Geophysical Studies of Marine Gas Hydrate in Northern Cascadia

Abstract

This paper reviews the extensive geophysical studies and Ocean Drilling Project (ODP) results that have provided constraints on the occurrence, distribution, and concentration of gas hydrate and underlying free gas beneath the continental slope of the northern Cascadia subduction zone. On this margin there is a large clastic accretionary sedimentary prism, the most common environment for high concentrations of marine gas hydrates. Most information on the gas hydrate has come from a wide range of seismic surveys, including mapping the area of the characteristic bottom-simulating reflector (BSR), determining the depth distributions of hydrate and underlying free gas, and the geological controls of hydrate formation. BSRs are evident beneath about half of the mid-continental slope. Special seismic studies include BSR reflection coefficients, the frequency dependence of the BSR amplitude, BSR amplitude-versus-offset (AVO), and full waveform inversions. Additional information on hydrate concentration is provided by electrical resistivity profiling, and measurements of seafloor compliance. Detailed heat flow surveys have mapped the thermal regime that controls the depth to which gas hydrate is stable. Ocean Drilling Program Leg 146 drilled through the gas hydrate on the mid continental slope and a reference hole in the adjacent deep sea Cascadia basin. Downhole geophysical logs and core analyses have constrained the concentrations of hydrate and free gas at the drill sites, and provided calibration of the regional geophysical data. In the region of the ODP site, the estimated concentration of hydrate from both geophysical and borehole data is 20-30% of the pore space (10-15% of total volume) over 100 m above the BSR, with an underlying 10-20 m layer of less than 1% free gas.