| Abstract |
Conduit drilling (Well USDP-4) was completed in 2004 in the Unzen Scientific Drilling Project (USDP). Some conduit materials of the 1990-95 eruption have been encountered at the bottom of Well USDP-4 (-150 m sea level), and a bottom temperature of 180° C was estimated using the logging data, although the temperature of about 700 oC had been inferred before the drilling. Accordingly, conduit cooling from the initial temperature (850 oC) to the observed temperature (180 oC) was evaluated by numerical simulation. The numerical model of our previous study was used for the initial model in this study, and revised by using the new information obtained from the conduit drilling. Drilling indicates that the N-S width of the conduit of the latest eruption is 20 to 30 m, and it occupies a zone of about 300 m, which includes the conduits of the past eruptions. However, the conduit width in the initial model was 100-300 m, which had been estimated by the result of seismic survey data. Therefore, the revised model has a conduit of 25 m width in the conduit zone of 300 m width in N-S direction. Two cases were calculated for the conduit width in E-W direction (100 and 300 m). In the simulation, the cooling process of the conduit, which has the initial temperature of 850 oC, from 1995 (the end of the eruption) to 2004 (completion of the conduit drilling) was calculated, and the temperatures of the conduit and the surroundings at -150 m sea level were monitored. To begin with, the permeability of the mountain body was fixed at 1 mdarcy and that of the conduit zone was set as a parameter and the values of 1, 10, 30 and 50 mdarcys were assigned. However, the temperature proved insensitive to the permeability of the conduit zone and was higher than the observed data. Therefore, the permeability of the mountain body was also treated as the parameter, and the same value with the conduit zone was assigned. As a result, the temperature of 180 oC was obtained in the conduit of the monitoring area under the permeability conditions of 10 mdarcys and 30 mdarcys in the case of the conduit width of 100 m in E-W direction. |