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Introduction

A fast finite-element software for gravity anomaly calculation in complex geologic regions Yongen Cai Department of Geophysics Peking University, Beijing, 100871 Chi-yuen Wang Department of Earth and Planetary Science University of California, Berkeley, CA 94720. Introduction.

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Introduction

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  1. A fast finite-element software for gravity anomaly calculation in complex geologic regionsYongen Cai Department of Geophysics Peking University, Beijing, 100871 Chi-yuen WangDepartment of Earth and Planetary ScienceUniversity of California, Berkeley, CA 94720

  2. Introduction • For geologically complex regions, forward computation of the gravity anomaly of a density model may be computationally demanding and the bottle-neck in gravity inversion. • We present a fast finite-element software for solving this problem.

  3. P(x,y,z) dv

  4. GBOX（R.J.Blakely,1995) P(0,0,0) x R y z

  5. Boundary value problem g (x, y, z) = －

  6. Boundary condition （Jeffreys, 1962）

  7. FEM formulation

  8. Accuracy verification Density model for verifying (c= 0.001 kg/m4 ）

  9. GBOX（average density） FFEM（distributed density）

  10. Application to Taiwan Source elements: 76,500 Source nodes: 83,448 Calculated gravity points GBOX:4636 points only at ground surface FEM: 285488 at all nodal points Computer: PC with 2.3 GHz CPUs Figure 7(b) (GBOX)

  11. Comparison between FFEM and GBOX mGal FFEM: used cpu time : 280 s GBOX: used cpu time : 6780 s Figure 7(b) (GBOX)

  12. Application to Sirrea Nevada (Cai, Zhang and Wang, 2006) Calculated Bouguer anomalies by FFEM Calculated Bouguer anomalies by classical method

  13. Conclusion • A software FFEM is provided which is more accurate and much faster than the classical integration method, if density in the material body is highly heterogeneous. • The computational efficiency for the FFEM method is more pronounced in regions with greater heterogeneities.

  14. Density model The density distribution can be obtained from the velocity from seismic tomograph.

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