Introduction of new Acoustic Transducer to the Seafloor Geodetic Monitoring System

Institute of Industrial Science, University of Tokyo and Hydrographic and Oceanographic Department, Japan Coast Guard have been constructing the geodetic observation network on the seafloor around Japan. The observation network, which consists of sixteen seafloor geodetic reference stations, has been built along the ocean trench regions by the end of 2003. Two more stations are planned to be added to cover whole area of the Nanakai trough and to focus on the area off Miyagi in 2004. The observation results were accumulated as the observations have been routinely repeated. We are on the stage that we can discuss and improve the system and the observation based on the results. Issues and knowledge that we have through the observations should be fed back to the system and further observation for improvement. One of such issues is one inherent in a cylindrical acoustic transducer that is employed both on the ship-board unit and on the seafloor transponder. Wave length ( 15cm ) of the ranging signal that we use is compatible to the dimension of the cylindrical transducer. This implies that possible ranging error is caused depending on the incident angle of the signal to the transducer. We are planning to take measures against this issue by introducing new acoustic transducer which is designed so that same response is secured within some range of the incident angle. Tank tests were conducted to evaluate the response of the newly developed acoustic transducer in advance of introducing it to the observation system. Ranging procedure has repeated by receiving a signal transmitted from the transducer with hydrophone at points keeping constant distance (150.0cm) from the transducer. Evaluation of the ranging response of the current transducer was also performed under the same conditions in order to compare these responses directly. Some corrections to retain continuity in the seafloor geodetic observation could be deduced from these comparisons. Results of the tests show that the new transducer provided 3-4 cm shorter ranges within the signal incident angles of 0 to 50 degrees than by the current transducer. Ranging with the current transducer indicate that there is a possibility of providing about 15 cm longer range in case of around 0 degree than it really is. We are planning to have further examinations on these transducers to understand their characteristic features fully and improve the seafloor geodetic observation system based on these data.