Development of seafloor thermometry system using optical fiber distributed temperature sensor for study of mid-ocean ridges

Hydrothermal activity at mid-ocean ridges is believed to have frequent temporal fluctuations. Therefore, it is necessary to monitor the temperature change on the seafloor to calculate the total heat flux released from the hydrothermal activity. A thermometry system using an optical fiber distribution sensor is expected to be the most suitable system for this purpose, since it enables us to measure temperature distribution for a long period of time. Because there was no such system designed for seafloor measurement, the authors began to develop a system for seafloor thermometry. The system consists of laser source/receiver, data processor, and a data logger contained in a pressure housing. Power saving and down-sizing of the system are crucial to its design. Basic specifications are as follows. Temperature sensor: Optical fiber cable. Temperature range: 0-100°C. Measuring distance: up to 4 km. Measuring temperature accuracy: ±0.3°C. Spatial resolution: up to 2.5 m. Maximum measuring time: 1 year (for 10 minutes/day). Power supply: Lithium battery. Data logging: Internal Hard disk. Recover system: Acoustic releaser and buoyancy system. Working water depth: up to 3000 m. A preliminary field experiment using the system was successfully carried out in lakes and coastal waters in Japan. These experiments have demonstrated that the system is ideal for such kind of water temperature measurement for a long period of time