1569 with coal micro-fracturing activity. Figure 5 shows the geophone arrangement. The sensor orientation was defined such that the vertical direction corresponded to the x-component, the horizontal direction to the ycomponent, and the borehole axial direction to the z-component. The installation location was positioned above the ignition point in the co-axial well. Although the borehole pick is primarily designed as a downhole receiver for PS logging to determine in-situ P-wave and S-wave velocities, it is also applicable for detecting micro-fracturing events and estimating their occurrence frequency and source locations. The instrument is coupled to the borehole wall by means of an inflatable packer expanded with compressed air or pressurized water, allowing for effortless installation and removal. Power supply to the geophone, signal reception, and amplification were performed using a Geode exploration seismograph (Geode NZ SmartSeis ES 3000, Geometrics Inc.). The vibration waveforms obtained by Geode were transmitted to a laptop, enabling realtime monitoring and data logging. In this study, waveform acquisition was conducted with a sampling interval of 250 µs. Product gas during the experiment was sampled every hour and analyzed using a micro gas chromatography (Micro GC 3000A, Inficon Inc.) after removal of moisture and tar components. The analyzed gases are concentrations of oxygen (O2), nitrogen (N2), carbon dioxide (CO2), hydrogen (H2), carbon monoxide (CO), methane (CH4), ethylene (C2H4), ethane (C2H6), propylene (C3H6), and propane (C3H8). Prior to the experiment, instrument calibration was conducted using certified standard gas mixtures containing known concentrations of each component. The micro GC has two analytical columns, employing helium as the carrier gas for one column and argon for the other. Although helium is commonly used as the carrier gas, argon was also adopted in this study to enable quantification of hydrogen. At the end of the experiment, nitrogen was injected into the co-axial well to extinguish and stop reaction. Figure 4 – Injection conditions of oxygen-enriched air Figure 5 – Geophone arrangement 3.2 Ignition of Coal Seams Figure 6 shows the coal ignition system. Coal ignition was performed using a double-structured injection pipe. LP gas was injected through the annular space between the inner and outer pipes, while oxygen-enriched air was supplied through the inner pipe. Ignition of the LP gas was achieved using a nichrome wire installed at the tip of the injection pipe. An ac transformer was used to pass a current of
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