4:00 PM - 4:15 PM
[SCG47-09] Simultaneous measurements of electrical resistivity, seismic velocity, and porosity of serpentinites from the Oman ophiolite
Keywords:Serpentinite, the Oman ophiolite, Electrical resistivity, Seismic velocity, Crack properties
Recent geophysical surveys have reported that seismic velocity and electrical resistivity are significantly reduced in the outer-rise region, which is considered to be caused by the serpentinization of the mantle due to seawater permeated from the outer-rise fault (e.g., Grevemeyer et al., 2007). The degree of serpentinization can be roughly estimated from seismic velocities because the velocity of solid phase decreases systematically as serpentinization progresses. However, serpentinization of mantle rock is accompanied by a volume increase which may result in the development of cracks, and seismic velocity is also affected by crack density and geometry, so it is difficult to correctly estimate the degree of serpentinization based on seismic velocity alone. On the other hand, since electrical resistivity is considered to be dominated by fluid pathways formed by interacted cracks compared to seismic velocity, it is possible to estimate the effects of solid phase alteration and crack properties separately by combining seismic velocity and resistivity. Therefore, we conducted simultaneous measurements of resistivity, seismic velocity, and porosity using serpentinite from the Oman ophiolite, and attempted to estimate the degree of serpentinization from the relationship between resistivity and seismic velocity considering the effect of crack.
The samples used in this study were serpentinites collected from mantle sections (BA3A, CM2B) on the Oman drilling project. We developed simultaneous measurement system of seismic velocity, resistivity and porosity under hydrostatic pressure in our laboratory. Seismic velocity, resistivity and porosity were measured under confining pressure up to 200 MPa and pore pressure fixed at 1 MPa using NaCl solution (0.5 mol/L). Electrical resistivity was calculated from the impedance and phase difference between current and voltage that were obtained by two terminal method, and seismic velocities (Vp, Vs) were measured by a pulse transmission method using piezoelectric transducers with a resonant frequency of 2 MHz. Porosity was calculated by initial porosity measured by pycnometer before compaction, and volume change of pore fluid after compaction using a syringe pump. The degree of hydration was estimated from the grain density of samples. Preliminary experimental results show that seismic velocity and resistivity increase systematically with confining pressure in all samples. Application of effective medium theory to seismic wave velocity results suggests that changes in resistivity and Vp/Vs reflect the effect of pore with large aspect ratio. By applying the effective medium theory and percolation model to these experimental results, velocity is affected by the degree of hydrating and crack density, while resistivity is affected by crack porosity and connectivity. Based on these results, it can be said that the degree of serpentinization should be correctly evaluated by combining resistivity and seismic velocity to account for the effects of crack.
The samples used in this study were serpentinites collected from mantle sections (BA3A, CM2B) on the Oman drilling project. We developed simultaneous measurement system of seismic velocity, resistivity and porosity under hydrostatic pressure in our laboratory. Seismic velocity, resistivity and porosity were measured under confining pressure up to 200 MPa and pore pressure fixed at 1 MPa using NaCl solution (0.5 mol/L). Electrical resistivity was calculated from the impedance and phase difference between current and voltage that were obtained by two terminal method, and seismic velocities (Vp, Vs) were measured by a pulse transmission method using piezoelectric transducers with a resonant frequency of 2 MHz. Porosity was calculated by initial porosity measured by pycnometer before compaction, and volume change of pore fluid after compaction using a syringe pump. The degree of hydration was estimated from the grain density of samples. Preliminary experimental results show that seismic velocity and resistivity increase systematically with confining pressure in all samples. Application of effective medium theory to seismic wave velocity results suggests that changes in resistivity and Vp/Vs reflect the effect of pore with large aspect ratio. By applying the effective medium theory and percolation model to these experimental results, velocity is affected by the degree of hydrating and crack density, while resistivity is affected by crack porosity and connectivity. Based on these results, it can be said that the degree of serpentinization should be correctly evaluated by combining resistivity and seismic velocity to account for the effects of crack.