UC San Diego

Gas hydrate, an ice-like solid formed from methane and water, is prevalent in marine slope sediments. Gas hydrate in sediments is a slope stability geohazard, a potential energy source, a carbon reservoir sensitive to warming oceans, and the base of chemosynthetic ecosystems. Critical to assessing these varied roles is identifying the location and extent of these dynamic hydrate deposits. Acoustic methods historically used to identify hydrate deposits are not as sensitive to the hydrate as they are the free gas sometimes associated with hydrate. Gas hydrate is electrically resistive which makes it a good target for controlled-source electromagnetic (CSEM) methods.

Presented in this thesis is a deep-towed CSEM transmitter and receiver array system built for identifying gas hydrate deposits, and the results of two surveys in the California Borderlands to assess and characterize hydrate-bearing sediments first at the Del Mar Seep and then in the Santa Cruz Basin. The Del Mar Seep, located in the Inner California Borderlands, is a natural methane seep, but little is known about the hydrate system underlying the seep. Inversion results showed that while the surface expression of the seep is only 200m in diameter, the hydrate system underlying the seep is imaged as a kilometer-wide resistive body. Using Archie’s equation, the total gas in place at the Del Mar Seep is 2 billion m3. The second survey took place in the Santa Cruz Basin, in the Outer California Borderlands, where indirect seismic indicators of hydrate were found in legacy seismic data. Six lines of CSEM data were collected in the Santa Cruz Basin coincident with seismic data. While the seismic data indicated that most of the hydrate should be in the center of the basin, inversions indicate that resistors are stronger and more prevalent on the flanks, associated with steeped migration pathways such as dipping beds and faults. Two shallow resistive bodies are interpreted as undiscovered methane seeps. These two surveys demonstrate that the deep-towed receiver array is capable of identifying and quantifying methane hydrate deposits.