14:45 〜 15:00
[SCG55-28] Seafloor Crustal Deformation Observations Utilizing Unmanned Surface Vehicles at Plate Subduction zones around the Japanese Islands
キーワード：海底地殻変動、GNSS-A、ウェーブグライダー、無人観測機、2011年東北地方太平洋沖地震
Since the development began in 2018, Japan Agency for Marine-Earth Science and Technology (JAMSTEC) and Tohoku University has been using an unmanned surface vehicle (USV), Wave Glider (WG), to observe seafloor crustal deformation using GNSS-acoustic (GNSS-A) measurements. Since 2020, there have been twice-yearly observation cruises that visit numerous (7–18) observation stations installed along the Japan and Kuril Trenches, northeastern Japan, during each cruise. A total of 11 long-term (20–80 days) cruises have already been completed.
The WG, developed by Liquid Robotics, Inc. (U.S.A.), is an automatic autonomous USV that generates propulsion by utilizing the difference in magnitude of vertical motion of seawater caused by waves between the sea surface and the seawater. The solar panels and rechargeable batteries on the float on the sea surface power for navigation control, operations of observation equipment, data recording, and satellite communication with land. As no fuel is required for the system, the WG can navigate for a prolonged period and conduct GNSS-A measurements when the solar panels generate sufficient power. Autonomous control is also provided, such as adjusting the angle of the rudder based on the direction of the ocean currents and the direction of travel of the WG, allowing the vehicle to follow a predetermined course and avoid collision with a vessel equipped with an Automatic Identification System.
From spring to summer, we conducted one-to two-month observation cruises covering observation stations off the Pacific coast of the Nemuro Peninsula, Hokkaido district, to off the coast of Ibaraki Prefecture, Kanto district, as well as less than one-month cruises focusing on visiting observation stations off Aomori to Miyagi Prefectures in the Tohoku district from summer to fall. The observation cruises were performed without major accidents that could have resulted in the loss or wreckage of the vehicle, such as contact with other vessels or serious malfunctions of internal equipment, making the GNSS-A measurement impossible. However, there have been instances where the vehicle was unable to follow the planned route due to strong ocean currents or when the observation time had to be shortened due to insufficient power generation from the solar panels. It has become clear that operational limitations exist due to the WG’s performance.
In terms of ocean currents, the WG’s cruising speed (speed through the water) is slower than 2 knots even when the auxiliary thruster is operated at maximum output, and the oceanic current is faster than it is in the central region of the Kuroshio Extension and the Tsugaru Warm Current. As a result, the WG struggles to follow the planned route as set at the expected speed. Although a moving survey at an observation station (i.e., establishing a navigation route as a circle with diameter equal to the station’s water depth) is required for highly accurate positioning, the station’s current oceanic state limits the availability of this operation. Furthermore, the availability of the moving survey is determined by the amount of remaining electric power and power generation, as using an auxiliary thruster consumes two to four times more power than not using one. Furthermore, based on our experience, solar radiation is often insufficient from late October onward, limiting the time of year when an observation cruise can be conducted.
However, GNSS-A observation cruises using USVs have advantages over ships in terms of cost-effectiveness and storm tolerance. Other than the WG, there are currently unmanned surface vehicles, such as the Saildrone (Saildrone Inc., U.S.A.) and Bluebottle (Ocius Technology Ltd., Australia) that navigate by wind power and are used for a variety of oceanographic observations. The use of such large vehicles allows for increased power generation and cruising speed. However, the vehicle’s size determines whether it is classified as an unmanned vessel or a drifting object in Japan. If the vehicle is treated as an unmanned vessel, operational costs rise, potentially jeopardizing the system’s cost-effectiveness. Recently, underwater wing-propelled USVs have been developed in Japan, and the possibility of GNSS-A observations using these vehicles is being explored.
We will discuss the ideal vehicle for the GNSS-A observation cruise to monitor the seafloor crustal deformation related to interplate locking and sliding around the Japanese Islands and other subduction zones. Drawing from our experience with the WG, we will also present the crustal deformation field data obtained from its recordings during the meeting.
The WG, developed by Liquid Robotics, Inc. (U.S.A.), is an automatic autonomous USV that generates propulsion by utilizing the difference in magnitude of vertical motion of seawater caused by waves between the sea surface and the seawater. The solar panels and rechargeable batteries on the float on the sea surface power for navigation control, operations of observation equipment, data recording, and satellite communication with land. As no fuel is required for the system, the WG can navigate for a prolonged period and conduct GNSS-A measurements when the solar panels generate sufficient power. Autonomous control is also provided, such as adjusting the angle of the rudder based on the direction of the ocean currents and the direction of travel of the WG, allowing the vehicle to follow a predetermined course and avoid collision with a vessel equipped with an Automatic Identification System.
From spring to summer, we conducted one-to two-month observation cruises covering observation stations off the Pacific coast of the Nemuro Peninsula, Hokkaido district, to off the coast of Ibaraki Prefecture, Kanto district, as well as less than one-month cruises focusing on visiting observation stations off Aomori to Miyagi Prefectures in the Tohoku district from summer to fall. The observation cruises were performed without major accidents that could have resulted in the loss or wreckage of the vehicle, such as contact with other vessels or serious malfunctions of internal equipment, making the GNSS-A measurement impossible. However, there have been instances where the vehicle was unable to follow the planned route due to strong ocean currents or when the observation time had to be shortened due to insufficient power generation from the solar panels. It has become clear that operational limitations exist due to the WG’s performance.
In terms of ocean currents, the WG’s cruising speed (speed through the water) is slower than 2 knots even when the auxiliary thruster is operated at maximum output, and the oceanic current is faster than it is in the central region of the Kuroshio Extension and the Tsugaru Warm Current. As a result, the WG struggles to follow the planned route as set at the expected speed. Although a moving survey at an observation station (i.e., establishing a navigation route as a circle with diameter equal to the station’s water depth) is required for highly accurate positioning, the station’s current oceanic state limits the availability of this operation. Furthermore, the availability of the moving survey is determined by the amount of remaining electric power and power generation, as using an auxiliary thruster consumes two to four times more power than not using one. Furthermore, based on our experience, solar radiation is often insufficient from late October onward, limiting the time of year when an observation cruise can be conducted.
However, GNSS-A observation cruises using USVs have advantages over ships in terms of cost-effectiveness and storm tolerance. Other than the WG, there are currently unmanned surface vehicles, such as the Saildrone (Saildrone Inc., U.S.A.) and Bluebottle (Ocius Technology Ltd., Australia) that navigate by wind power and are used for a variety of oceanographic observations. The use of such large vehicles allows for increased power generation and cruising speed. However, the vehicle’s size determines whether it is classified as an unmanned vessel or a drifting object in Japan. If the vehicle is treated as an unmanned vessel, operational costs rise, potentially jeopardizing the system’s cost-effectiveness. Recently, underwater wing-propelled USVs have been developed in Japan, and the possibility of GNSS-A observations using these vehicles is being explored.
We will discuss the ideal vehicle for the GNSS-A observation cruise to monitor the seafloor crustal deformation related to interplate locking and sliding around the Japanese Islands and other subduction zones. Drawing from our experience with the WG, we will also present the crustal deformation field data obtained from its recordings during the meeting.