at 8 knots, with data acquisition partly occurring at 15 knots in direct mechanical thrust mode. A swath angle of 40/40 degrees was set during the survey, and the "Very deep mode" was operationally tested near the Challenger Deep. An expendable conductivity, temperature, and depth probe 4 (XCTD4) observed the ocean structure down to 1,914 m, calculating seawater sound velocity profiles combined with ocean model values (global dataset of LEVITUS) in the deeper layer.
On the east-west survey line, data was acquired well and densely with only one survey line. Minor data loss was confirmed on the other side of the small ridge structure. On the north-south survey line, the basin bottom was smoothly captured at the beam directly under the ship (at a distance of ~1km). On the other hand, the data is noisier on the outer swath, probably due to the wobble of the bow and the roll motion caused by the ocean current. As a result, the east-west survey line shows better quality, but there is still a tendency for the depth values in the outer swath to be deeper around a specific point in the swath (probably the seam of the beam sector), and this was observed about once every few pings. There was no difference in ping quality between different thrust modes or vessel speeds.
Preliminary results of obtained bathymetry distinctly indicate that the central basin is the shallowest in the Challenger Deep. The shape of the three basins is consistent with previous studies. Although preliminary, the maximum depth of eastern and western basins indicates a range of 10,915–10,930 m and 10,910–10,920 m, respectively. These values are generally consistent with bathymetric values most recently examined by Greenaway et al. (2021), but are thought to be a few meters to 20 meters shallower. This verification requires at least a close examination of the seawater sound velocity structure.
Finally, it should be emphasized that the present observations have shown that even with a single survey line at cruising ship speeds up to 15 knots and seawater sound velocity profiles with shallow XCTDs and deep ocean models, fairly realistic bathymetric values can be obtained even at the deepest ocean floor on Earth.