Abstract:
Marine gas hydrate is seismically mapped since the base of its stability zone marks an acoustic impedance contrast, which generally mimics seafloor topography and is associated with a bright, negative-polarity Bottom Simulating Reflector (BSR). However, limitations of seismic methods include uncertainty in the origin of the BSR, blanking, and lack of clear upper boundary reflections, hindering hydrate assessment. We employ two supplementary geophysical imaging techniques for hydrate evaluation: a deep-towed controlled-source electromagnetic (CSEM) and a seafloor compliance experiment. These methods are sensitive to physical properties of the sedimentary section modified by the presence of gas hydrate, namely the resistivity and the bulk shear modulus depth profile, respectively. CSEM data are gathered by inline receivers towed behind an AC transmitter; high precision timing allows measurement of the EM field propagation time through sediments which is proportional to resistivity increased by the presence of insulating hydrate. Seafloor compliance is the transfer function between pressure induced on the seafloor by surface gravity waves and the associated deformation. It is mostly sensitive to shear modulus anomalies. Hydrates can cement grains together, increasing shear modulus. As part of a 2004 research cruise in northern Cascadia, in the vicinity of ODP Site 889 and proposed new IODP transect, reflection seismic, CSEM and compliance data were collected and thus these three methodologies can be compared. Methane vent sites, identified seismically, coincide with significant resistive anomalies and increases in shear modulus. The size and nature of these anomalies are investigated through 3D modeling.