Resistivity and seismic structure at southern Hydrate Ridge

Deep methane seep environments are areas of intense methane flow that provide pathways for methane transport through the sediments into the water column and potentially the atmosphere. The presence of methane hydrate, an ice-like clathrate of methane and water, are traditionally inferred from a seismic feature known as the bottom simulating reflector. However this feature alone cannot be used to determine hydrate distribution within the hydrate stability zone. Marine controlled source electromagnetics can be used to locate resistive regions in the sediment column; however it is not yet possible to distinguish free gas from hydrate, both of which are resistive. Either method alone produces an incomplete picture of hydrate distribution. Combining seismic and electromagnetic surveys allows better characterization of hydrate and free gas within the hydrate stability zone. One of the best studied methane seeps is Hydrate Ridge, located on the ocean slope 90km off the Oregon coast. Using a deployed undersea electromagnetic source instrument (DUESI) a new image of the resistive structure of southern Hydrate Ridge was constructed. An array of 5 electromagnetic receivers and 2 source instruments were deployed along an east-west transect across the summit of southern hydrate ridge. These results are compared to seismic velocities obtained through inversion of P-wave travel times recorded at an array of ocean bottom seismometers, which show regions of high free gas content in the sediment. Additionally, the seismic data provide information on the reflectivity of sediments. Both carbonates and hydrates are highly reflective, but hydrates are resistive while carbonates common to this environment are fractured and therefore conductive. Free gas results in high electrical resistivity, high seismic attenuation and low seismic velocity. A pseudosection of resistivities at southern Hydrate Ridge show shallow resistive structures beneath the summit. The sediment column underlying a tall carbonate pinnacle to the west of the summit exhibits shallow conductive structures overlying a deeper resistive formation. These results compare well with borehole logging resistivities taken during ODP leg 204. The tomographic p-wave velocity model shows possible correlation between high resistivity and low velocity, which is consistent with the presence of free gas. Through the use of multiple geophysical techniques, a greater understanding of methane hydrate and free gas distribution in the sediment column of southern Hydrate ridge is possible.