14:15 〜 14:30
[SVC33-03] 比抵抗構造を利用した火山熱水系の数値モデリング
Numerical simulation is a useful method for studying the magmatic–hydrothermal systems of volcanoes, which generally shows very complex behavior, since the method attempts to directly reproduce hydrothermal convection within the volcano edifice. However, most previous studies on the numerical modeling of such systems use highly simplified structures, and few studies have taken into account the natural heterogeneity of permeability distribution and topographic variation. This is because it is difficult to establish the distribution of permeability in subsurface regions, which have a significant impact on fluid flow within the volcano.
If information on the resistivity structure is available, on the other hand, the uncertainty in constructing the permeability structure can be greatly reduced. Electrical resistivity and permeability are parameters related to the transport characteristics of completely different physical quantities (electric current and fluid, respectively), but both can be said to be similar parameters in that they are closely related to the degree of connectivity of pore fluid. Therefore, hydrothermal flow simulations using the permeability structure directly converted from the resistivity structure are expected to be useful, but no comprehensive scheme has been established for such a conversion.
In this study, as the first step toward establishing a method for reproducing magmatic–hydrothermal processes by integrated analysis of permeability and resistivity, we attempted to reproduce the behavior of the hydrothermal system of Kusatsu-Shirane Volcano (KSV) of central Japan by numerical simulations based on the three-dimensional resistivity structure revealed by magnetotelluric (MT) survey (Matsunaga et al., 2022). In the modeling, three low-permeability zones were assigned to the computational domain based on the interpretation of the resistivity structure: 1) a low-resistivity clay lich layer in the shallow part of southeastern flank, 2) sealing layers associated with mineral precipitation surrounding a deep sub-vertical low-resistivity zone below the summit area, and 3) highly resistive basement rock in the deep part. In addition, highly permeable conduit was created inside the sealing zone, and hydrothermal circulation within the volcanic edifice was simulated by injecting saline water, which represents a magmatic fluid composition from the bottom of the conduit. Then, simulations with structures, property of each rock type and composition of injected fluid modified from reference case were performed to identify the key parameters that characterizing the hydrothermal system of KSV.
Because of several assumptions and simplifications, the simulation results do not reproduce all of the observed data, such as the heat discharge rate. Nevertheless, the results reproduced some important observations. For example, when a sealing zone was assumed around the conduit, the sub-vertical conductive region like in the observed resistivity structure model was well reproduced. Also, the fluid that ascended to near the summit area flowed in the east and west due to the downward flows toward the foot of the volcano caused by the topographical effect and discharged mainly along the valleys. This well reproduces the distribution pattern of hot springs around KSV, where hot springs are concentrated on the east and west flanks of the volcano.
Although the constructed permeability structure was relatively simple, the simulation results closely reproduced some observations, suggesting that the uncertainty in generating permeability structures in hydrothermal fluid flow simulations can be greatly reduced by using resistivity structures. Therefore, if further studies establish a method for converting resistivity structures into permeability structures, then exploration studies of resistivity structures should be able to make an important contribution to the evaluation of volcanic activity.
If information on the resistivity structure is available, on the other hand, the uncertainty in constructing the permeability structure can be greatly reduced. Electrical resistivity and permeability are parameters related to the transport characteristics of completely different physical quantities (electric current and fluid, respectively), but both can be said to be similar parameters in that they are closely related to the degree of connectivity of pore fluid. Therefore, hydrothermal flow simulations using the permeability structure directly converted from the resistivity structure are expected to be useful, but no comprehensive scheme has been established for such a conversion.
In this study, as the first step toward establishing a method for reproducing magmatic–hydrothermal processes by integrated analysis of permeability and resistivity, we attempted to reproduce the behavior of the hydrothermal system of Kusatsu-Shirane Volcano (KSV) of central Japan by numerical simulations based on the three-dimensional resistivity structure revealed by magnetotelluric (MT) survey (Matsunaga et al., 2022). In the modeling, three low-permeability zones were assigned to the computational domain based on the interpretation of the resistivity structure: 1) a low-resistivity clay lich layer in the shallow part of southeastern flank, 2) sealing layers associated with mineral precipitation surrounding a deep sub-vertical low-resistivity zone below the summit area, and 3) highly resistive basement rock in the deep part. In addition, highly permeable conduit was created inside the sealing zone, and hydrothermal circulation within the volcanic edifice was simulated by injecting saline water, which represents a magmatic fluid composition from the bottom of the conduit. Then, simulations with structures, property of each rock type and composition of injected fluid modified from reference case were performed to identify the key parameters that characterizing the hydrothermal system of KSV.
Because of several assumptions and simplifications, the simulation results do not reproduce all of the observed data, such as the heat discharge rate. Nevertheless, the results reproduced some important observations. For example, when a sealing zone was assumed around the conduit, the sub-vertical conductive region like in the observed resistivity structure model was well reproduced. Also, the fluid that ascended to near the summit area flowed in the east and west due to the downward flows toward the foot of the volcano caused by the topographical effect and discharged mainly along the valleys. This well reproduces the distribution pattern of hot springs around KSV, where hot springs are concentrated on the east and west flanks of the volcano.
Although the constructed permeability structure was relatively simple, the simulation results closely reproduced some observations, suggesting that the uncertainty in generating permeability structures in hydrothermal fluid flow simulations can be greatly reduced by using resistivity structures. Therefore, if further studies establish a method for converting resistivity structures into permeability structures, then exploration studies of resistivity structures should be able to make an important contribution to the evaluation of volcanic activity.