5:15 PM - 6:45 PM
[SSS11-P03] Atmospheric Methane Concentration Anomalies at the Aosawa Fault
Keywords:Shonai-heiya-toen Fault Zone, Aosawa Fault, gas migration, methane anomaly, Cavity Ring-Down Spectroscopy
In the boundary region between the Shonai Plain and the Dewa Hills in Yamagata Prefecture, Japan, several faults including the Aosawa Fault, Sakata Thrust Fault, and Kannonji-Oishi Fault align from east to west. The fault groups (named Shonai-heiya-toen Fault Zone), bordering the western edge of the Dewa Hills, have been extensively studied for their roles in fault movement (advancement), fault activity migration and its impact on landscape formation, their seismic functions, and their connection to the oil accumulation mechanism in the Amarume Oil Field.
This study focuses on the role of faults as pathways for gas migration from the deep subsurface and aims to detect trace gas seepage along these faults. For this purpose, methane concentrations near the surface across these fault lines were measured using Cavity Ring-Down Spectroscopy (CRDS), a type of absorption spectroscopy facilitated by a laser resonator.
CRDS involves introducing laser light into a cavity (measurement container) and observing the decay of light intensity due to multiple reflections between a pair of high-reflectivity mirrors. This process, known as the 'ring-down' curve, occurs immediately after the laser pulses and is used to determine the gas concentration within the cavity. A portable CRDS measurement device (G4301, manufactured by Picarro, USA) was mounted on a vehicle. By introducing outdoor air into the device through a tube and driving at a low speed (about 20 km/h), near-surface methane concentrations were accurately measured. This device is capable of measuring methane concentrations with a precision of 3 ppb and a response time of less than one second. Measurement data were verified in real-time on a tablet connected via Wi-Fi, with the collected data stored on a PC hard disk alongside GPS location information.
Field measurements of methane concentration were conducted along the paths crossing the Sakata Thrust Fault from the Shonai Plain and another path crossing the Aosawa Fault at the western edge of the Dewa Hills. No significant methane anomalies were detected across the Sakata Thrust Fault group. In contrast, notable increases in methane concentration were observed at anticipated fault locations on two transects across the Aosawa Fault, approximately 3 km apart. Atmospheric methane levels peaked at 15 ppm, significantly exceeding the background level (around 2 ppm) and the device's measurement accuracy (3 ppb). The reproducibility of these findings was confirmed through repeated measurements at nearly identical locations. To our knowledge, this is the first report of methane anomalies at the Aosawa Fault, made possible by utilizing CRDS technology.
The measurement results suggest that among the faults in the region, the Aosawa Fault functions as a pathway for gas migration connected to deep underground methane sources, offering new perspectives on fault structures and their relation to the petroleum system in this area.
Future work will aim to expand the measurement area and gather further information on the extent, seepage behavior, and origins of methane through flux measurement and chemical and isotopic analysis of gas samples.
This study focuses on the role of faults as pathways for gas migration from the deep subsurface and aims to detect trace gas seepage along these faults. For this purpose, methane concentrations near the surface across these fault lines were measured using Cavity Ring-Down Spectroscopy (CRDS), a type of absorption spectroscopy facilitated by a laser resonator.
CRDS involves introducing laser light into a cavity (measurement container) and observing the decay of light intensity due to multiple reflections between a pair of high-reflectivity mirrors. This process, known as the 'ring-down' curve, occurs immediately after the laser pulses and is used to determine the gas concentration within the cavity. A portable CRDS measurement device (G4301, manufactured by Picarro, USA) was mounted on a vehicle. By introducing outdoor air into the device through a tube and driving at a low speed (about 20 km/h), near-surface methane concentrations were accurately measured. This device is capable of measuring methane concentrations with a precision of 3 ppb and a response time of less than one second. Measurement data were verified in real-time on a tablet connected via Wi-Fi, with the collected data stored on a PC hard disk alongside GPS location information.
Field measurements of methane concentration were conducted along the paths crossing the Sakata Thrust Fault from the Shonai Plain and another path crossing the Aosawa Fault at the western edge of the Dewa Hills. No significant methane anomalies were detected across the Sakata Thrust Fault group. In contrast, notable increases in methane concentration were observed at anticipated fault locations on two transects across the Aosawa Fault, approximately 3 km apart. Atmospheric methane levels peaked at 15 ppm, significantly exceeding the background level (around 2 ppm) and the device's measurement accuracy (3 ppb). The reproducibility of these findings was confirmed through repeated measurements at nearly identical locations. To our knowledge, this is the first report of methane anomalies at the Aosawa Fault, made possible by utilizing CRDS technology.
The measurement results suggest that among the faults in the region, the Aosawa Fault functions as a pathway for gas migration connected to deep underground methane sources, offering new perspectives on fault structures and their relation to the petroleum system in this area.
Future work will aim to expand the measurement area and gather further information on the extent, seepage behavior, and origins of methane through flux measurement and chemical and isotopic analysis of gas samples.