Physics of Earth

#Zharikov A.V., Lebedev E.B., Dorfman A.M., Shmonov V.M., Sokolova N.T. Experimental study of elastic wave velocities, porosity and permeability in water saturated and dry rocks under high pressure and temperature.

key words [elastic waves porosity permeability] IGEM, GEOHKI, IEM RAS, zhar@igem.msk.ru

Compressive wave velocities were measured in the experiments on sandstone, amphibolite and basalt under water or gas (dry conditions) pressure of 300 MPa and temperature up to 850oC. After the experiments porosity and permeability of the samples were determined. The data obtained under water pressure and gas pressure differs strongly.

Under water pressure in amphibolites a deep minima of Vp with its value decrease by ~ 1,5 km/s, comparing with the initial conditions, occurs at temperature about 650oC.

Under water pressure in basalts a Vp increase (by about 0.5 km/s), which is caused by ceolite formation in intergranular space of rock, occurs at temperatures from 200 to 300oC. In dry conditions such increase does not occur. In the presence of alkaline and neutral solutions the velocity increase in basalt is higher than in the presence of acid ones. At temperature higher than 350oC elastic wave velocities in basalts decrease strongly due to ceolite dehydration.

Under pressure of 300 MPa and temperatures from 20 to 850oC the Vp variations in sandstone are dependent on fluid composition: neutral, alkaline and acid. Fluid composition in the range: NaOH-Na2CO3-NaHCO3-H2CO3 regularly effects the elastic properties of sandstone as a result of mineral reactions and changes in the rock microstructure.

After the experiments porosity, permeability and pore sizes distribution were measured on quenched samples. In most cases rock porosity and permeability changes and variations of elastic wave velocities are synchronous.

Porosity changes in sedimentary rock samples comprise 12-18%, in methamorphic and magmatic rock samples - 1-3%.

The analysis of temperature trends of elastic properties of fluid saturated rocks shows that they are dependent on physical properties of rock matrix and fluid, and on changes in rock structure caused by fluid driving geochemical reactions at high temperature and pressure. Mineral reactions occurring in the rocks in fluid presence lead to dissolution under pressure and new mineral formations. These minerals seal rock pores and microcracks. Changes in microstructure lead to porosity and permeability variations, and as a result rock elastic properties change as well.

Temperature trends of elastic properties of rocks saturated by water-alkaline-acid fluids can be used as geochemical interpretation of seismic discontinuities at the middle depths of the Earth crust.


# This work is supported by the Russian Foundation for Basic Research (project N 98-05-64768).


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Pilyugin N.N. Investigation of non-equilibrium processes in the trace left behind the chondrite model flying along a ballistic route.

Institute of Mechanics of the Moscow State University, 119899 Moscow, Michurin pr., 1.

The electron concentrations and the frequencies of the electron-atom collisions have been measured in the trace after the spheric model produced from the Tsarev chondrite flying in a mixture of air and xenon, the Mach number being M=12.04 and the total pressure in the ballistic route p=36 Torr. The analogous measurements have been given, for comparison, for non-destructive models and models with destruction of aluminium. It has been shown that for the models of chondrite and Al the far trace exhibits the occurrence of condenced SiO2 and Al2O3 particles. The electric potential of SiO2 microparticles is found to be 0.183 V. The effective sticking coefficient of electrons to SiO2 microparticles is found to be 384s-1torr-1, and for Al2O3 576 s-1torr-1 that the several times as great as the sticking coefficient in pure air.

The kinetic model of nonequilibrium reactions in the trace after the chondrite has been constructed. It has been shown that the presence of Si+ ions arising due to the body mass escape is determinant in the electron concentration decrease in the trace as compared with air ions. It has been found that the effective constant of the triple recombination is governed by the processes allowing for the excitation of the atomic states, and its value is

cm6/c,

where T is the gas temperature (in deg). This keff value agrees well with the data for analogous processes with argon and aluminium ions.

The author expresses his gratitude to O.I. Yakovlev for the chondrite sample kindly given to him for the investigations and for the useful discussions.


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