The influence of dry and water saturated cracks on seismic velocities of crustal rocks - A comparison of experimental data with theoretical model |
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| Authors: | T Popp H Kern |
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| Institution: | (1) Mineralogisch-Petrographisches Institut, Olshausenstra e 40, 24098 Kiel, Germany |
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| Abstract: | The pressure dependence of P- and S-wave velocities, velocity anisotropy, shear wave splitting and crack-porosity has been investigated in a number of samples from different crustal rock types for dry and wet (water saturated) conditions. At atmospheric pressure, P-wave velocities of the saturated, low-porosity rocks (< 1%) are significantly higher than in dry rocks, whereas the differences for S-wave velocities are less pronounced. The effect of intercrystalline fluids on seismic properties at increased pressure conditions is particularly reflected by the variation of the Poisson's ratio because P-wave velocities are more sensitive to fluids than S-wave velocities in the low-porosity rocks. Based on the experimental data, the respective crack-density parameter ( ), which is a measure of the number of flat cracks per volume unit contained within the background medium (crack-free matrix), has been calculated for dry and saturated conditions. There is a good correlation between the calculated crack-densities and crack-porosities derived from the experimentally determined volumetric strain curves. The shear wave velocity data, along with the shear wave polarisation referred to a orthogonal reference system, have been used to derive the spatial orientation of effective oriented cracks within a foliated biotite gneiss. The experimental data are in reasonable agreement with the self consistent model of O'Connell and Budiansky (1974). Taking the various lithologies into account, it is clear from the present study, that combined seismic measurements ofV
p
andV
s
, using theV
p
V
s
-ratio, may give evidence for fluids on grain boundaries and, in addition, may provide an estimate on the in-situ crack-densities. |
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| Keywords: | Crustal rocks seismic properties pore fluid saturation crack-porosity aspect ratio poisson's ratio modelling of the elastic behaviour |
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