During the October cruise of KS16-16 ⁽¹⁾ a research team with members from the Kobe University Graduate School of Science, the University of Tokyo Earthquake Research Institute and the Japan Agency for Marine-Earth Science and Technology (JAMSTEC) ⁽²⁾ tested a newly-developed island volcano monitoring system in the seas around Nishinoshima, where eruptions have been continuing since November 2013.

The monitoring system uses a wave glider that can operate autonomously relying solely on wave power. The glider is equipped with cameras for visual observation of the volcano, a GPS wave gauge that can detect tsunami caused by volcanic collapse, and a gauge that checks for earthquakes and air vibrations by measuring sonic waves in the air and water. During the system’s test run around the island researchers were able to confirm that these features were functioning correctly.

In order to monitor in real time, they continuously transmitted data from the wave gauge and earthquake/air tremor gauge to a server on the mainland 1000km from Nishinoshima using satellite transmissions.

Based on this test run, the development stages of the island volcano monitoring system are almost complete, and the group plans to start preparing the system for practical use in monitoring Japan’s numerous island volcanoes.

Research background

Nishinoshima

Nishinoshima, part of the Ogasawara island chain, has been experiencing active volcanic eruptions since November 2013. The new island formed by the lava flow has continued to grow, expanding to 2km in diameter as of August 2015. After late November 2015 the eruptions that produce ash deposits and lava flows stopped, and further eruptions that will affect large areas of the island are unlikely to occur.

These volcanic activities on Nishinoshima are a valuable opportunity to understand the processes behind submarine volcanic eruptions and the growth of new volcanic islands. Until recently, observation of the volcano was mainly carried out by artificial satellites and a monthly aircraft visit by the Japan Coast Guard.

The island volcano monitoring system

The research team started to develop a system to improve continuous monitoring of volcanic activity on far-flung islands. Japan has many volcanic islands, and the team aims to use this system to contribute to disaster preparedness and prevention.

Using the wave-powered glider as a platform, the system can move unmanned without refueling, ideal for observing faraway islands. The plan is to monitor volcanic activity by transmitting data to the mainland in real time using satellite communications. Using this system, they will create a framework that constantly monitors the activity of island volcanoes, starting with Nishinoshima.

Development stages

To achieve these goals, in February 2015 they participated in the KR15-03 cruise ⁽³⁾. At the time Nishinoshima was still very active, so from on board ship 7km from the crater they took video footage, air vibration measurements, volcanic activity measurements using hydrophones, and carried out a transmission test using the satellite communications system ⁽⁴⁾.

The group stayed in the ocean area near Nishinoshima for about two days, carrying out geomorphic investigations, sonic observations using air vibration gauges and hydrophones, visual observations and video. This enabled them to analyze the continuous eruptions. They also investigated the possibility of using Thuraya satellite phones for transmitting data in real time.

Based on the above tests, they developed sensor equipment for the island volcano monitoring system and attached it to a wave glider. The KS-16-16 cruise marks the first time this system was used at Nishinoshima.

The KR15-03 cruise in February 2015 also provided an opportunity for preparatory observations in order to set up the Vector Tsunameter real time observation system using a wave glider currently being developed by JAMSTEC. For this purpose, during the KR15-03 cruise, they set up an ocean floor electromagnetic observation device approximately 12km from Nishinoshima at a depth of 2200m.

Research contents and results

On October 20 during the KS16-16 cruise, the team launched the island volcano monitoring system on a trajectory that circles Nishinoshima. The system autonomously sailed around Nishinoshima in a circular path with a 5km radius, and they successfully recalled it on the following day.

Details of the system

The wave glider used for this system consists of a float above water connected by a 5.8m umbilical cable to an underwater glider that propels the wave glider. As well as being equipped with the necessary controllers for the wave glider, the float also carries several cameras, a device to detect earthquakes and air vibrations, a GPS tsunami meter, and two sets of transmission devices including a satellite communications terminal to transmit the observation data via satellite.

The device is also equipped with 2 microphones at the front and back to detect air vibrations, and a hydrophone is attached to the underwater glider, detecting sonic waves at a depth of 6m. Four time-lapse cameras are attached to the wave glider at 90 degree intervals to enable continuous visual observation of Nishinoshima over long periods of time and capture Nishinoshima by taking images across 360 degrees. The up-and-down motion of the wave glider can be adjusted for to within 5cm using the Doppler shift of the GPS carrier wave.

Results

Using the above devices, the wave glider was able to continuously record the predetermined data while it circled Nishinoshima. The team confirmed that their sonic observation was accurate enough to detect earthquakes and air vibrations that may occur on Nishinoshima. The observation data from the air vibration gauge and hydrophone and the wave data from the GPS wave gauge was successfully transmitted in real time to a mainland server via satellite transmissions using Thuraya satellites.

Future developments

Based on the test run at Nishinoshima of the island volcano monitoring system and the collection of data, the development stages of this system are almost complete, but in order to carry out practical operations in the future, various points for improvement were clarified. Namely, the constraints on the speed and safety of satellite transmissions for sending data in real time, and limits on the amount of electricity that can be used by this system.

Due to the restraints on satellite transmissions, visual data could not be sent to the mainland this time. The team is aiming to enable the transmission of images by improving the speed and safety of transmissions and compressing the visual data.

Electricity consumption has also presented some issues. In order to accurately predict the size of a tsunami, the wave gauge attached to this system must carry out underwater acoustic communication with the Vector Tsunameter. Underwater acoustic communication with the vector tsunami gauge is already being carried out in the sea near the Tohoku region of Japan ^{(see 5)}. However, this time because of the restrictions on electricity consumption, it was difficult to equip the volcanic activity monitoring system with acoustic transmission equipment in addition to its other functions. From next year, the team plans to improve the wave glider and carry out real time tsunami observation using this system in the sea around Nishinoshima. Through these improvements, the team will prepare the island volcano monitoring system for practical use in disaster preparedness and prevention.

Technical terms

Vector TsunaMeter (VTM)

A device that combines a deep pressure gauge (DPG) that can measure seismic movements and movements in the earth’s crust that accompany earthquakes, and changes in water level that accompany tsunami as changes in pressure, and an ocean bottom electromagnetometer (OBEM) that detects changes in the electromagnetic field induced by seawater flow caused by tsunami propagation.

The Vector TsunaMeter can separate and observe changes in water level that accompany tsunami propagation, seawater flow, the speed of tsunami propagation, the direction of propagation, and movements in the earth’s crust that accompany earthquakes. This enables researchers to grasp the details of the tsunami occurrence process within the seismic center, and tsunami propagation within complex geological formations. It could contribute to more reliable tsunami predictions along the coast.

Data

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