Interpretation of corrected sea floor HVSR data on a gas emitting structure in the Sea of Marmara

Ocean bottom measurements of ambient vibrations at a gas emitting area in the Marmara region are analyzed. The overall stability of average Horizontal to Vertical Spectral Ratios (HVSR) of ambient vibrations values above 0.2 Hz obtained in different sea conditions suggests that the relevant pattern is weakly affected by oceanic disturbances and can be considered informative about the subsoil structure. A procedure based on the removal of the water column effect from sea floor HVSR data is illustrated which allows the application to off-shore data of inversion tools developed for inland measurements. On this basis, sea floor HVSR measurements are used to tentatively constrain the local seismostratigraphical configuration in terms of Vs and V p profiles. On this basis three main seismic impedance contrasts have been identified (respectively around 10, 100 and 500 m below the sea floor) in good correspondence with geological unconformities revealed by seismic reflection data. Moreover, the interpretation of the body wave profiles and, in particular, of the Vs/V p , ratios suggest the presence of unconsolidated material down to a depth of about 500 m below the sea level with an estimated porosity of the order of 30 %. Based on the Biot-Gassmann model, the body wave profile has been used for a preliminary estimate of the degree of gas saturation which reaches 70 % in the depth range 150–500 m of depths below the sea floor. Beyond these figures, results obtained suggest that a methodology base on the interpretation of HVSR data at sea bottom may represent a new important tool for the characterization of the sea bottom subsoil structure in correspondence of gas reservoirs.