There are probes and sampling systems for both bottom stationed monitoring and via ROV and or AUV vehicles on a mission specific basis or even hand held for spot sampling.  What are your study targets? 

I want to determine the gases come out from the bottom of seabed is geothermal activity or what else. The local people call this location as boiling valley and 10km from that location, 1 hot spring was recorded.

I agree with Brian. Recently, I've read a work of the Geological Survey of Canada about a study on undersea gas releases. Charlie Paull and his colleagues used a brand new undersea robot to observe seafloor features and collect gas samples. It was not properly a geothermal research but I think it would be interesting for your future approach to collect gases in a marine environment.

http://www.mbari.org/news/homepage/2012/paull-arctic/paull-arctic.html

At the TU Bergakademie Freiberg in Germany a Scientific Diving Center is doing Excursions to the mediterranean sea. Gas sampling under water (up to 40m) at geothermal active regions close to the island Panare (Italy ) is one of their work.

They developed a device to measure the gas volume and the temperature of fumaroles. They also analyse the gas composition.

You can contact them under: http://tu-freiberg.de/sdc/team ( [email protected] or [email protected] )

Thank you sir

I personally have sampled gas at the sea bottom near Aeolian islands and Pozzuoli bay (Phleraean fields, Italy) for both volcanic surveillance and geothermal exploration. You can use different methods, according with the depth, specific expertise, number of gas to be sampled, chemical analyses (major components, stable isotopes, etc.). I sampled in apnea up to shallow depth (5-10 m) and scuba diving up to 35-40 m. More recently, I also used a robot that I designed for gas sampling at a muddy bottom of a crater lake (38 m). however I found useful connect the sampling bottle to an upside-down funnel by means of a gooseneck siphon for having a goog separation of the sea water from the gas in a bottle having a valve at each side.

Generally my research group use to fill by water the sampling bottles before the immersion to prevent any air contamination or admixture, then the gas coming from the siphon enter the bottle from the upper side, while the water is pushed down and goes out from the bottom side of the bottle.

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