10:45 AM - 12:15 PM
[SCG58-P03] Seismic velocity and electrical resistivity of altered mantle inferred from field mapping of the rodingite veins in serpentinite
Keywords:Seismic velocity, Electrical resistivity, Crack properties, Serpentinite
Mantle serpentinization results in the formation of cracks due to volume expansion, which have a significant effect on seismic velocity and electrical resistivity. There is a large gap of more than several orders of magnitude in scale between the laboratory measurements and geophysical surveys for measuring these physical properties, due to the difference in frequency. Therefore, estimating the degree of serpentinization of the mantle from geophysical observations requires not only laboratory scale results, but also field scale observations of fractures. In this study, we investigated distribution and shape of rodingite veins in serpentinite outcrops and measured seismic velocities and electrical resistivities of serpentinite in laboratory to estimate field scale seismic velocity and electrical resistivity.
We measured density distribution, aspect ratio, strike and dip of rodingite veins, in the serpentinite outcrops along the coastline for approximately 1 km on the Nagasaki metamorphic rocks in Japan. From these measurements, we determined porosity and aspect ratio due to rock veins at field scale. On the other hand, seismic velocity and electrical resistivity of serpentinite collected at this area were measured under hydrostatic pressure. Laboratory measurement have carried under confining pressure up to 200 MPa and pore pressure fixed at 1 MPa using NaCl solution as a pore fluid. Electrical resistivity was estimated from the impedance and phase shift between current and voltage that were obtained by the two terminal method, and seismic velocities (Vp, Vs) were measured by the pulse transmission method using piezoelectric transducers with a resonant frequency of 2 MHz.
The density distribution varied from place to place and was not constant, with an average of 14 veins per 100 m. The aspect ratio in the study area averaged about α = 0.01, but not all of the measured veins had visible ends, so this aspect ratio is considered to be a lower limit. Vein porosity determined from the density and aspect ratio of rodingite veins is 1.3 %. The orientation of the veins was nearly random, because the strikes and dips are not concentrated in certain orientation and vein density is almost similar in orthogonal directions. Whereas experimental results show that seismic velocity (Vp = 6.3 km/s, Vs = 3.6 km/s) increase slightly with confining pressure, but electrical resistivity (ρ = 4.3 × 104 Ω m) remained almost unchanged. The change of seismic velocity is attributed to the closure of thin crack with small aspect ratio, while the electrical resistivity did not change due to the low crack connectivity. Using effective medium theory and percolation model, seismic velocities and electrical resistivities of rocks containing fractures can be expressed in terms of crack porosity and aspect ratio. Based on the spatial distribution and geometry of the veins we measured in field and the matrix values measured in our laboratory, we estimated the field scale seismic velocities and electrical resistivity in the study area (Vp = 5.9 km/s, Vs = 3.4 km/s, ρ = 7 × 103 Ω m).
We measured density distribution, aspect ratio, strike and dip of rodingite veins, in the serpentinite outcrops along the coastline for approximately 1 km on the Nagasaki metamorphic rocks in Japan. From these measurements, we determined porosity and aspect ratio due to rock veins at field scale. On the other hand, seismic velocity and electrical resistivity of serpentinite collected at this area were measured under hydrostatic pressure. Laboratory measurement have carried under confining pressure up to 200 MPa and pore pressure fixed at 1 MPa using NaCl solution as a pore fluid. Electrical resistivity was estimated from the impedance and phase shift between current and voltage that were obtained by the two terminal method, and seismic velocities (Vp, Vs) were measured by the pulse transmission method using piezoelectric transducers with a resonant frequency of 2 MHz.
The density distribution varied from place to place and was not constant, with an average of 14 veins per 100 m. The aspect ratio in the study area averaged about α = 0.01, but not all of the measured veins had visible ends, so this aspect ratio is considered to be a lower limit. Vein porosity determined from the density and aspect ratio of rodingite veins is 1.3 %. The orientation of the veins was nearly random, because the strikes and dips are not concentrated in certain orientation and vein density is almost similar in orthogonal directions. Whereas experimental results show that seismic velocity (Vp = 6.3 km/s, Vs = 3.6 km/s) increase slightly with confining pressure, but electrical resistivity (ρ = 4.3 × 104 Ω m) remained almost unchanged. The change of seismic velocity is attributed to the closure of thin crack with small aspect ratio, while the electrical resistivity did not change due to the low crack connectivity. Using effective medium theory and percolation model, seismic velocities and electrical resistivities of rocks containing fractures can be expressed in terms of crack porosity and aspect ratio. Based on the spatial distribution and geometry of the veins we measured in field and the matrix values measured in our laboratory, we estimated the field scale seismic velocities and electrical resistivity in the study area (Vp = 5.9 km/s, Vs = 3.4 km/s, ρ = 7 × 103 Ω m).