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| Titel |
Relations between electrical resistivity, carbon dioxide flux, and self-potential in the shallow hydrothermal system of Solfatara (Phlegrean Fields, Italy). |
| VerfasserIn |
Svetlana Byrdina, Jean Vandemeulebrouck, Carlo Cardellini, Aurelie Legaz, Christian Camerlynck, Giovanni Chiodini, Thomas Lebourg, Jean Letort, Ghislan Motos, Aurore Carrier, Pascale Bascou |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250090943
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| Publikation (Nr.) |
 EGU/EGU2014-5204.pdf |
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| Zusammenfassung |
| In the frame of the Geo-Supersite Med-Suv project, we present the results of an electric
resistivity tomography (ERT) survey, combined with mappings of diffuse carbon dioxide flux,
ground temperature and self-potential (SP) at Solfatara, Phlegrean Fields, Italy.
This ensemble of methods aims to image the hydrothermal system of Solfatara,
understand the geometry of the fluid circulation, and precise the extension of the
hydrothermal plume evidenced by Bruno et al. (2007). Solfatara is the most active crater of
Phlegrean Fields, characterized by an intense carbon dioxide degassing, about 1500
T/day (Chiodini et al, 2005). Its main structures are Bocca Grande fumarole and
several lesser fumaroles aligned along two normal faults, and Fangaia mud pool
where the aquifer reaches the surface. Solfatara appears as a globally conductive
structure, with resistivity in the range 1 - 100 Ohmm. Comparison between spatial
variations of resistivity and gas flux rate indicates that resistivity changes at depth
are related to gas ratio content and the fluid temperature. Broad negative anomaly
of self-potential in the inner part of Solfatara with a minimum in the area of the
Bocca Grande suggests a significant downward flow of condensing liquid water.
Our results delineate several distinct zones: 1) a vegetation-covered area, relatively
undisturbed by a hydrothermal activity and characterized by a high resistivity (up to 100
Ohm-m) of the shallow layer (vadose zone), and low carbon dioxide flux. In this area,
self-potential takes zero or positive values with little spatial variations. 2) In the central
part, below a superficial vadose zone, a resistive layer (20 - 100 Ohm-m), between
30 - 100 m depth, interpreted as a gas-saturated body, is systematically overlain by a
conductive aquifer (1 - 5 Ohm-m). In this area, the self-potential displays a negative
anomaly with an average value of -100 mV and the carbon dioxide flux is > 1000 g
m-2day-1. 3) Close to Bocca Grande fumarole, the self-potential reaches its minimum
(-160 mV), the CO2 flux is extremely high (5000-20 000 g m-2day-1) and the
aquifer is interrupted by a quasi-vertical structure (20 Ohm-m) representing in our
interpretation a channel for a hot mixture of CO2/vapour/condensing water; 4) Around
Fangaia mud-pool, the resistivity model clearly identifies a very conductive plume
(1 - 5 Ohm-m). The self-potential around Fangaia takes an almost constant value of
~-90 mV; 5) The flanks of Solfatara crater are characterized by highest values
of resistivity up to 500 Ohm-m and strong diffuse degassing. We also perform a
3-D resistivity model of Fangaia hydrothermal plume using 23 high-resolution
Wenner-Schlumberger ERT profiles. Assuming that the shallow resistivity variations in
the vicinity of the plume are mainly due to saturation variations, we propose a 2D
axis-symmetric numerical model coupling Richards equation for fluid flow in conditions of
partial saturation with the resistivity calculation as function of saturation only. The
numerical model allows the estimation of the permeability of the shallow layers
below Fangaia as 5 x 10-13 m 2, which belongs to the upper part of the range of
permeabilities given by laboratory studies for pyroclastic rock samples (Vanorio et al,
2002). |
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