Time-domain finite-element wave form inversion of acoustic wave equation

The paper derives the finite element equation for acoustic wave in time domain and presents a transparent-plus-attenuation boundary condition. Forward modeling demonstrates that the boundary condition absorbs boundary reflection wave very well. On these bases, we derive the equation satisfied by elements of Jacobi matrix used in the inversion of the physical property parameters of acoustic media. In fact, the equation is the same as that of forward modeling in form. Only the right force item is different. So with the same method of forward modeling, we can get the elements of Jacobi matrix. Because the elements are variable with time and the present inversion does not permit too many unknowns. We integrate the finite elements with the same physical property as one unknown structure unit (for example, a horizontal layer or an oblique layer, etc.) and inverse the physical property parameters of these unknown structure units instead all element's unknown parameters. The method greatly reduces calculation time and saves computer memory. Also, it improves the accuracy of the inversion results and improves the stability of the solving process. The inversion equations are solved with QR decomposition method. Model results prove that the full wave equation inversion method in time domain is effective.