[1] The interpretation of electromagnetic anomalies associated with volcanic activity requires a good understanding of two rock properties, the electrical conductivity and the streaming potential coupling coefficient. We measured these properties on 22 consolidated tuff samples containing clays and zeolites typically found in hydrothermal systems or in other areas of high water-rock interactions associated with active volcanic areas. These rocks exhibit unusually high surface conductivity and electrical cementation exponent (2.5-3.3). These features are explained by the highly complex texture of these rocks due to hydration/dissolution processes of the volcanic glass and the formation of clay minerals and zeolites as alteration products. At a pore fluid conductivity of 1.0 ± 0.2 S m À1 , the streaming potential coupling coefficient ranges from À3.55 to À10.7 mV MPa À1 . The zeta potential (a key electrochemical property of the pore water/mineral interface) determined from these measurements at T 0 = 20°C and pH $6-7 and corrected for surface conductivity is $À28 ± 8 mV at 0.1 M NaCl. Two clay-free samples exhibit a zeta potential $À16 ± 5 mV, a value associated with zeolites (clinoptilolite). The temperature dependence of the z potential is rather strong with z(T ) = z(T 0 ) [1 + n z (T À T 0 )], where n z % 4.2 Â 10 À2 C À1 in the temperature range 20-60°C. These data are applied to the understanding of large-scale self-potential anomalies located at the ground surface of Miyake-Jima volcano.
Abstract. The geophysical investigation of gas hydrate accumulations within marine sediments and the sediments of permafrost regions suffer from a lack of information on the influence of the hydrate content on the physical sediment properties. The estimation of the hydrate content using downhole electrical measurements based on Archie's law requires the knowledge of the saturation exponent. The saturation exponent is an empirical parameter that includes influences from the internal rock structure such as pore shape, connectivity and constrictivity of the pore network, and the distribution of the conducting phase. Based on different models that account for the different morphological forms of gas hydrates found during gas hydrate sampling in various research wells, the influence of gas hydrate content on the electrical properties of the hydrate bearing sediment was investigated. For all studied forms of hydrate occurrence, disseminated in the pore space, nodular, and layered, the saturation exponent depends on the sediment properties and on saturation itself. The growth of gas hydrate nodules, lenses, and layers is a process that is assumed to result in the displacement and compaction of the surrounding sediment. Because of this change of sediment properties during hydrate generation, the saturation exponent for these forms of hydrate occurrence depends strongly on the relationship between porosity and formation resistivity factor, expressed in the form of Archie's cementation exponent. For the case that hydrate occurs disseminated in the pore space and the assumption that capillary effects are important for hydrate generation, the saturation exponent depends on grain size and grain size sorting. For the parameters chosen for these model calculations, the saturation exponent aries between 0.5 and 4. The use of a constant mean value for the saturation exponent of approximately 2 can result in both underestimation and overestimation of the hydrate content.
[1] An experimental device designed and developed to grow methane hydrate in the pore space of a sediment was successfully used with a glass bead sample. The underlying idea for the experiment is that methane dissolved in water is transported with upward moving fluids from its place of origin at greater depths to formations within the hydrate stability field where the methane is removed from the pore water to form hydrate. This process is simulated in a closed loop flow system where methane charged water from a gas/ water reservoir outside the hydrate stability field is pumped into the sediment sample cell in the stability field for methane hydrate. The fluid depleted of methane, then flows back into the gas/water reservoir to be recharged with methane. When the experiment was terminated due to blockage of flow by hydrate formation, hydrate saturation was about 95%. Citation: Spangenberg, E., J. Kulenkampff, R. Naumann, and J. Erzinger (2005), Pore space hydrate formation in a glass bead sample from methane dissolved in water, Geophys.
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