HR: 1340h
AN: S23B-1396    [Abstracts]
TI: Simultaneous Imaging of Q Structure and Velocity Structure by Full Waveform Inversion
AU: * Kamei, R
EM: rie@geoladm.geol.queensu.ca
AF: Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, ON K7L 3N6, Canada
AU: Pratt, R G
EM: pratt@geol.queensu.ca
AF: Department of Geological Sciences and Geological Engineering, Queen's University, Kingston, ON K7L 3N6, Canada
AB: Seismic inversion is a multiparameter problem: the observed waveforms contain information not only on velocity structure, but also on various other parameters, including attenuation and density (and the possibility of anisotropy). Among these parameters, the attenuation (or its inverse, the seismic Q value) is strongly related to useful geological variables such as rheology, fluid flow, pore fluid content and fractures. In this work, we investigate methodologies for obtaining velocity and attenuation images simultaneously. In contrast to ray-based inversions of first arrival times and/or amplitudes, Full Waveform Inversion incorporates scattered, refracted and reflected waves, and consequently improves image resolution and accuracy. In frequency domain implementations, instead of inverting Q values directly, complex valued velocities are estimated using steepest descent algorithms. The real and imaginary parts of the velocity fields, (v_r, v_i), have differing sensitivities, making the simultaneous inversion of (v_r, v_i) with steepest descent methods difficult. Although Newton methods may enable a true simultaneous inversion, these are computationally expensive, and the instability of the Hessian matrix remains problematic. A possible solution to the simultaneous inverse problem is the subspace method (a Newton method in a chosen subspace), in which we search for an optimal update direction in a 2D subspace spanned by each of the two model type steepest descent vectors: P-wave velocity and Q values. The proposed 2D subspace method requires only one additional forward modelling over the steepest descent method, and inverting the 2 × 2 projected Hessian is trivial. The off-diagonal terms of the projected Hessian matrix indicate the coupling of data sensitivities to the two model parameter types. In this study, we implemented the subspace method into frequency-domain full waveform inversion, and compared the results to the standard steepest descent method. Because the model parameterization may influence robustness and resolution of the inversion procedure, we also examined several model parameterizations, including (v_r, v_i), (v_r, Q), and their respective inverses.
DE: 7203 Body waves
DE: 7270 Tomography (6982, 8180)
SC: Seismology [S]
MN: 2007 Fall Meeting