Japan Geoscience Union Meeting 2015

Presentation information


Symbol S (Solid Earth Sciences) » S-CG Complex & General

[S-CG57] Structure, evolution and dynamics of mobile belts

Wed. May 27, 2015 2:15 PM - 4:00 PM A06 (APA HOTEL&RESORT TOKYO BAY MAKUHARI)

Convener:*Toru Takeshita(Department of Natural History Sciences, Graduate School of Science, Hokkaido University), Hiroshi Sato(Earthquake Prediction Research Center, Earthquake Research Institute, The University of Tokyo), Koichiro Obana(Research and Development Center for Earthquake and Tsunami, Japan Agency for Marine-Earth Science and Technology), Takuya NISHIMURA(Disaster Prevention Research Institute, Kyoto University), Yukitoshi Fukahata(Disaster Prevention Research Institute, Kyoto University), Aitaro Kato(Graduate School of Environmental Studies, Nagoya University), Jun Muto(Department of Earth Sciences, Tohoku University), Katsushi Sato(Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University), Shuichi Kodaira(Institute for Research on Earth Evolution Japan Agency for Marine-Earth Science and Technology), Takeshi Sagiya(Disaster Mitigation Research Center, Nagoya University), Tatsuya Ishiyama(Earthquake Research Institute, University of Tokyo), Makoto MATSUBARA(National Research Institute for Earth Science and Disaster Prevention), Yasutaka Ikeda(Department of Earth and Planetary Science, Graduate School of Science, University of Tokyo), Chair:Eiji Kurashimo(Earthquake Research Institute, the University of Tokyo)

3:42 PM - 3:45 PM

[SCG57-P07] Measurements of elastic wave velocity and conductivity in a brine-saturated sandstone under confining pressures

*Minako NAMBU1, Tohru WATANABE1 (1.Department of Earth Sciences, University of Toyama)

Keywords:seismic velocity, conductivity, resistivity, sandstone, fluid

Geophysical mapping of fluids is critical for understanding crustal processes. Seismic velocity and electrical resistivity structures have been revealed to study the fluid distribution. However, the fluid distribution has been still poorly constrained. Observed velocity and resistivity should be combined to make a quantitative inference on fluid distribution. The combined interpretation requires a thorough understanding of velocity and resistivity in fluid-saturated rocks. We have studied elastic wave velocities and electrical conductivity in a brine-saturated sandstone under different confining and pore-fluid pressures.
Berea sandstone (OH, USA) was selected as a rock sample for its high porosity (~20%) and permeability (~10-13 m2). It is mainly composed of subangular quartz grains, with small amounts of feldspar grains. Microstructural examinations showed that clay minerals (e.g., kaolinite) and carbonates (e.g., calcite) fill many gaps between grains. The grain size is 100-200 micrometers. Cylindrical samples (D=26 mm, L=30 mm) were saturated with 0.1 M KCl aqueous solution. Measurements have been made using a 200 MPa hydrostatic pressure vessel, in which confining and pore-fluid pressures can be separately controlled. An aqueous pore-fluid is electrically insulated from the metal work by using a plastic devices. Elastic wave velocity was measured with the pulse transmission technique (PZT transducers, f=2 MHz), and electrical conductivity the four-electrode method (f=100 mHz - 100 kHz) to minimize the influence of polarization on electrodes.
Confining and pore-fluid pressures work in opposite ways. Increasing confining pressure closes pores, while increasing pore-fluid pressure opens them. For a given pore-fluid pressure, both compressional and shear velocities increase with increasing confining pressure, while electrical conductivity decreases. When confining pressure is fixed, velocity decreases with increasing pore-fluid pressure while conductivity increases. The closure and opening of pores can explain observed changes of velocity and conductivity. In contrast to a granitic rock, a brine-filled sandstone showed only relatively small changes in conductivity. These contrasting behaviors might reflect the difference in pore geometry between two rock samples.