9:30 AM - 9:45 AM
[SCG41-03] Effect of pore geometry on ultrasonic velocity of epidosites collected from the Oman Drilling Project
Keywords:Epidosite, Pore geometry, Vp/Vs, Aspect ratio
Geophysical survey suggests a large amount of pore water exists in crack at the upper oceanic crust where seismic wave velocity is slow. However, anomalous low Vp/Vs ratio observed in central ridge and young oceanic crust (Spudich and Orcutt 1980; Kim et al., 2019), and such low Vp/Vs structure can’t be explained by crack alone. Theorical model predict that crack with small aspect ratio increase Vp/Vs of rocks. On the other hand, pore with large aspect ratio decrease Vp/Vs (Kuster and Toksoz, 1974). Therefore, these anomalies can be explained by thick pore with large aspect ratio (shearer 1988). In this study, we focused on epidosite suggested to form on the hydrothermal alteration process of oceanic crust (Schiffman et al., 1987). We measured physical properties of epidosite and diabase collected from the Oman ophiolite, and we investigate pore aspect ratio from simultaneous measurement of velocity and porosity.
We developed simultaneous measurement system of ultrasonic velocity and porosity under hydrostatic pressure, using Intra-vessel deformation and fluid flow apparatus at Hiroshima University (Shimamoto et al., 2006). Ultrasonic velocity and porosity were measured under confining pressure up to 200 MPa at water-saturated condition. Pore pressure was fixed at 1 MPa. Ultrasonic velocity was measured by a pulse transmission method using both P- and S-wave 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 water after compaction using syringe pump. After each experiment, the measurement specimens were cored (10 mm in length, 6 mm in diameter) to perform X-ray microtomography in order to obtain 3D images of the pore shape. Microtomography analyses were performed using Xradia 410 Versa at Kochi Core Center. We used epidosite and diabase collected from gabbro-dike transition sections at Hole GT3A on the Oman Drilling project.
The experimental result shows that both P-wave and S-wave velocities increase with decreasing of porosity due to closing of pore. Diabase shows an increasing of Vp/Vs with increasing porosity whereas, sample with epidosite shows a decreasing of Vp/Vs with porosity. The microstructure of the measurement specimen showed that diabase is consisted crack-like pore shape, while the epidosite showed a porous pore shape.
We investigate pore aspect ratio from measured velocities and porosities based on application of effective medium theory by Kuster and Toksoz (1974). As a result, this implies that different aspect ratio in diabase and epidosite, pore aspect ratio is more than an order of magnitude higher in epidosite. This implies that there is possibility that anomalous low Vp/Vs observed in the central ridge and young oceanic crust might be explained by the effect of pore water in epidosite.
We developed simultaneous measurement system of ultrasonic velocity and porosity under hydrostatic pressure, using Intra-vessel deformation and fluid flow apparatus at Hiroshima University (Shimamoto et al., 2006). Ultrasonic velocity and porosity were measured under confining pressure up to 200 MPa at water-saturated condition. Pore pressure was fixed at 1 MPa. Ultrasonic velocity was measured by a pulse transmission method using both P- and S-wave 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 water after compaction using syringe pump. After each experiment, the measurement specimens were cored (10 mm in length, 6 mm in diameter) to perform X-ray microtomography in order to obtain 3D images of the pore shape. Microtomography analyses were performed using Xradia 410 Versa at Kochi Core Center. We used epidosite and diabase collected from gabbro-dike transition sections at Hole GT3A on the Oman Drilling project.
The experimental result shows that both P-wave and S-wave velocities increase with decreasing of porosity due to closing of pore. Diabase shows an increasing of Vp/Vs with increasing porosity whereas, sample with epidosite shows a decreasing of Vp/Vs with porosity. The microstructure of the measurement specimen showed that diabase is consisted crack-like pore shape, while the epidosite showed a porous pore shape.
We investigate pore aspect ratio from measured velocities and porosities based on application of effective medium theory by Kuster and Toksoz (1974). As a result, this implies that different aspect ratio in diabase and epidosite, pore aspect ratio is more than an order of magnitude higher in epidosite. This implies that there is possibility that anomalous low Vp/Vs observed in the central ridge and young oceanic crust might be explained by the effect of pore water in epidosite.