Imaging Gas Hydrate with Marine EM at Hydrate Ridge, Offshore Oregon

We present results from a marine controlled source electromagnetic experiment (CSEM) conducted at Hydrate Ridge, Oregon in August, 2004. Transmitted EM fields were measured by a dense array of 25 seafloor EM receivers spaced at 600 m intervals, allowing for redundant sampling of the spatially varying EM fields. The data heterogeneity is represented in simple form by using pseudosection approach, which allows for a quick approximation of the subsurface lateral resistivity variations. The pseudosections reveal lateral resistivity contrasts that are consistent with other geophysical data collected at Hydrate Ridge and indicate several regions likely to contain significant amounts of gas hydrates. We validate the pseudosection approach with a rigorous 2.5D CSEM forward model study. We augment our data driven experiment with forward model studies that suggests that typical hydrate bearing formations give the largest signal at high frequencies (>10 Hz) and short ranges (< 500 m). We explore the electrical response of hydrate models due to changes in the bottom layer thickness, apparent resistivity and the depth to the top of the hydrate layer in frequency-range space. An important factor to consider for hydrate detection is the trade off with the larger hydrate anomaly produced at high frequencies and the larger window of detectable ranges at lower frequencies.