17:15 〜 19:15
[STT42-P06] DAS field data using subsea fiber optic cable offshore Kamaishi
キーワード：分布型光ファイバ振動センサ、二酸化炭素地下貯留
Currently, nine CCS projects are underway in Japan as part of the Ministry of Economy, Trade and Industry's "Advanced CCS Project." As the number of planned CO2 storage sites spreads along Japan's coastline, it will be important to monitor these multiple storage sites, prevent CO2 leakage, and ensure safety.
A method used in oil and gas industry to monitor reservoirs is the time-lapse seismic survey. To carry out this survey, in case of marine areas, it is common to tow a hydrophone array several kilometers long with an acoustic wave transmitter called an air gun. This practice is expensive (billions of yen), and not time-continuous. It may not be possible to respond immediately to a sudden CO2 leak. Further, air gun is not friendly to sea animals. Another practice is Ocean Bottom Seismometer (OBS) in order to obtain data continuously. However, installing tens or hundreds of OBS wide-scale is very costly.
To overcome above-mentioned issues, authors have developed a monitoring system that combines an underwater speaker that emits weak vibrations and a DAS (Distributed Acoustic Sensor) seismometer. With regard to an underwater speaker, authors have reported in the reference paper. As well known, DAS is a measurement technology that uses optical fiber cables, that detects vibration at tens of thousands of points along the optical cables in real time with high spatial resolution.
To evaluate the performance of the developed system, field tests were conducted using subsea optical fiber cable of the Earthquake Research Institute (ERI) of the University of Tokyo installed off the coast of Kamaishi, Iwate. This optical fiber cable is extended for more than 100 km offshore to the east and has multi fiber optic cores. This test continued for 2 weeks.
Meantime, ERI is measuring earthquake by their DAS, made in Europe, being famous for its leading performance. Taking this opportunity, ERI and our group agreed to compare performance between Europe and Japan-made. For simplicity, we evaluated base noise and earthquake (Signal to Noise), at 30km offshore which was silent (near onshore was very noisy because of waves). Major measurement parameters are: gauge length 50 meters, sampling speed 800Hz (because of total cable length), no frequency filter (no high/low cut), sampling time length 30 seconds. Data processing is in accordance with international standard of IEC 61757-3-2 “Fiber optic sensors – Part 3-2: Acoustic sensing and vibration measurement – Distributed sensing”, chapter 6.5 Self-noise. Amplitude (vertical axis) is expressed as pico strain / √Hz.
One of the example data was shown in attached figure. We took 3 base-noises and 2 earthquakes. Base-noises show that both DAS have noise level as approximately 40 pico strain at 1Hz and 10 pico strain at 10Hz. For reference, typical underwater speaker shoots frequency-swept sine waves from 200Hz to 10Hz band (down sweep). Earthquake 1 shows that both DAS could detect 6Hz peaked waves. Earthquake 2 shows that both DAS detected 2Hz peaked waves.
One of the functions we compiled in our DAS software enables us to choose measuring distance section arbitrarily. For example, having total cable length of 100 km, we can allocate 10,000 spatial sampling points to 40 – 50 km section, which means 1 m spatial sampling, and shorten gauge length to increase data density. Shorter gauge length also brings us capability to detect bigger vibrations, that is very critical in earthquake monitoring (cycle skip matter). Furthermore, if the seismic source device moves, we just shift the focused observation section on the optical fiber accordingly.
Using this function at Kamaishi offshore, clear reflected waves could be confirmed up to 700 meters deep and offset distance (distance between speaker and logging point) of about 1500m. From this achievement, it is considered that reflected waves from the depth of the CO2 storage can also be detected.
Reference
T. Tsuji, T. Imam, A. Ahmad, K. Sakamoto, A. Kioka, M. Ueki, T. Ito, Y. Machijima, Y. Ebihara, F. Hutapea (2024): Continuous monitoring system of the geologically stored CO2 at offshore CCS sites - Development of offshore small seismic source and distributed acoustic sensing, Journal of the JIME, 59(5), 42-47.
A method used in oil and gas industry to monitor reservoirs is the time-lapse seismic survey. To carry out this survey, in case of marine areas, it is common to tow a hydrophone array several kilometers long with an acoustic wave transmitter called an air gun. This practice is expensive (billions of yen), and not time-continuous. It may not be possible to respond immediately to a sudden CO2 leak. Further, air gun is not friendly to sea animals. Another practice is Ocean Bottom Seismometer (OBS) in order to obtain data continuously. However, installing tens or hundreds of OBS wide-scale is very costly.
To overcome above-mentioned issues, authors have developed a monitoring system that combines an underwater speaker that emits weak vibrations and a DAS (Distributed Acoustic Sensor) seismometer. With regard to an underwater speaker, authors have reported in the reference paper. As well known, DAS is a measurement technology that uses optical fiber cables, that detects vibration at tens of thousands of points along the optical cables in real time with high spatial resolution.
To evaluate the performance of the developed system, field tests were conducted using subsea optical fiber cable of the Earthquake Research Institute (ERI) of the University of Tokyo installed off the coast of Kamaishi, Iwate. This optical fiber cable is extended for more than 100 km offshore to the east and has multi fiber optic cores. This test continued for 2 weeks.
Meantime, ERI is measuring earthquake by their DAS, made in Europe, being famous for its leading performance. Taking this opportunity, ERI and our group agreed to compare performance between Europe and Japan-made. For simplicity, we evaluated base noise and earthquake (Signal to Noise), at 30km offshore which was silent (near onshore was very noisy because of waves). Major measurement parameters are: gauge length 50 meters, sampling speed 800Hz (because of total cable length), no frequency filter (no high/low cut), sampling time length 30 seconds. Data processing is in accordance with international standard of IEC 61757-3-2 “Fiber optic sensors – Part 3-2: Acoustic sensing and vibration measurement – Distributed sensing”, chapter 6.5 Self-noise. Amplitude (vertical axis) is expressed as pico strain / √Hz.
One of the example data was shown in attached figure. We took 3 base-noises and 2 earthquakes. Base-noises show that both DAS have noise level as approximately 40 pico strain at 1Hz and 10 pico strain at 10Hz. For reference, typical underwater speaker shoots frequency-swept sine waves from 200Hz to 10Hz band (down sweep). Earthquake 1 shows that both DAS could detect 6Hz peaked waves. Earthquake 2 shows that both DAS detected 2Hz peaked waves.
One of the functions we compiled in our DAS software enables us to choose measuring distance section arbitrarily. For example, having total cable length of 100 km, we can allocate 10,000 spatial sampling points to 40 – 50 km section, which means 1 m spatial sampling, and shorten gauge length to increase data density. Shorter gauge length also brings us capability to detect bigger vibrations, that is very critical in earthquake monitoring (cycle skip matter). Furthermore, if the seismic source device moves, we just shift the focused observation section on the optical fiber accordingly.
Using this function at Kamaishi offshore, clear reflected waves could be confirmed up to 700 meters deep and offset distance (distance between speaker and logging point) of about 1500m. From this achievement, it is considered that reflected waves from the depth of the CO2 storage can also be detected.
Reference
T. Tsuji, T. Imam, A. Ahmad, K. Sakamoto, A. Kioka, M. Ueki, T. Ito, Y. Machijima, Y. Ebihara, F. Hutapea (2024): Continuous monitoring system of the geologically stored CO2 at offshore CCS sites - Development of offshore small seismic source and distributed acoustic sensing, Journal of the JIME, 59(5), 42-47.