[SCG56-P05] Pore geometry in diabase collected by the Oman DP inferred from elastic wave velocity
Keywords:Oceanic crust, Ophiolite, Vp/Vs ratio, Aspect ratio
Geophysical survey observed anomalous low Vp/Vs ratio in oceanic upper crust (Spudich and Orcutt 1980). Although, it is previously explained by felsic rock such as trondhjemite is one of interpretation for such low Vp/Vs. Occurrence of trondhjemite is highly localized in ophiolite complexes (Christensen and Smewing 1981). Another possibly to explain is thick pore with large aspect ratio (shearer 1988). In this study, we measured physical properties of 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 elastic wave velocity and porosity under hydrostatic pressure, using Intra-vessel deformation and fluid flow apparatus at Hiroshima University. Elastic wave velocity and porosity were measured under confining pressure up to 200 MPa at fluid-saturated condition. Elastic wave 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 fluid after compaction using syringe pump. We used porous altered diabase with various porosity collected from 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. Sample with low porosity shows an increasing of Vp/Vs with increasing porosity whereas, sample with high porosity shows a decreasing of Vp/Vs with porosity. And then, 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 these samples, pore aspect ratio is more than an order of magnitude higher in the high porosity diabase. This implies that there is possibility that anomalous low Vp/Vs observed in the oceanic upper crust might be explained by the presence of pore with large aspect ratio.
We developed simultaneous measurement system of elastic wave velocity and porosity under hydrostatic pressure, using Intra-vessel deformation and fluid flow apparatus at Hiroshima University. Elastic wave velocity and porosity were measured under confining pressure up to 200 MPa at fluid-saturated condition. Elastic wave 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 fluid after compaction using syringe pump. We used porous altered diabase with various porosity collected from 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. Sample with low porosity shows an increasing of Vp/Vs with increasing porosity whereas, sample with high porosity shows a decreasing of Vp/Vs with porosity. And then, 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 these samples, pore aspect ratio is more than an order of magnitude higher in the high porosity diabase. This implies that there is possibility that anomalous low Vp/Vs observed in the oceanic upper crust might be explained by the presence of pore with large aspect ratio.