(2021/10/08)

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Low-cost, continuous seismic monitoring system to support emission
reduction efforts
New system enables closer monitoring of underground activity in geothermal reservoirs andsequestered carbon dioxide for improved safety and efficiency
Kyushu University researchers demonstrated a new method for continuously monitoring theenvironment just below Earth’s surface using a small seismic source and fiber-optic cables.
Accurate and frequent measurements of changes in the ground enabled by the system can aid
in the safe and efficient implementation of measures to fight climate change such as carbon
storage and geothermal energy generation.
“Storing captured carbon dioxide underground and generating electricity from geothermalenergy are two important technologies for reducing greenhouse gases,” says Takeshi Tsuji, leadauthor of the study published in Scientific Reports.
“However, for further adoption of these technologies, we urgently need better monitoring tools
to mitigate and respond to related risks such as leakage and induced earthquake activity.”
Because of cost constraints, conventional seismic monitoring is typically performed at long timeintervals. This gap between data means that rapid, unexpected changes in carbon dioxide orgeothermal reservoirs can easily be missed.
The new method developed by the research team uses a small seismic source that rotates a
17-kg off-centered weight to create vibrations for probing the ground.
Based on a technique called distributed acoustic sensing, the vibrations are then detected at
hundreds of points along a buried fiber-optic cable—which can be tens of kilometers long—by
measuring changes in light passing through the fibers.
Analyzing the signals, the researchers could identify changes in pore pressure—the pressure offluids within voids in the ground. In systems deployed in the field, such changes could indicate
hazards such as leakage of carbon dioxide or changes in a geothermal reservoir.
“With our newly developed source system, we can very accurately control the timing of thegeneration of continuous waveforms over a wide range of frequencies,” Tsuji explains.
“Since we know exactly when they originated, we can stack multiple signals over time toimprove the signal-to-noise ratio. This allows us to use a small, comparatively weak source tocontinuously monitor a large area—within a radius of 80 kilometers from the source—at arelatively low cost.”
To test the effectiveness of this new system, the researchers conducted field experiments at
two sites: a geothermal field on the island of Kyushu, southwestern Japan, and a marine
formation off Kamaishi in northeastern Japan.
The results of these field experiments showed that the new system could identify changes inthe signals of the monitored subsurface formations with greater than 99.99% accuracy. The
detected signal changes represented changes in pore pressure associated with geothermal
operations and rainfall.
“Our study shows that, by combining our small seismic source with distributed acoustic sensing
technology, multiple reservoirs can be monitored over an extensive area at the fraction of thecost of conventional systems,” says Tsuji.
“By giving us the means to detect and respond more quickly to leaks and other hazards, thecontinuous monitoring our system enables will also help in gaining the public acceptance thatis vital for advancing carbon capture and storage and geothermal energy development
projects.”###For more information about this research, see “Continuous monitoring system for safe
managements of CO2 storage and geothermal reservoirs,” Takeshi Tsuji, Tatsunori Ikeda,
Ryosuke Matsuura, Kota Mukumoto, Fernando Lawrens Hutapea, Tsunehisa Kimura, Koshun
Yamaoka, and Masanao Shinohara, Scientific Reports (2021). https://doi.org/10.1038/s41598021-97881-5

This schematic image shows what a next-generation system for continuous monitoring of undergroundgeothermal and sequestered carbon dioxide reservoirs could look like using a low-cost approach developedby researchers at Kyushu University. Through distributed acoustic sensing (DAS), fiber-optic cables detectsignals from small seismic sources. The system could also be combined with traditional seismometers andautonomous surface vehicles (ASVs) that produce acoustic waves offshore to further expand themonitored region. Credit: Takeshi Tsuji et al., Scientific Reports 11, 19120 (2021). CC BY

[Contact]Takeshi Tsuji, Professor
Department of Earth Resources Engineering, Faculty of Engineering
International Institute for Carbon-Neutral Energy Research
Tel: +81-92-802-3316 Fax: +81-92-802-3316
E-mail: tsuji@mine.kyushu-u.ac.jp