Precise Distributed Acoustic Sensing measurements by using the seafloor optical fiber cable system offshore Sanriku for earthquake monitoring

Distributed Acoustic Sensing (DAS) measurements which utilize an optical fiber itself as a sensor can be applied for various purposes. An observation of earthquakes using an optical fiber deployed on the seafloor with DAS technology is attractive because DAS measurements enable a dense seismic observation as a long linear array. Spatial resolution of the observation reaches a few meters. The length of the array is determined by the measurement range of the DAS interrogator deployed on the optical fiber, and a fine spatial sensor interval can be configured. DAS measurements have become increasingly accurate and the current state of technology exhibit high signal quality. Therefore, various sophisticated data analyses can be applied to such DAS data. However, data processing needs to consider that DAS measures strain wave fields and, thus is different from conventional seismic measurements using devices based on the pendulum principle.

In 1996, a seafloor seismic tsunami observation system using an optical fiber cable was deployed off the coast of Sanriku by Earthquake Research Institute, the University of Tokyo. The system has six spare (dark) optical fibers which are dispersion shifted single mode type, and have been incorporated for future extension of the observation system. We have started development of a seafloor seismic observation system utilizing DAS technology on the Sanriku cable observation system as a next generation of marine seismic observation system. In June 2019, we performed DAS measurements using a dark fiber from Sanriku seafloor observation system. Before DAS measurements commenced, we confirmed the satisfactory condition of the fiber using OTDR. An interrogator was installed in the cable landing station temporarily, and the length of data collection (array aperture) was set to 5 km. Data were recorded with various values of parameters, such as gauge length, ping rate, acquisition offset, for evaluation of data quality and signal to noise ratios. The total recording period was approximately 3 days. As a result, many earthquakes including micro-earthquakes were recorded. The obtained data will be used to develop data processing techniques for seismic observations utilizing DAS measurements.