Page 425 - Caldera Volcanism Analysis, Modelling and Response

400 Micol Todesco

descriptions, including heterogeneous, anisotropic or time-dependent rock
properties. Different strategies have been used to describe flow through fractures:
considering a single, fracture-dominated continuum; accounting for discrete
fracture networks; defining an equivalent (fracture and matrix) continuum; or
through a dual porosity approach in which fracture and matrix are considered
separately (Evans et al., 2001). More recent advances involve coupling of non-
isothermal fluid flow and chemical reactions (Xu and Pruess, 2001; Xu et al., 2001;
Kiryukhin et al., 2004) or hydrothermal circulation and deformation of the porous
medium (Rutqvist et al., 2002). These modelling techniques are now widely
applied to a variety of problems that involve underground flows ranging from site
testing for nuclear waste storage to mining engineering, environmental restoration,
vadose zone hydrology and more recently for investigating carbon dioxide
sequestration (O’Sullivan et al., 2001).
Despite such development, applications to volcanological problems are not
common. Volcanological applications face the complexity of volcanic settings and
involve extreme and highly transient physical conditions, as well as the lack of
appropriate modelling-oriented data sets required to constrain subsurface properties
and conditions. Surface measurements of geochemical and geophysical parameters
are usually carried out as a part of surveillance programs, but do not necessarily
involve the definition of hydraulic properties of subsurface rocks. Although these
surface data can be integrated with subsurface data from few sparse deep drill holes,
large uncertainties commonly remain in the definition of the conceptual model. In
spite of these difficulties, numerical modelling has been performed to study
hydrothermal fluid circulation in volcanic areas (Ingebritsen and Sorey, 1985, 1988;
Bonafede, 1991; Ingebritsen and Rojstaczer, 1993, 1996; Todesco, 1995, 1997;
Gaeta et al., 1998, 2003; Kissling, 1999; Hurwitz et al., 2002, 2003; Chiodini et al.,
2003; Todesco et al., 2003a, b, 2004; Reid, 2004; Todesco and Berrino, 2005;
Villemant et al., 2005). Early studies mostly focused on the description of the
natural state and were aimed at the study of some theoretical aspects of fluid flow
in volcanic regions. When long-term, high-quality data sets are available it is
possible to set up reliable conceptual models for the evolution of the entire volcanic
system. This in turn allows implementation of more sophisticated numerical
models designed to elucidate site-specific features and details of case histories.
When sophisticated modelling tools and high-quality data are both available, the
ideal condition of being able to compare and constrain modelling results with
observations becomes possible.

4. Hydrothermal Systems and Volcano Monitoring

Hydrothermal fluid circulation plays a significant role during both eruptive
and non-eruptive unrest events. Volcanic monitoring around the world commonly
records changes in geochemical and geophysical parameters, many of which can be
directly or indirectly related to the activity of hydrothermal fluids. In some cases these
changes result from the evolution of the magmatic system at depth. In other cases the
observed variations only reflect the natural evolution of the hydrothermal system or