Page 285 - Caldera Volcanism Analysis, Modelling and Response

260 J. Martı ´ et al.

Such information can play important roles in the assessment of the physico-
chemical state of a magmatic system. However these tools are inappropriate in
constructing a 3-D image of the subsurface.
Geophysical investigations can currently be regarded as the unique tool for
constructing a regional image of the subsurface at an active caldera. Geophysical
images beneath volcanoes are generally constructed from data obtained by seismic,
gravimetric, magnetic, or electrical investigations. Instrumentation is usually
deployed on the ground surface as well as in boreholes, although both air- and
space-borne potential measurements may become important for the analysis of
volcanic systems in the future (CHAMP and GRACE missions, Thompson et al.,
2004; Wahr et al., 2004). Geophysical imaging is the result of collection of
geophysical data and their evaluation in order to obtain a spatial image of the
substructure. These measurements are performed in static mode and, hence, aim to
resolve the substructure in the space domain. In contrast, dynamic geophysical
investigations aim at identifying changes in the subsurface over time, and informa-
tion on the substructure are hence obtained in the time domain. Recent
investigation have also successfully explored a 4-D approach to geophysical imaging
(Foulger et al., 2003).
In addition to their undisputable value for economic exploration in volcanic
environments, results from (static) geophysical imaging serve three important
scientific aspects in analysing dynamic processes at active caldera volcanoes:
(i) subsurface images help validate results from analogue and mathematical
modelling of processes accompanying caldera reactivation; (ii) subsurface images
play a major role in the interpretation of results from dynamic geophysical
investigations; and (iii) subsurface images are hence critical for the assessment of
hazards associated with caldera unrest and for the mitigation of risks.
Information from geophysical imaging of depth and shape of magmatic
reservoirs, for example, are crucial for the validation of mathematical or analogue
models aiming at investigating the dynamics of post-collapse processes such as
resurgence or active faulting (Acocella et al., 2000; Walter and Troll, 2001; Roche
et al., 2000; Folch and Martı ´, 2004). The identification of magmatic bodies at depth
is also of great importance for the validation of causative bodies inferred from
geodetic and gravimetric dynamic investigations (Battaglia et al., 2003; Beauducel
et al., 2004; Gottsmann et al., 2006b; Gottsmann and Battaglia, 2008). Due to the
non-unique nature of results from inversion of time-series data obtained during
caldera unrest, geophysical images provide critical constraints on the plausibility of
inversion results. In assessing subsurface dynamics at active calderas, geophysical
imaging not only facilitates the interpretation of results from other investigations
but also provides information against which other results can be critically assessed.
As a direct consequence, hazard assessment during periods of unrest at active
calderas is improved via a combination of space- and time-domain investigations
(Gottsmann and Battaglia, 2008).
The aim of this section is not to provide a comprehensive review on individual
imaging techniques and their application. Detailed information can be found in
general geophysical literature: seismic imaging (e.g. Scales, 1994; Sheriff and
Geldart, 1982a, 1982b), gravimetric and magnetic imaging (Blakely, 1996; Wahr,