A cabled monitoring module for gas seepage: the first experiment in a pockmark (Patras Gulf, Greece)

A new seafloor observatory, the Gas Monitoring Module (GMM), has been developed for continuous and long-term measurements of methane concentration in seawater at the benthic boundary layer, as a result of marine environmental geology and technology synergy. The module is designed to host a series of sensors controlled and managed by a data acquisition and control system able to perform first-level data quality checks. The prototype includes semiconductor methane sensors, an H2S sensor and a CTD for temperature, salinity, pressure recording, as well as interfaces for additional sensors. 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. 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.