4:45 PM - 5:00 PM
[SVC29-12] Modeling of multidisciplinary observations during an inter-eruptive period by using a hydrothermal simulation with post-processors
Keywords:hydrothermal system, hydrothermal simulation, unrest event
Recently, some volcanic unrest events in inter-eruptive periods have been observed with multidisciplinary monitoring (e.g. Tokachidake volcano, Kuchinoerabu-jima volcano, Kuju volcano). We focus on the simultaneous changes in ground deformation, total magnetic field, and thermal discharge rate, which are considered to be good indicators of subvolcanic hydrothermal activity. The ground deformation and total magnetic field mainly reflect the subsurface changes in pressure and temperature, respectively. Therefore, these observations are important to understand the activity of the hydrothermal system. In this study, we performed the simulation under several conditions to clarify the behavior of the hydrothermal system and the multidisciplinary observations by coupling the hydrothermal simulation with the post-processors which calculate a ground deformation and changes in the magnetic field.
The numerical simulation was carried out with the TOUGH2 geothermal simulator (Pruess et al. 1999). The calculation region was set axisymmetric 2D. A high permeability region (radius: 100 m, permeability: 1.0×10-12 m2) was set near the symmetry axis to represent the fumarolic conduit. The rest of the calculation region represents the host rock and was set the permeability in two ways (1.0×10-14 m2 and 5.0×10-15 m2). For simplicity, the topography was assumed to be flat. In the subsequent simulations, the initial condition was obtained by simulating a constant injection of hydrothermal fluids (H2O, 350 ºC, 1500 ton/day) from the bottom of the fumarolic conduit for 10k years, representing a quasi-steady state. After that, we imposed an elevated hydrothermal fluid input (3000 ton/day) from the bottom and/or a reduced permeability (1.0×10-14 m2) at a particular depth in the middle of the conduit to monitor the subsequent changes.
The ground deformation was calculated by using the partial differential equation module of COMSOL Multiphysics (COMSOL, 2012). The ground deformation corresponding to changes in pressure and temperature was calculated following the extended Hooke’s law (Jaeger et al., 2007), assuming the thermoporoelasticity. For the magnetic field changes, we take the thermomagnetic effect only and calculated the total magnetic field following the uniformly magnetized prism model (Bhattacharyya, 1964), where we assumed a temperature-dependence of the rock magnetization as 0.01 A/m/K. The heat discharge rate from the crater was calculated by summing up the heat discharge rate of the most upper cells of fumarolic conduit in the hydrothermal simulation.
When only an elevated injection rate was applied, the increase in the subsurface pressure and temperature resulted in the combination of inflation, demagnetizing field changes, and a larger heat discharge from the crater. In the case of a depleted permeability in the conduit, inflation, re-magnetizing field changes, and a reduced heat discharge was observed together. This combination was resulted from the simultaneous occurrence of cooling above the choked zone and pressurization below it. Subsequently, the cooled zone in the upper part of the conduit turned into heating and the continued pressurization below the choked point resulted in inflation, demagnetization, and recovery in the heat discharge.
As a next step, we will perform more calculations under various conditions to classify the simultaneous changes in ground deformation, total magnetic field, and thermal activity, in which it is necessary to examine the effects of a caprock and topography. It enables us a direct comparison between hydrothermal simulations and field observations to better understand the behavior of a subvolcanic hydrothermal system during an inter-eruptive period.
This work was supported by MEXT “Integrated Program for Next Generation Volcano Research and Human Resource Development”.
The numerical simulation was carried out with the TOUGH2 geothermal simulator (Pruess et al. 1999). The calculation region was set axisymmetric 2D. A high permeability region (radius: 100 m, permeability: 1.0×10-12 m2) was set near the symmetry axis to represent the fumarolic conduit. The rest of the calculation region represents the host rock and was set the permeability in two ways (1.0×10-14 m2 and 5.0×10-15 m2). For simplicity, the topography was assumed to be flat. In the subsequent simulations, the initial condition was obtained by simulating a constant injection of hydrothermal fluids (H2O, 350 ºC, 1500 ton/day) from the bottom of the fumarolic conduit for 10k years, representing a quasi-steady state. After that, we imposed an elevated hydrothermal fluid input (3000 ton/day) from the bottom and/or a reduced permeability (1.0×10-14 m2) at a particular depth in the middle of the conduit to monitor the subsequent changes.
The ground deformation was calculated by using the partial differential equation module of COMSOL Multiphysics (COMSOL, 2012). The ground deformation corresponding to changes in pressure and temperature was calculated following the extended Hooke’s law (Jaeger et al., 2007), assuming the thermoporoelasticity. For the magnetic field changes, we take the thermomagnetic effect only and calculated the total magnetic field following the uniformly magnetized prism model (Bhattacharyya, 1964), where we assumed a temperature-dependence of the rock magnetization as 0.01 A/m/K. The heat discharge rate from the crater was calculated by summing up the heat discharge rate of the most upper cells of fumarolic conduit in the hydrothermal simulation.
When only an elevated injection rate was applied, the increase in the subsurface pressure and temperature resulted in the combination of inflation, demagnetizing field changes, and a larger heat discharge from the crater. In the case of a depleted permeability in the conduit, inflation, re-magnetizing field changes, and a reduced heat discharge was observed together. This combination was resulted from the simultaneous occurrence of cooling above the choked zone and pressurization below it. Subsequently, the cooled zone in the upper part of the conduit turned into heating and the continued pressurization below the choked point resulted in inflation, demagnetization, and recovery in the heat discharge.
As a next step, we will perform more calculations under various conditions to classify the simultaneous changes in ground deformation, total magnetic field, and thermal activity, in which it is necessary to examine the effects of a caprock and topography. It enables us a direct comparison between hydrothermal simulations and field observations to better understand the behavior of a subvolcanic hydrothermal system during an inter-eruptive period.
This work was supported by MEXT “Integrated Program for Next Generation Volcano Research and Human Resource Development”.