A methodology for detecting free gas in marine sediments

The existence of occluded or continuous-phase pore gas at shallow depths beneath the seabed can lead to acoustic turbidity or blanking, resulting in poor definition of seismo-stratigraphic features. Shallow gas can dramatically affect the engineering behaviour of marine sediment by increasing the bulk compressibility and the rate of settlement, can alter the undrained shear strength mobilized at failure, can reduce permeability, and can delay the pore pressure response to cyclic loading, giving rise to residual pore pressures. A method is presented that overcomes previous uncertainties in seabed gas detection, facilitating calculation of the in situ degree of saturation. Gaseous and dissolved molar concentrations are evaluated under atmospheric conditions using gas chromatography and are corrected for in situ pressure and temperature and compared to dissolved limit (gas solubility) curves. The volumes of gases existing at concentrations in excess of their dissolved limits are determined to evaluate the overall in situ degree of saturation.