Annual Report 2000 - WIT

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Àzy
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The eigenvectors of this matrix yield the orientations of the diffusivity tensor. The
magnitudes of the tensor are obtained by equating the irregular body to an ideal ellipsoid.
The result relates the ì/%•Æì/o)•ÆìÔ” eigenvalues of the irregular body and the the
ê1•h&¨• lengths of the half-axes of the ellipsoid:
Ò (5)
ì/l‘
ê Ò • ì/o­‘
& Ò • ìÔ” ‘
Further transformations lead to solutions of the »P×;× expressed by the known eigenvalues
ì/•Æì/o0•ÆìÔ” :
Í(ì/
(6)
‘ê Ò ‘
Ç7Ç
ö “(Nq£ Ò Ì¨Î Õ ì/¦ì/oìÔ”
Í(ì/o
(7)
‘r& Ò ‘
Ò7Ò
ö “(Nq£ Ò Ì¨Î Õ ì/¦ì/oìÔ”
Í(ìÔ”
È (8)
Õ7Õ
Ò ‘
The orientations of the tensor axes are obtained from the eigenvectors of the
pseudo-covariance matrix. They are given – as we will show in the next section –
as strike and dip of the vector. The strike is counted clockwise from N and the dip
positively downwards.
ö “Nq£ Ò Ì¨Î Õ ì/¨ì/oÔìÔ”
In order to obtain the permeability tensor we use relations between hydraulic diffusivity
and permeability introduced by (Biot, 1962):
(—d˜
»½—d˜Á‘tsvu
w (9)
where (—d˜ is the permeability tensor, »½—d˜ the hydraulic diffusivity tensor, w is the
pore-fluid dynamic viscosity and svu
is the poroelastic modulus. The latter comprises
several other parameters. For crystalline rocks with low porosity one can simplify the
poroelastic modulus and thus obtains the approximation (Shapiro et al., 1997)
Å1Ç
(10)
sxu
‘ £
|{
± À
where ¬~} . ‘úÌ The parameters are the bulk modulus of the drained rock À , the
{
bulk modulus of the pore À€y fluid , the bulk modulus of the À‹ solid , the porosity and
}U
the coefficient of effective pressure . Generally, the ¦(Àzy term can not be neglected
À€y Z À {
since (Shapiro et al., 1997).