Rapid Rayleigh scattered wave frequency analysis in distributed optical fiber sensing for broadband geophysical observation in the seafloor.

Distributed Acoustic Sensing (DAS) has significantly high sensitivity and suitable for observing rapid phenomena, being widely used in many fields including seismological and volcanological observation. Observations in these fields often require very broadband (1/years ~ 0.01Hz) spectrum and dynamic range (< nano strain ~ milli strain), beyond current range of DAS technology. Especially, continuous phase tracking necessitated from phase DAS technology is problematic in observation of slow and small changes (such as crustal deformation) overwrapped with rapid and large change (such as earthquakes). Therefore, we also applied another DOFS technology called TW-COTDR in our geophysical observation in the seafloor. TW-COTDR is a DOFS technology that analyses the same Rayleigh scattered wave as DAS, but analyzes Rayleigh Intensity Pattern (RIP) in optical frequency domain to measure fiber strain change between measurements by their frequency shift. We applied TW-COTDR with an instrument manufactured by Neubrex, Co. Ltd. (NBX-7031) and our off Muroto seafloor fiber optic cable which extends about 86 km offshore reaching to deep seafloor since April 2022. We scanned Rayleigh scatter wave over 30 GHz at 0.002 GHz interval in 20 minutes to obtain seafloor fiber strain data up to ~ 80 km offshore every 1 m interval. This observation provided valuable information regarding small seafloor temperature fluctuations occurring ~ 0.1 degree over day even in large seismic events. The observation at the same time, posed significant limitation with the TW-COTDR technology that field strain change during frequency scan would make it difficult to evaluate frequency shift between each scan. In the seafloor measurement, significant strain event by ocean wave called microseisms, persists all the time. Amplitude of microseisms can be as large as ~1 micro strain and its period ~ a few seconds, obscuring our TW-COTDR measurements.

              To deal with such strain change during frequency scan, we jointly developed with Neubrex a new instrument called Rayleigh Frequency Acoustic and Strain (RFAS) which uses chirped frequency optical laser pulse to evaluate RIP every 2 msec, where field strain change from microseisms would be negligible. We started long-term field test with the new RFAS instrument (NBX-7800) and the same off Muroto seafloor cable from March 23, 2024. Initial field test result with an interval of 0.3 second at 1.4 GHz scan clearly observed microseismic strain changes in 62-67 km offshore comparable to our DAS measurement using a separate fiber of the same seafloor cable, showing effective improvement is achieved by the chirp laser pulse. In comparison with the DAS in the field test, the RFAS measured the strain every 1 m, which is much finer than 20~80 m in DAS. Thus, the RFAS would be especially suitable for targets such as seafloor downhole measurement where greater spatial resolution is necessary.