BRGM has been involved for many years in the study of the Bouillante geothermal
system (Guadeloupe, West Indies). Bouillante has been used as a natural laboratory to
test, validate and improve geophysical and geochemical methodologies aimed at
improving our understanding of both the structure and dynamic behaviour of a
hydrothermal system under controlled depressurisation (there is no re-injection of
cold fluid). Marine seismic and terrestrial geophysical surveys performed by the BRGM
have improved our knowledge of the 3D structure of this hydrothermal system.

Géothermie Bouillante has been exploiting the Bouillante hydrothermal system since
1988 for local power supply. In October 2004, the extraction rate of the hydrothermal
fluids was increased up to 15 MWe to reach 10% of the electrical power supply needs
of Guadeloupe. To monitor possible seismicity induced by stress changes, BRGM started
the operation of a network of 5 Güralp CMG-40T broadband seismometers (0.02 - 60 s)
in August 2004. Data recorded with this network reveal various types of signals, none
of which show a clear relationship to the geothermal field. The data mainly consist
of local seismicity and regional tectonic earthquakes, including aftershocks of the
Mw 6.3 earthquake of 21 November 2004 near the Island of Les Saintes.

Interestingly, however, the records obtained at the northernmost station in the
Bouillante network reveal the existence of events of small amplitudes with an average
repeat time of ~30s, whose spectra are characterized by dominant frequencies in the
range 0.5 - 10 Hz. Cross-correlation analyses performed on a record of several months
duration show that these events are highly repeatable in time, which is suggestive of
a stable source location and non-destructive source mechanism. Most events display
resonant signatures reminiscent of long-period events seen in other hydrothermal
systems. To better characterize the origin of the events seen at Bouillante, we
carried out a survey in a 1 x 1 km area in the immediate vicinity of the northernmost
station of the permanent network. During this survey, 20-min-long records were
obtained with a single broadband seismometer temporarily deployed at 19 sites within
the selected area. Each period of observation common to the permanent and temporary
stations is marked by the presence of events that are clearly recorded at the
permanent station, but which are not always recorded by the temporary station. Based
on the highly repeatable character and frequency of occurrence of the events, we
infer their source location by using event arrival times measured at individual
stations, combined with polarity analyses and temporal and spectral amplitude
measurements. We discuss possible mechanisms at the origin of the events and their
implications in light of the known tectonic features and available geochemical data.
We also compare these data with similar events observed at Yellowstone, U.S.A.

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