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| Titel |
2-D FDTD computation of seismoelectric fields excited by an underground double couple in a horizontally layered formation |
| VerfasserIn |
Z. Wang, W. Guan, Y. Gao, H. Hu |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250064638
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| Zusammenfassung |
| Electromagnetic signals have been recorded during earthquakes (e.g. Karakelian et
al., 2002). One important mechanism for the coupling between the elastic and the
electromagnetic energies is the electrokinetic effect. Gao and Hu (2010) simulated the
electromagnetic fields excited by a double couple by solving analytically the set of equations
derived by Pride (1994), which combines the Biot equations with the Maxwell
equations. However, analytical solution is not available when the geological structure
is complex. Numerical methods are thus needed to solve for the seismoelectric
fields.
In the present work, seismoelectric fields excited by an underground double couple in a
horizontally layered geological structure are computed by solving the Pride equations with a
finite-difference time-domain (FDTD) algorithm with 2-D grids. A double couple
source represents a small fault, and it has no axisymmetric nature. However, as the
layered formation is axisymmetric, we only need to solve a 2-D problem by Fourier
transforming the seismoelectric fields from the azimuthal angle θ domain to the
corresponding wavenumber m domain in cylindrical coordinates. Further, we can
prove that m -¤ 2 for a double couple source. 2-D FDTD grid is developed, and
the perfectly matched layer technique (Guan and Hu, 2008) is applied to truncate
the computational region. The radiation pattern of the double couple is computed.
The seismic and the electromagnetic fields on the surface of the layered formation
are obtained and compared to the analytical results given by Hu and Gao (2011).
Good agreements between the FDTD results and the analytical solutions show
the validity of our FDTD algorithm. Extension to a general 3-D problem is under
way.
A key issue involved in our modeling of the earthquake source in a porous medium is to
find out the body forces in the Pride equations. We point out that if Biot (1956) theory (which
is one base of Pride equations) is used, no equivalent force should be loaded on the fluid
phase, and the equivalent double couple is loaded only on the solid phase. That is because in
the constitutive equations used by Biot (1956) the shear stresses due to the rate of the shear
strains of the fluid are neglected.
This study is supported by the Natural Science Foundation of China by project
No.41174110.
References
Gao, Y., and Hu, H., 2010, Seismoelectromagnetic waves radiated by a double couple
source in a saturated porous medium, Geophys. J. Int., 181, 873-896
Guan, W., and Hu, H., 2008, Finite-difference modeling of the electroseismic
logging in a fluid-saturated porous formation, Journal of computational physics, 227,
5633-5648
Hu, H., and Gao, Y., 2011, Electromagnetic field generated by a finite fault due to
electrokinetic effect, Journal of geophysical research, 116, B08302, 1-14
Karakelian, D., Beroza, G. C., Klemperer, S. L., et al., 2002, Analysis of
ultralow-frequency electromagnetic field measurements associated with the 1999
M7.1 Hector Mine, California, earthquake sequence., Bull. Seism. Soc. Am., 92,
1513-1524
Pride, S. R., 1994, Governing equations for the coupled electromagnetics and acoustics of
porous media, Phys. Rev. B., 50, 15678-15696 |
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