Monitoring of a methane-seeping pockmark by cabled benthic observatory (Patras Gulf, Greece)
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 near-real 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.
KeywordsProxy Signal Monitoring Campaign Submarine Cable Public Switch Telephone Network Methane Sensor
GMM was developed and experimentally tested within the framework of the ASSEM project (Array of Sensors for long-term Seabed Monitoring of geohazards) funded by the European Commission (contract no. EVK3-2001-00038). Thanks are due to Claude Millot and Martin Hovland for helpful comments on the dataset.
- Christodoulou D, Papatheodorou G, Ferentinos G, Masson M (2003) Active seepage in two contrasting pockmark field in Patras and Corinth Gulfs, Greece. In: Woodside JM, Garrison RE, Moore JC, Kvenvolden KA (eds) Proc 7th Int Conf Gas in Marine Sediments. Geo-Mar Lett 23(3/4):194–199Google Scholar
- Ferentinos G, Zacharias I, Kastanos N (1993) Patras Outfall Investigations: water quality, water circulation patterns and modelling effluent dispersion. Technical Report for Patras Public Company for Water and Sewerage, pp 145Google Scholar
- Ferentinos G, Papatheodorou G, Lambrakis N, Christodoulou D, Ravasopoulos G, Panagopoulos G (2004) Oceanographical study for the monitoring of Patras gulf receiving the biologically treated sewage of Patras city, Greece. Technical Report for the Patras Public Company for Water and Sewerage, pp 188Google Scholar
- Greinert J, Klaucke I, Gimpel P (2002) Side-scan sonar investigations and hydroacoustic ‘bubble’ quantification in an area of surface-near gas hydrate occurrences: recent studies from Hydrate Ridge, offshore Oregon. In: Proc Int Conf Oceans 2002 MTS/IEEE, October 2002, vol 2, pp 1143–1147Google Scholar
- Kvenvolden KA, Lorenson TD, Reeburgh W (2001) Attention turns to naturally occurring methane seepage. EOS Trans AGU 82:457Google Scholar
- Lascaratos A, Salusti E, Papageorgaki G (1989) Wind-induced upwellings and currents in the gulfs of Patras, Nafpaktos and Korinthos, Western Greece. Oceanonologica Acta 12(3):159–164Google Scholar
- McGinnis DF, Wuest A, Schubert CJ, Klauser L, Lorke A, Kipfer R (2005) Upward flux of methane in the Black Sea: does it reach the atmosphere? In: Lee JHW, Lam KM (eds) Environmental hydraulics and sustainable water management. Taylor and Francis, London, pp 423–429Google Scholar