Misae Imano

Progress in the Project for Development of GPS/Acoustic Technique Over the Last 4 Years

GPS/Acoustic (GPS/A) survey is the most promising way to detect crustal deformation in the ocean far from the coast, where a dense onshore GPS network is not available. Monitoring seafloor deformation is crucial to understand the tectonic state in regions of geophysical significance such as subduction zones. We, Tohoku University, together with Nagoya University and Japan Coast Guard have been dedicated to GPS/A survey around the Japanese Islands and developing its instruments for more than a decade. Especially in 2010, a new project for the development of the GPS/A technique commenced, and since 2012 following the Tohoku earthquake, further acceleration of the project has been taken place. Tohoku and Nagoya Universities have been working on this project for 4 years. In the project, Tohoku University worked on several topics, such as realtime/continuous monitoring of crustal deformation using a moored buoy, automatic survey using an Autonomous Surface Vehicle (ASV), which makes the survey as efficient as possible, and constructing a new GPS/acoustic survey network along the Japan Trench and their intensive survey using a chartered ship. In this paper, we summarize the achievements in each of the topics above.

Improvement in the Accuracy of Real-Time GPS/Acoustic Measurements Using a Multi-Purpose Moored Buoy System by Removal of Acoustic Multipath

A component of the multi-purpose moored buoy has been improved for the instantaneous detection of seafloor displacement and possible tsunami generation upon the occurrence of large earthquakes. Here, improvements to the acoustic ranging component of this buoy, which is a key element of on-demand GPS/acoustic (GPS/A) measurements, are demonstrated. A 1-m positioning accuracy is required in GPS/A measurements using the buoy system for the detection of the horizontal seafloor crustal deformation associated with large earthquakes. Owing to the limitation of collecting data on only a limited range of sub-surface depth from a single point located far from the optimal location, resulting from the slack mooring system, obtaining the positioning accuracy is challenging. To overcome this challenge, we developed an automatic travel-time algorithm that reliably excludes the multipath from acoustic waves. Applying the algorithm to the data from 4 months of sea trial, we revealed that the short-period repeatability of the positioning improved from 4 to 0.5 m, while the long-term repeatability improved from 8 to 4 m, which is still beyond the required accuracy of 1 m. Because acoustic ranging under sub-optimal conditions will propagate any error in the data at intermediate steps into the uncertainty of the final positioning, approaches to reduce the errors at each step must be undertaken, such as determination of the pre-defined geometry of seafloor transponders more precisely.