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Monitoring of a methane-seeping pockmark by cabled benthic observatory (Patras Gulf, Greece)
Author(s)
Language
English
Status
Published
JCR Journal
JCR Journal
Peer review journal
Yes
Title of the book
Issue/vol(year)
5 / 26 (2006)
Publisher
Springer-Verlag
Pages (printed)
297-302
Issued date
2006
Keywords
Abstract
A new seafloor observatory, the gas monitoring
module (GMM), has been developed for continuous and
long-term measurements of methane and hydrogen sulphide
concentrations in seawater, integrated with temperature (T),
pressure (P) and conductivity data at the seafloor. GMM
was deployed in April 2004 within an active gas-bearing
pockmark in the Gulf of Patras (Greece), at a water depth of
42 m. Through a submarine cable linked to an onshore
station, it was possible to remotely check, via direct phone
connection, GMM functioning and to receive data in nearreal
time. Recordings were carried out in two consecutive
campaigns over the periods April–July 2004, and September
2004–January 2005, amounting to a combined dataset
of ca. 6.5 months. This represents the first long-term
monitoring ever done on gas leakage from pockmarks by
means of CH4+H2S+T+P sensors. The results show
frequent T and P drops associated with gas peaks, more
than 60 events in 6.5 months, likely due to intermittent,
pulsation-like seepage. Decreases in temperature in the
order of 0.1–1°C (up to 1.7°C) below an ambient T of ca.
17°C (annual average) were associated with short-lived
pulses (10–60 min) of increased CH4+H2S concentrations.
This seepage “pulsation” can either be an active process
driven by pressure build-up in the pockmark sediments, or a
passive fluid release due to hydrostatic pressure drops
induced by bottom currents cascading into the pockmark
depression. Redundancy and comparison of data from
different sensors were fundamental to interpret subtle proxy
signals of temperature and pressure which would not be
understood using only one sensor.
module (GMM), has been developed for continuous and
long-term measurements of methane and hydrogen sulphide
concentrations in seawater, integrated with temperature (T),
pressure (P) and conductivity data at the seafloor. GMM
was deployed in April 2004 within an active gas-bearing
pockmark in the Gulf of Patras (Greece), at a water depth of
42 m. Through a submarine cable linked to an onshore
station, it was possible to remotely check, via direct phone
connection, GMM functioning and to receive data in nearreal
time. Recordings were carried out in two consecutive
campaigns over the periods April–July 2004, and September
2004–January 2005, amounting to a combined dataset
of ca. 6.5 months. This represents the first long-term
monitoring ever done on gas leakage from pockmarks by
means of CH4+H2S+T+P sensors. The results show
frequent T and P drops associated with gas peaks, more
than 60 events in 6.5 months, likely due to intermittent,
pulsation-like seepage. Decreases in temperature in the
order of 0.1–1°C (up to 1.7°C) below an ambient T of ca.
17°C (annual average) were associated with short-lived
pulses (10–60 min) of increased CH4+H2S concentrations.
This seepage “pulsation” can either be an active process
driven by pressure build-up in the pockmark sediments, or a
passive fluid release due to hydrostatic pressure drops
induced by bottom currents cascading into the pockmark
depression. Redundancy and comparison of data from
different sensors were fundamental to interpret subtle proxy
signals of temperature and pressure which would not be
understood using only one sensor.
Type
article
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