Numerical models for ground deformation and gravity changes during volcanic unrest: simulating the hydrothermal system dynamics of a restless caldera
Numerical models for ground deformation and gravity changes during volcanic unrest: simulating the hydrothermal system dynamics of a restless caldera
复制标题
DOI：
10.5194/se-7-557-2016
复制
发表时间：
2016-01-01
期刊：
影响因子：
3.4
通讯作者：
Bunney, S.
中科院分区：
文献类型：
--
作者：
Coco, A.;Gottsmann, J.;Bunney, S.
在动荡时期，不稳定的火山口的地面变形和重力变化可能预示着即将爆发，因此必须正确解释以进行危害评估。区分与岩浆源热液从深部上升有关的浅层热液活动引起的岩浆迁移引起的体积和压力变化有关的地球物理观测值变化是至关重要的。本文提出了一个数值模型，通过模拟孔隙压力和热流体（水和二氧化碳）深层注入相关的热膨胀，来评估火山口环境下热液系统的热孔弹性响应。采用多组分多孔介质流体流动多相模拟器TOUGH2对热液流体循环进行了模拟。然后评估孔隙压力和温度的变化，并将其输入到热孔弹性模型（单向耦合）中，该模型基于为无界域轴对称问题设计的有限差分数值方法。根据Campi Flegrei火山口（意大利）的限制条件，一系列模拟评估了流体注入速率和力学特性对热液系统、隆升和重力的影响。与环状断层的存在有关的水文和力学性质的非均质性是流体流动模式的关键决定因素，因此也是地球物理观测结果的关键决定因素。地表计算的隆起和重力变化剖面的峰值（绝对值）位于注入点附近（即模型和断裂带的中心）。地面变形的时间演化表明，相对于孔隙压力的贡献，热效应对总隆升的贡献在动荡的头几年几乎可以忽略不计，但随着时间的推移而增加，并在长时间的模拟后成为主导。在动荡的头几年里，重力变化经过短暂的增加后，变为负值，并单调地向稳态值下降。由于所研究的热液系统的物理性质类似于任何充满流体的储层，例如油田或封存产生的二氧化碳储层，因此该模型的通用公式将使其能够用于监测和解释与对人类活动构成风险的其他地球物理危害相关的变形和重力数据。
Ground deformation and gravity changes in restless calderas during periods of unrest can signal an impending eruption and thus must be correctly interpreted for hazard evaluation. It is critical to differentiate variation of geophysical observables related to volume and pressure changes induced by magma migration from shallow hydrothermal activity associated with hot fluids of magmatic origin rising from depth. In this paper we present a numerical model to evaluate the thermo-poroelastic response of the hydrothermal system in a caldera setting by simulating pore pressure and thermal expansion associated with deep injection of hot fluids (water and carbon dioxide). Hydrothermal fluid circulation is simulated using TOUGH2, a multicomponent multiphase simulator of fluid flows in porous media. Changes in pore pressure and temperature are then evaluated and fed into a thermo-poroelastic model (one-way coupling), which is based on a finite-difference numerical method designed for axi-symmetric problems in unbounded domains.Informed by constraints available for the Campi Flegrei caldera (Italy), a series of simulations assess the influence of fluid injection rates and mechanical properties on the hydrothermal system, uplift and gravity. Heterogeneities in hydrological and mechanical properties associated with the presence of ring faults are a key determinant of the fluid flow pattern and consequently the geophysical observables. Peaks (in absolute value) of uplift and gravity change profiles computed at the ground surface are located close to injection points (namely at the centre of the model and fault areas). Temporal evolution of the ground deformation indicates that the contribution of thermal effects to the total uplift is almost negligible with respect to the pore pressure contribution during the first years of the unrest, but increases in time and becomes dominant after a long period of the simulation. After a transient increase over the first years of unrest, gravity changes become negative and decrease monotonically towards a steady-state value.Since the physics of the investigated hydrothermal system is similar to any fluid-filled reservoir, such as oil fields or CO2 reservoirs produced by sequestration, the generic formulation of the model will allow it to be employed in monitoring and interpretation of deformation and gravity data associated with other geophysical hazards that pose a risk to human activity.