14:00 〜 14:15
[SVC35-02] High-resolution imaging of the Menengai geothermal field using ambient noise surface wave tomography
キーワード：seismic interferometry, ambient noise, surface wave velocity, geothermal, shallow heat source
Abstract
A network of twenty broadband seismometers was deployed for three months, from April 29, 2024, to image the three-dimensional S wave velocity (Vs) structure beneath the Menengai volcano using seismic noise interferometry. We calculated daily cross-correlation functions (CCFs) between station pairs and stacked all the CCFs obtained from the three component recordings over the three months of seismic noise. We then applied the multimode and multicomponent method to estimate the phase velocity dispersion curves between station pairs, extracting 190 phase velocity dispersion curves within the 1.6 – 10 s period band. Subsequently, we estimated the 3D Vs structure by applying the direct surface wave inversion method. Our results revealed three significant S-wave anomalies with depth. The first is a low-velocity anomaly extending from the crustal surface to 0.9 km above sea level. This low-velocity anomaly is attributed to the hydrothermal alteration of the young post-caldera lava flows, resulting in the formation of clay minerals. This region is considered the caprock of the geothermal field. The second anomaly is a high-velocity anomaly occurring between 0.8 km above sea level and 1.2 km below sea level. This zone is characterized by intrusive trachytic material and dikes within the caldera and is considered a high-temperature zone characterized by high-temperature mineral assemblages. Additionally, low-velocity belts within the depth range of 0.8–1.2 km below sea level are possible hydrothermal fluid pathways that facilitate the transfer of brine and volcanic gases to the surface, occasionally manifesting as fumaroles. This depth range can be considered as the region hosting the geothermal reservoir. The third anomaly, between 1.2km - 3.8km below sea level, indicates a low-velocity zone possibly associated with magmatic activity at depth. The presence of magmatic activity at this depth range suggests a potential heat source. Our findings demonstrate the effectiveness of seismic interferometry in imaging small-scale features, less than 5 km in size, within geothermal regions.
A network of twenty broadband seismometers was deployed for three months, from April 29, 2024, to image the three-dimensional S wave velocity (Vs) structure beneath the Menengai volcano using seismic noise interferometry. We calculated daily cross-correlation functions (CCFs) between station pairs and stacked all the CCFs obtained from the three component recordings over the three months of seismic noise. We then applied the multimode and multicomponent method to estimate the phase velocity dispersion curves between station pairs, extracting 190 phase velocity dispersion curves within the 1.6 – 10 s period band. Subsequently, we estimated the 3D Vs structure by applying the direct surface wave inversion method. Our results revealed three significant S-wave anomalies with depth. The first is a low-velocity anomaly extending from the crustal surface to 0.9 km above sea level. This low-velocity anomaly is attributed to the hydrothermal alteration of the young post-caldera lava flows, resulting in the formation of clay minerals. This region is considered the caprock of the geothermal field. The second anomaly is a high-velocity anomaly occurring between 0.8 km above sea level and 1.2 km below sea level. This zone is characterized by intrusive trachytic material and dikes within the caldera and is considered a high-temperature zone characterized by high-temperature mineral assemblages. Additionally, low-velocity belts within the depth range of 0.8–1.2 km below sea level are possible hydrothermal fluid pathways that facilitate the transfer of brine and volcanic gases to the surface, occasionally manifesting as fumaroles. This depth range can be considered as the region hosting the geothermal reservoir. The third anomaly, between 1.2km - 3.8km below sea level, indicates a low-velocity zone possibly associated with magmatic activity at depth. The presence of magmatic activity at this depth range suggests a potential heat source. Our findings demonstrate the effectiveness of seismic interferometry in imaging small-scale features, less than 5 km in size, within geothermal regions.